Chiral magnetism plays an increasingly important role in condensed matter investigations driven by the discovery of exotic spin textures, such as the topologically protected chiral skyrmions that can form a lattice under magnetic fields. The nature and universality of the phase diagram in cubic chiral magnets has been the subject of numerous experimental and theoretical investigations, including the transition to the helimagnetic state. In MnSi, the archetypal system in this family, this transition is of first order and involves a precursor phase, where strong chiral fluctuating correlations build up. This work presents an experimental investigation of the structural and dynamical aspects of the phase transition in Fe${}_{1-x}$Co${}_{x}$Si, a system that belongs to the same family as MnSi but with the additional possibility to tune important physical interactions and parameters through variation of the Fe and Co concentration. In this system, the combination of small-angle neutron scattering and neutron spin echo spectroscopy uncovers that the scenario of the transition is qualitatively very different from that in MnSi. This goes beyond what can be expected from a comparison of the relevant length scales and thus challenges the validity of a universal approach to the helimagnetic transition in cubic chiral magnets.

[Phys. Rev. B 95, 144433] Published Fri Apr 28, 2017

]]>The strong spin-spin exchange interaction in some low-dimensional magnetic materials can give rise to a high group velocity and thermal conductivity contribution from magnons, which are energy quanta of collective spin excitations. Examples are the incommensurate layered compounds (Sr,Ca,La)${}_{14}$Cu${}_{24}$O${}_{41}$, with strong antiferromagnetic interaction along the ladders. The effects of grain boundaries and defects on quasi-one-dimensional magnon transport in these compounds are not well understood. Here, the authors report the microstructures and anisotropic thermal transport properties of textured Sr${}_{14}$Cu${}_{24}$O${}_{41}$. TEM clearly reveals nanolayered grains and the presence of dislocations and planar defects. The thermal conductivity contribution and mean free paths of magnons in the textured samples are evaluated with the use of a kinetic model for one-dimensional magnon transport, and found to be suppressed significantly compared to single crystals at low temperatures. The experimental results can be explained by a one-dimensional magnon-defect scattering model, provided that the magnon–grain boundary scattering mean free path in the anisotropic magnetic structure is smaller than the average length of these nanolayers along the $c$ axis. The finding suggests low transmission coefficients for energy-carrying magnons across grain boundaries.

[Phys. Rev. B 95, 144310] Published Thu Apr 27, 2017

]]>Recently, it was suggested that metastable spin supercurrents (spin superfluidity) are possible in the magnon condensate observed in yttrium iron garnet (YIG) magnetic films under strong magnon pumping. In YIG, magnetic anisotropy is rather weak, and this material can be treated as an isotropic ferromagnet. Its order-parameter space is a sphere of radius equal to the spontaneous magnetization $M$. A current state maps on the equatorial circumference on the sphere, and topology of the sphere allows us to continuously transform the equatorial circumference to the point. This rules out metastable current states. Only easy-plane anisotropy reducing the order-parameter space from the sphere to the equatorial circumference makes current states topologically stable. However, by pumping magnons to the isotropic ferromagnet in a magnetic field, it is possible to support a strongly nonequilibrium state with fixed average ${M}_{z}$. This confines the precessing spin to an “easy plane” of dynamical rather than topological origin and makes metastable spin currents possible. But final judgment requires evaluation of the Landau superfluidity criterion as done in the present work. The conclusion is that spin superfluidity in YIG films is possible in principle, although the recently published claim of its observation is not justified.

[Phys. Rev. B 95, 144432] Published Thu Apr 27, 2017

]]>The tenfold way classification of noninteracting topological insulators represents a great success towards understanding topological states of matter. The question the authors try to address is: can all possible topological insulators be represented by the prototypes in the tenfold way classification (perhaps enriched with some extra symmetries), or are there exceptions fundamentally different from those prototypes? The Hopf insulator was considered as a special type of TI that is not obviously represented by any prototype in the tenfold way classification, but its stability and classification were never clarified. In this work, the authors identify a generalized particle-hole symmetry that gives the Hopf insulator and its higher-dimensional analog a rigorous definition and classification. Moreover, they provide a very heuristic understanding of the minimal models for the 3$d$ and 4$d$ Hopf insulators, based on which possible experimental realization of a Hopf insulator can be found.

[Phys. Rev. B 95, 161116(R)] Published Thu Apr 27, 2017

]]>Recently, there has been a surge of interest in effects arising from the interplay between the electron-electron interaction and spin-orbit coupling (SOC). One such effect is a novel type of collective spin excitations (chiral spin waves), which are oscillations of the magnetization that exist even without an external magnetic field. However, a typical experimental setup in a semiconductor heterostructure includes a magnetic field as well as both Rashba and Dresselhaus types of SOC. In this case, the spectrum of chiral spin waves becomes fairly complicated, with some branches running into or splitting off the continuum of spin-flip excitations. The authors show that this complicated physics can be understood by exact mapping of the quantum kinetic equation for a two-dimensional Fermi liquid onto an effective one-dimensional tight-binding model. This mapping is a useful tool that helps us to understand the nature of the collective modes in a Fermi liquid of arbitrary type.

[Phys. Rev. B 95, 165140] Published Wed Apr 26, 2017

]]>Experiments with ultracold fermionic atoms in optical lattices have attracted much interest in condensed matter physics as they serve as a model for electrons in solids. While experiments now allow the observation of nonequilibrium transport processes with single-site resolution, the theoretical description, especially for strongly correlated fermions, remains very challenging. Standard methods, such as the density matrix renormalization group (DMRG) and nonequilibrium Green functions (NEGF), have been primarily applied to one-dimensional systems and weak to moderate coupling, respectively. Here, the authors perform a detailed comparison of DMRG and NEGF and demonstrate that both can be benchmarked against each other. They observe complementary applicability ranges suggesting that a combination of both will allow to substantially expand the range and duration of first-principles quantum dynamics simulations for fermionic lattice systems.

[Phys. Rev. B 95, 165139] Published Tue Apr 25, 2017

]]>This paper reports on the observation of the magnetic quantum ratchet effect in (Cd,Mn)Te- and CdTe-based quantum well structures with an asymmetric lateral dual grating gate superlattice subjected to an external magnetic field ($B$), applied normal to the quantum well plane. An electric current excited by terahertz laser radiation shows 1/$B$-periodic oscillations with amplitude much larger than the photocurrent at zero magnetic field. It is shown that the photocurrent generation is due to the combined action of a spatially periodic lateral potential and the spatially modulated radiation due to the near-field effects of light diffraction. The magnitude and direction of the photocurrent are determined by the degree of the lateral asymmetry controlled by the variation of voltages applied to the individual gates. The observed magneto-oscillations with enhanced photocurrent amplitude result from Landau quantization. For (Cd,Mn)Te-based structures at low temperatures, a beating-like pattern of the oscillations is observed, caused by the interplay of Zeeman and Landau splitting. A theoretical analysis, considering the magnetic quantum ratchet effect in the framework of a semiclassical approach, describes quite well the experimental results.

[Phys. Rev. B 95, 155442] Published Mon Apr 24, 2017

]]>Among various superconductors, the series of the layered organic superconductors, $\lambda $-(BETS)${}_{2}$Fe${}_{1-x}$Ga${}_{x}$Cl${}_{4}$, where BETS=bis(ethylenedithio)tetraselenafulvalene, is known to show very unique phase diagrams because of the exchange interaction between the conduction electrons in the BETS layer and the localized 3$d$ spins of the Fe ions in the insulating layers. The large internal field (${H}_{\text{int}}$) by the 3$d$ spins is theoretically predicted to induce a peculiar superconducting phase diagram, where a paramagnetic phase with unconventional vortices appears. By systematic measurements of the magnetic torque and resistance, the authors determine the magnetic phase diagram for $x$=0.37 for an in-plane field, which has a maximum ${T}_{c}$ at 14 T (=${H}_{\text{int}}$). They also find anomalously large energy dissipation due to Josephson vortex dynamics for $H$=${H}_{\text{int}}$, in which microscopic distributions of the paramagnetic and super-currents play an essential role. These results will help develop a new field in vortex matter physics.

[Phys. Rev. B 95, 165133] Published Mon Apr 24, 2017

]]>Graphene, a two-dimensional monatomic layer of crystal carbon, has recently emerged as a potential material for next-generation optoelectronic devices owing to unique properties arising from its massless Dirac fermions. However, the intrinsic carrier dynamics of graphene has remained a mystery even for the simple case of carrier cooling after the photoexcitation. This is because the observed temporal variations of the nonequilibrium carriers in graphene have been thoroughly described in terms of a defect-induced extrinsic effect known as “supercollision” (SC). The SC process is based on defect-mediated electron-acoustic phonon scattering and theoretically has been predicted to reduce with the increase of mobility of a material. Here, the authors have prepared extremely high mobility graphene and traced the dynamics of photoexcited carriers in the Dirac bands directly by time- and angle-resolved photoemission spectroscopy. They successfully observed suppression of SC and extracted the intrinsic dynamical properties of graphene, such as anharmonic decay of the optical phonons and the bottleneck relaxation at the Dirac point. Breaking the limit of SC, their research also has technological significance in developing graphene-based optoelectronic devices.

[Phys. Rev. B 95, 165303] Published Wed Apr 19, 2017

]]>Many-body localization plays an increasing role in condensed matter theory, both because it challenges the fundaments of statistical physics, and because it allows us to engineer several new, exotic, stable phases of matter. In this paper, the authors address the issue of the stability of a many-body localized material in contact with an ergodic grain, i.e., an imperfect bath made of a few interacting degrees of freedom. Thanks to detailed microscopic analysis and numerics, they conclude that such an ergodic grain eventually destabilizes the localized phase in the following cases: if the spatial dimension is higher than one, or if the spatial dimension is one but the localization length of the localized material is larger than a fixed threshold value. In realistic materials, these ergodic grains are always present as Griffiths regions where the disorder is anomalously small, and hence, the authors conclude that the localized phase in such materials is unstable, strictly speaking. Transport and thermalization are however exponentially suppressed in the distance between ergodic grains.

[Phys. Rev. B 95, 155129] Published Tue Apr 18, 2017

]]>Essentially all high temperature superconductors display a “strange metal” regime above their critical temperatures, where electrical and thermal transport cannot be explained within the traditional Fermi liquid paradigm of quasiparticle excitations. This paper focuses on the first, and newly discovered, solvable theory of a disordered metal without quasiparticle excitations, obtained by extending the Sachdev-Ye-Kitaev (SYK) models to finite spatial dimensions. The complete thermoelectric conductivity matrix is computed, and a surprising exact relation is found between the Seebeck coefficient and the derivative of the thermodynamic entropy with respect to the density. These computations are then compared with holographic theories which map strange metals onto black holes in theories of quantum gravity with one extra spatial dimension.

[Phys. Rev. B 95, 155131] Published Tue Apr 18, 2017

]]>Researchers predict new two-dimensional materials whose structures differ from their three-dimensional counterparts.

[Phys. Rev. B 95, 155426] Published Tue Apr 18, 2017

]]>Time-periodic driving enables new nonequilibrium quantum phases of matter with topologically protected and quantum coherent motion that would be forbidden in thermal equilibrium. This work uncovers two new classes of driven topological phases in driven two-dimensional spin systems: i) Floquet symmetry protected topological phases, which are driven and interacting analogs of topological insulators whose edge states are protected against disorder and localization; and ii) Floquet enriched topological orders, in which time-dependent driving “breaks” the spins into anyonic particles with fractional statistics and oscillating topological charge. The physics of these phases is explored by constructing toy models that can be explicitly solved despite the presence of strong interactions.

[Phys. Rev. B 95, 155126] Published Mon Apr 17, 2017

]]>The Weyl fermion – originally proposed to describe neutrinos – arises in a topological semimetallic phase in condensed matter systems, exhibiting such novel properties as surface Fermi arcs and a chiral anomaly. Although Weyl fermions have been observed, it is challenging to find materials that exhibit them near the Fermi level. The authors prove that Weyl fermions can be created in band-inverted materials in a large class of crystal systems by applying a magnetic field along various symmetry axes of the crystal. As the field direction is changed, the Weyl points move in momentum space, during which time pairs are created and annihilated. However, in the highly symmetric ${T}_{d}$ point group, the Weyl points cannot completely disappear: at least one pair must remain for any direction of the magnetic field. Furthermore, a semiclassical analysis shows that the magnetoresistance scales differently for Weyl fermions created by a magnetic field compared to intrinsic Weyl points. The ability to create Weyl fermions will lead to new material candidates in these crystal systems; controlling their positions opens the possibility to track and manipulate Fermi arcs.

[Phys. Rev. B 95, 161306(R)] Published Mon Apr 17, 2017

]]>Skyrmions, swirling magnetic textures with a topological character, have been gaining much attention in spintronics due to the fundamental interest as well as their touted utility as information carriers in ultradense and low-power memory devices. Generally, a skyrmion behaves as a massive particle moving in a viscous medium and experiencing a Magnus force, which is proportional to its winding number and the spin polarization of the magnet. There is a class of tunable ferrimagnets, such as rare-earth transition-metal alloys, exhibiting the angular momentum compensation point at which the spin density changes sign. This raises the possibility to engineer both the magnitude and the sign of the Magnus force acting on the skyrmions in the ferrimagnets. The authors exploit this to suggest that ferrimagnetic skyrmions can exhibit snake trajectories along the line of the vanishing spin density, analogous to the snake orbits of electrons in a nonuniform magnetic field. This can be utilized as dynamically self-focusing racetracks for skyrmions, paving the way for skyrmion-based memory devices.

[Phys. Rev. B 95, 140404(R)] Published Fri Apr 14, 2017

]]>Quantum effects in magnetic materials are prevalent when the magnetic ions support spin-½ moments, as in Cu${}^{2+}$-based materials. Strong quantum effects can also be achieved when the large angular momentum states are reduced to an effective spin-½ subspace (${S}_{\text{eff}}$=½) by the combined effect of spin-orbit coupling and the crystal electric field. A consequence of this is the introduction of anisotropy to the g tensor and the effective exchange interactions. Such anisotropic ${S}_{\text{eff}}$=½ models have recently been applied successfully to rare-earth based frustrated pyrochlore materials, where they are found to lead to particularly rich phenomenology for XY-like $g$ tensors. Here, it is shown that a similar quantum model should apply to the recently discovered “high temperature” Co${}^{2+}$ pyrochlores, NaCaCo${}_{2}$F${}_{7}$ and NaSrCo${}_{2}$F${}_{7}$, based on a fit to their single ion levels as measured via inelastic neutron scattering. The effect of the intrinsic crystalline disorder on the anisotropy of the ${S}_{\text{eff}}$=½ moments in these materials is also estimated.

[Phys. Rev. B 95, 144414] Published Thu Apr 13, 2017

]]>This work studies the manipulation of photoelectron emission from metallic nanostructures by intense single-cycle terahertz transients. Specifically, the authors employ streaking spectroscopy to study the kinetic energy of photoelectrons emitted from metal nanotips exposed to tailored static and terahertz electrical fields. Supported by detailed numerical simulations, the measurements provide quantitative information on the temporal and spatial properties of terahertz near fields at metal nanotips. The study illustrates far-reaching control over the trajectories and phase-space density evolution of photoelectron wavepackets acted upon by strong static and dynamic near fields. The results are relevant for applications of nanoscopic photoelectron sources employed in ultrafast electron diffraction and microscopy.

[Phys. Rev. B 95, 165416] Published Thu Apr 13, 2017

]]>In ferromagnetic layers, spin-wave modes can be excited by a laser pulse launching the magnetization out of equilibrium and into precession. The trajectory of the magnetization vector can be fully reconstructed using different magneto-optical effects, generally admitted to give a faithful picture of the dynamics. The authors show that this is not always the case and that magneto-optical effects can indeed be deceiving. For this, they study the case of a weakly absorbing ferromagnet, the semiconductor GaMnAs, in which perpendicular standing spin waves are excited by a laser pulse. Quite counterintuitively, the optical detection shows the first two excited modes to be of opposite chirality, whereas theory tells us the spin waves actually rotate in the same direction. The paper demonstrates that this unexpected and surprising effect is a pure optical illusion. It can perfectly be explained by taking into account absorption and optical phase shift inside the layer, the latter being particularly strong in weakly absorbing layers. These results provide a correct identification of spin-wave modes, enabling a trustworthy estimation of their respective weight as well as an unambiguous determination of the spin stiffness parameter.

[Phys. Rev. B 95, 144411] Published Wed Apr 12, 2017

]]>Topological pumping, which was originally proposed by Thouless, is a beautiful manifestation of quantum effects in transport phenomena. Thouless’s topological pumping is characterized by the topology of Bloch states in the space of momentum and time, and can be viewed as a dynamical analog of the integer quantum Hall effect of noninteracting fermions. Here, the authors propose a systematic procedure to construct nontrivial classes of topological pumping from strongly correlated quantum Hall states on a thin torus. In particular, this procedure is applied to the bosonic integer quantum Hall (BIQH) state formed by two species of bosons. The BIQH state is an example of a symmetry-protected topological (SPT) state of interacting bosons, and is characterized by nontrivial Hall responses. The authors find that the thin-torus counterpart of the BIQH state is the Haldane state of emergent spin-1 degrees of freedom, which is also a SPT state. The authors further show that an adiabatic change between the Haldane phase and trivial Mott insulators constitute an “off-diagonal” topological pumping, in which the translation of the lattice potential for one component induces a current in the other.

[Phys. Rev. B 95, 165116] Published Wed Apr 12, 2017

]]>In a set of papers, the authors present a comprehensive investigation of the magnetic, structural, and dynamic properties of thin film Ni-Co, Ni-Fe, and Co-Fe alloys and uncover new phenomena in systems previously thought to be well understood and commonplace. High-precision measurements of saturation magnetization, effective magnetization, spectroscopic $g$ factor, inhomogeneous linewidth broadening, and damping are performed, allowing precise testing of \textit{ab initio} calculations for the orbital magnetic moment and the intrinsic magnetic damping, which is the most integral motivation of this work. Quantitative comparison between experiment and theory is shown and discussed for magnetic damping spanning a broad range of material parameters. Furthermore, a large set of data has been created that will serve as a glossary for alloy magnetic properties in thin films.

[Phys. Rev. B 95, 134410] Published Fri Apr 07, 2017

]]>One of the challenges in quantum information science is to achieve ultralong spin coherence in naturally grown solid-state systems. So far, isotope engineering is generally needed to suppress the main relaxation mechanism caused by the interaction with nuclear spins. The authors demonstrate here that this ambitious goal can be achieved in binary compounds with natural isotope abundance too. They attain a spin-locked subspace with a drastically reduced spin-decoherence rate through the combination of two effects. First, the suppression of heteronuclear spin cross-talk is achieved by applying a moderate magnetic field. This leads to two dilute and weakly interacting spin baths. Second, because the interaction between nuclei becomes weak, the mutual spin flip-flop processes occur at lower rate, which can be viewed as a reduction of the high-frequency part of the noise spectrum. As a result, dynamic decoupling protocols demonstrate high performance. Using this approach, the authors are able to preserve a coherent spin superposition above 20 ms, which is an improvement by more than one order of magnitude compared to the earlier reported value in SiC.

[Phys. Rev. B 95, 161201(R)] Published Fri Apr 07, 2017

]]>The historical origins of topological effects in wave equations can be traced to the relativistic Dirac equation, which includes fundamental symmetries related, for example, to spin and charge. Dirac devised his equation to eliminate the deficiencies of a previous theory, and achieved this by effectively taking the square root of the corresponding wave equation. Here, the authors explore the analogue of these considerations for tight-binding models. They show that starting from a suitable parent system, the square-root operation can induce nontrivial topological effects when it reduces the crystal symmetry, as this makes room for additional independent components and emerging fundamental symmetries. The resulting models display an enriched band structure, including additional gaps and controls to induce states into them. These models retain their scope for practical implementations, as the authors explore in the setting of silicon photonics.

[Phys. Rev. B 95, 165109] Published Thu Apr 06, 2017

]]>Vortices are known to nucleate when classical fluids or superfluids flow past an impenetrable obstacle. Solid-state planar ferromagnets are manifestly not fluids, but their magnetization dynamics can be formulated in terms of dispersive hydrodynamic equations that exhibit peculiar properties such as broken Galilean invariance and support large-amplitude spin-density waves. Here, the authors leverage this fluid interpretation of magnetization dynamics to explore novel large-amplitude nonlinear states. The fluidlike behavior of magnetization dynamics includes quantized vortex-antivortex pair shedding at subsonic conditions and the formation of nonlinear wavefronts and a Mach cone at supersonic conditions. These results are observed both when a spin-density wave interacts with a static obstacle and when an obstacle moves across a homogeneously magnetized ferromagnet. Qualitative agreement with observations in superfluids and quantitative agreement with analytical calculations are found. The results here demonstrate that vortex-antivortex complexes can be nucleated in planar ferromagnets and they exhibit features analogous to fluids and superfluids. The hydrodynamic interpretation of planar magnetic materials yields new insights into magnetization dynamics.

[Phys. Rev. B 95, 134409] Published Wed Apr 05, 2017

]]>Spin Hall phenomena are a collection of relativistic spin-orbit coupling effects that permit control and detection of magnetization dynamics in magnetic materials with electrical currents. In spintronics, they have been studied extensively in bilayer structures consisting of a magnetically ordered material interfaced by a current-carrying paramagnetic metal with strong spin-orbit coupling. In this work, the authors propose the magnetic phase qubit, the macroscopic quantum spintronic device that can be built from such bilayers and that permits full electrical control and readout via spin Hall phenomena. The device is an example of a macroscopic qubit that can be constructed from solid-state materials and so should offer the same advantages as the superconducting qubits of strong inter-qubit coupling and scalability. An estimate of the relevant physical parameters based on current spintronic technology gives a qubit operational temperature that is more than an order of magnitude higher than for existing superconducting qubits, thus opening the possibility of macroscopic quantum information processing at temperatures above the dilution refrigerator range. The authors also show that a coupled array of these qubits can realize a quantum annealer, which could be used to solve certain hard optimization problems and machine learning tasks.

[Phys. Rev. B 95, 144402] Published Tue Apr 04, 2017

]]>Quantum systems are known to exhibit several types of exotic behavior beyond classical physics, including quantum entanglement, which describes a web of nonlocal correlations among the constituents of the system. Despite numerous advances in understanding and quantifying the bipartite entanglement entropy of a pure state, where the whole system is described by a wave function, less is understood about quantum entanglement of a mixed state, where the system is described by a density matrix. The partial transpose of the density matrix, in which one takes the transpose only for a subsystem, and its corresponding entanglement measure – called the (logarithmic) negativity – have been introduced as an effective probe for the quantum entanglement in bosonic mixed states. In this work, the authors present a scheme to compute an analog of the entanglement negativity in the fermionic mixed states using a partial time-reversal transformation. Various examples are investigated. In particular, it is shown that the partial time reversal is an intrinsically fermionic construction, in that it can capture the formation of the edge Majorana fermions, while the partial transpose obtained from the bosonic partial transpose through the Jordan-Wigner transformation fails.

[Phys. Rev. B 95, 165101] Published Mon Apr 03, 2017

]]>Computer simulations show that a uniform but not periodic arrangement of tiny, wave-scattering pillars can host an efficient acoustic channel of arbitrary shape.

[Phys. Rev. B 95, 094120] Published Fri Mar 31, 2017

]]>Quantum mechanics nominally works in a space of infinite dimensions. In practice, however, the computer-based solution of quantum-mechanical problems relies on a restriction of that space to a high but finite number of dimensions. This is important since computational quantum-mechanical simulations have a huge presence in today’s physical science. In many instances, the finite-dimensional space needs to evolve during a simulation. In this work, the authors extend the formalism used for finite spaces to that of evolving spaces. Here, well-known equations acquire a new meaning in the language of the curved spaces familiar to general relativity. The authors provide a novel geometric approach to calculating tensorially well-defined derivatives of quantum-mechanical state and operator representations with respect to arbitrary external parameters. Contributions due to basis function evolution play the role of the Christoffel symbol terms of Riemannian geometry. The curvature of the resulting geometry is equivalent to Berry’s curvature, but generalized for possibly nonorthogonal, smoothly varying basis functions. This work provides insights into dynamical quantum simulations that enable the development of improved simulation algorithms. It also opens up new scientific horizons, as is ever the case when establishing formal links between different subfields of physics.

[Phys. Rev. B 95, 115155] Published Fri Mar 31, 2017

]]>Weyl points in photonic systems have gained much attention for their novel properties, such as topologically protected robustness and gapless surface states. However, many Weyl photonic crystal structures designed previously are probably too complicated for nanofabrication. Here, the authors discover that Weyl points can be found in metallic chiral woodpile photonic crystals, which can be fabricated using current nanotechnology. Weyl points with topological charge not only 1 but also 2 can be found in these photonic crystals. When the constituent materials change from the air-in-metal to metal-in-air configuration, the sign of the topological charge will change, leading to a topological phase transition, and the bands change from topologically trivial to nontrivial. Gapless surface states exist in the latter case and robust transport properties have also been demonstrated numerically. When the metallic component is replaced by dielectric materials, such as silicon, the Weyl points still exist. These woodpile photonic crystals should become promising platforms for exploring Weyl point related physics at optical frequencies.

[Phys. Rev. B 95, 125136] Published Wed Mar 29, 2017

]]>The Sachdev-Ye-Kitaev (SYK) model is a quantum mechanical model for randomly interaction fermions in a quantum dot. This paper studies the properties of the many-body spectrum of the SYK model and finds a periodicity of the spectral properties in term of the number of fermion modes in the quantum dot. In particular, the level statistics of SYK spectrum are investigated. It is found that the many-body level spacing of the SYK model follows Wigner-Dyson statistics, consistent with the thermalizing nature of the SYK model. Interestingly, the level statistics goes through those of different random matrix ensembles periodically. The periodicity can be explained by viewing the SYK model as an effective model for the boundary of 1D interacting fermionic symmetry-protected topological states. The periodicity in the SYK spectrum is therefore tied to the classification of fermionic topological states in one dimension.

[Phys. Rev. B 95, 115150] Published Tue Mar 28, 2017

]]>The superconducting ferromagnet UCoGe is a versatile laboratory tool to investigate the interplay of ferromagnetism (${T}_{\text{c}}$ = 3.0 K) and superconductivity (${T}_{\text{sc}}$ = 0.6 K). Here, the authors report the superconducting and magnetic phase diagram in the field-temperature plane determined by thermal expansion measurements. The use of the thermal expansion technique has the advantage that it involves a thermodynamic bulk probe. The experiments were carried out on a single crystal in fixed magnetic fields applied along the principal axes of the orthorhombic crystal. Since the phase diagram depends sensitively on the orientation of the magnetic field with respect to the crystal axes, the thermal expansion cell was mounted on a piezoelectric rotator to enable $i\phantom{\rule{0}{0ex}}n$ $s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}u$ tuning of the magnetic field angle. The authors observe that the Curie point for $B$ || $b$ shifts gradually to lower temperatures and present bulk-sensitive evidence for the unusual S-shaped ${B}_{\text{c2}}$ curve for the same field orientation. The results lend further support to theoretical proposals of spin-fluctuation mediated enhancement of superconductivity in a magnetic field ($B$ || $b$).

[Phys. Rev. B 95, 115151] Published Tue Mar 28, 2017

]]>Localization and transport of a quantum particle in fractal lattices with random-site connectivity and without any diagonal disorder are investigated. It is the random hopping terms that introduce disorder-causing localization of a single-particle wave function. The lattices are generated so that their fractal (Hausdorff and spectral) dimensions are independently controlled. Therefore, it is possible to analyze how different fractal dimensions influence the localization properties, which is the main purpose of this paper.

[Phys. Rev. B 95, 104206] Published Mon Mar 27, 2017

]]>Skyrmions are topological knots in the magnetization, and they can be stabilized as particle-like states in a ferromagnetic environment. Due to their noncollinear spin texture, the local density of states is modified, and they are thus electronically different from their ferromagnetic surroundings. Exploiting the spatial and energy resolution of scanning tunneling microscopy and spectroscopy this work demonstrates how the local spin configuration within a skyrmion modifies the respective differential conductance. This noncollinear magnetoresistance (NCMR) can reach values of 50% in Pd/Fe/Ir(111) and allows imaging of skyrmions with nonmagnetic probe electrodes. Because of its large signal and the reduced technical requirements, NCMR is expected to be useful for the detection of the magnetic state with planar tunnel junctions in skyrmionic spintronics applications.

[Phys. Rev. B 95, 104433] Published Mon Mar 27, 2017

]]>Entanglement entropy is a widely applicable indicator of the onset of quantum chaos. In particular, the entanglement entropy of the wave function after a quench from an initial state with low entanglement can be used to study the thermalization of the system. Here, the authors propose an initial-state independent quantity called the “operator entanglement entropy” (opEE) to extract the properties of the unitary evolution operator. They study the growth of the opEE in Floquet, chaotic, and many-body localized systems. They respectively have a linear, power-law, and logarithmic growth before reaching extensive saturation values. The most chaotic Floquet spin model has the maximal saturation value among the three classes and is identical to the value of a random unitary operator (the Page value). The authors interpret the opEE as the state EE of a quenched state living in a doubled Hilbert space, thus establishing its consistency with the existing state EE results. They conclude that the EE of the evolution operator should characterize the propagation of information in these systems.

[Phys. Rev. B 95, 094206] Published Thu Mar 23, 2017

]]>Quantum spin liquids can be well described in the language of gauge theory. While most theoretical effort has been focused on gauge theories with a familiar vector gauge field, there exists a stable class of quantum spin liquids described by higher-rank tensor gauge fields. Here, the authors focus on a class of stable three-dimensional spin liquids described by symmetric tensor $U$(1) gauge fields. They find that these spin liquids feature an exotic class of excitations that are restricted to motion along lower-dimensional subspaces, a phenomenon seen earlier in fracton models. They show how this subdimensional behavior follows naturally from a set of higher-moment charge conservation laws that place severe restrictions on particle motion. This work opens up an exciting new direction in the field of spin liquids.

[Phys. Rev. B 95, 115139] Published Wed Mar 22, 2017

]]>Berry connection is conventionally defined as a static gauge field in the Brillouin zone. Here, the authors show that for three-dimensional (3D) time-reversal invariant superconductors, a generalized Berry gauge field behaves as a fluctuating field of a momentum-space Chern-Simons gauge theory, which is the effective theory of the system. Furthermore, the gapless nodal lines in the momentum space which may arise in such superconductors play exactly the role of Wilson loop observables, and their linking and knot invariants modify the gravitational theta angle which can be evaluated from the Chern-Simons action. This angle induces a topological gravitomagnetoelectric effect where a temperature gradient induces a rotational energy flow. The authors also show how this idea may be potentially generalized to realize topological strings in six-dimensional phase space, where the physical space defects play the role of topological D-branes.

[Phys. Rev. B 95, 094512] Published Mon Mar 20, 2017

]]>The dynamics of the phase separation of very dilute ${}^{3}$He-${}^{4}$He solid solutions is studied using high-sensitivity low-temperature NMR techniques. This paper compares the results observed at very low concentrations (down to 16 ppm ${}^{3}$He) with previous results up to 2%. The growth of ${}^{3}$He nanodroplets after phase separation is analyzed and shown to follow a one-third power-law dependence at long times as expected for Ostwald ripening. The time scale for the dynamics is governed by the relatively fast quantum diffusion in solid helium, allowing the long-term behavior of the ripening process to be explored by experiments lasting a few days rather than several months, as in the case of metallic alloys.

[Phys. Rev. B 95, 104107] Published Mon Mar 20, 2017

]]>Here, the authors make connection to the foundation of quantum mechanics. As stated by Schrödinger, entanglement is “the characteristic trait of quantum mechanics” and seemingly at odds with the description of nature. In this joint experimental and theoretical work, the authors discuss the possibility of spin entanglement of two freely propagating electrons. These have been created by a collision of a spin-polarized primary electron with the Shockley surface state of the Cu(111) surface. The contribution of this state can clearly be identified in the experiment and allows use of its particular features. The electron scattering study done here resembles the scenario envisioned by the seminal works of Einstein-Podolsky-Rosen and Schrödinger to work out the peculiar nature of entanglement.

[Phys. Rev. B 95, 115134] Published Fri Mar 17, 2017

]]>Evidence for hydrodynamic flow of electrons was recently reported in a number of transport experiments in high-mobility semiconductor and graphene nanodevices. The electron liquid in these systems moves in the presence of a long-range disorder potential, which makes the entropy per electron position dependent. The resulting nonisentropic flow is markedly different from Stokesian flow: the resistivity is governed not only by the viscosity of the electron fluid but also by its thermal conductivity. Here, the authors show that the weak-field magnetoresistance (MR) in this hydrodynamic regime is caused by the modification of the flow pattern by the Lorentz force. The resulting MR is positive and is controlled by the shear viscosity alone. This may help separate the viscous and the thermal contributions to the resistivity.

[Phys. Rev. B 95, 121301(R)] Published Thu Mar 16, 2017

]]>The problem of the half-filled Landau level has recently received renewed interest. The ground state of the half-filled lowest Landau level is believed to be a Fermi liquid of an emergent “composite fermion” quasiparticle. In the past, Halperin-Lee-Read (HLR) theory was employed to describe the low-energy physics of this Fermi-liquid state. Recently, an alternative theory based on the idea of the Dirac composite fermion has been proposed, in which particle-hole symmetry is manifestly realized. In this paper, a previously unknown exact relationship between response functions at finite frequency and wave number is derived. It is shown that this relationship can be accommodated by the Dirac composite fermion theory, but is violated by the HLR theory.

[Phys. Rev. B 95, 125120] Published Wed Mar 15, 2017

]]>Antiferromagnetic spin systems have been extensively studied but still yield surprising new physics, especially on geometrically frustrated lattices and in the quantum regime. Less well studied are ferrimagnets, whose net magnetization is positive but less than that of the fully polarized state. Here, the authors study a frustrated ferrimagnetic spin Hamiltonian on a kagome lattice with both ferromagnetic and antiferromagnetic couplings, inspired by density functional theory analysis of the material Cu${}_{3}$V${}_{2}$O${}_{7}$(OH${}_{2}$)$\cdot $2H${}_{2}$O (volborthite). This model displays a very broad plateau in the magnetization curve at 1/3-full magnetization. We incorporate quantum effects via a semiclassical 1/$S$ expansion and find that they further stabilize the plateau. When the frustrating coupling becomes strong enough to almost destabilize the plateau, we find an instability to bound-magnon “exciton” condensation, which results in an exotic “chiral liquid” phase with relativistic excitations that breaks inversion (but no other) symmetry.

[Phys. Rev. B 95, 104411] Published Mon Mar 13, 2017

]]>Modeling the dislocation glide through atomic-scale simulations in pure solids and alloys, the authors show that in the course of the plastic deformation the variation of the crystal’s zero point energy (ZPE) and the dislocation potential energy barriers are of opposite sign. The multiplicity of situations in which this same trend has been observed allows us to conclude that quantum fluctuations, giving rise to crystal ZPE, make dislocation glide easier in most materials.

[Phys. Rev. B 95, 094103] Published Wed Mar 08, 2017

]]>An exciting recent development is the ability to drive a quantum material into a highly excited, but still phase coherent state, using intense laser sources. Although such a state can be extremely short-lived, it can be long enough for detection using ultrafast spectroscopy. The authors here show how intense laser pulses can generate a new type of nonequilibrium superconducting phase. The authors demonstrate theoretically that the pump induces a “twisted” BCS state that subsequently evolves coherently after the cessation of the pulse. They show how the nonlinear coupling of the pump pulse light can induce coherent dynamics of $\mathrm{\Delta}(t)$, consistent with the experiment [R. Matsunaga et al. Phys. Rev. Lett. 111, 057002 (2013)]. Moreover, the authors show that more intense pump pulses can create a far-from-equilibrium phase of gapless superconductivity, originally predicted in the context of ultracold atomic gases. The terahertz pump is found to be much more efficient than the interaction quench at producing this gapless phase. These results open the door to engineering and observing new dynamical phases and phase transitions in quantum materials.

[Phys. Rev. B 95, 104507] Published Wed Mar 08, 2017

]]>Here, the authors discover a missing link between antiferromagnetism and the Hall effect by introducing a theoretical framework based on a novel concept, cluster multipole (CMP), to characterize macroscopic magnetization of antiferromagnets. Whereas the anomalous Hall effect (AHE) is usually observed in ferromagnets and explained as an outcome of the macroscopic dipole magnetization, CMP theory reveals that a certain type of antiferromagnetic (AFM) structure induces the AHE despite no net magnetization. The new order parameters enable us to characterize the AHE in the AFM states and explain the AHE in the AFM states of Mn${}_{3}$Ir and Mn${}_{3}Z$ ($Z$ = Sn, Ge), for which the large AHE has recently been studied. Furthermore, the theory can deal with the AHE in antiferromagnets on an equal footing with that in simple ferromagnets. The authors compare the AHE in antiferromagnetic Mn${}_{3}Z$ Mn${}_{3}Z$ and ferromagnetic bcc Fe based on first-principles calculations and find out their similarity with respect to the CMP moments. The theory brings on a significant step forward in our current understanding of anomalous current in condensed matter, and the obtained knowledge could be crucial in the future for the design of antiferromagnetic devices, e.g., with possible spintronics-related applications.

[Phys. Rev. B 95, 094406] Published Tue Mar 07, 2017

]]>Traveling-wave parametric amplifiers may be fabricated from superconducting films that exhibit highly nonlinear kinetic inductance. The coplanar waveguide of such a microwave device, extending to a meter or more in length but compacted to reside on a chip of the order of a square centimeter, is engineered with periodic variations in its width. These width variations, or loadings, alter the dispersion characteristics of a nonlinear current propagating along the waveguide, changing its group velocity and modulation behavior. A strong pump and a small signal injected into one end of the waveguide mix parametrically in the presence of the nonlinear kinetic inductance. Engineered dispersion induces the favorable conditions of overall phase matching, leading to generation of idler products as well as signal amplification of wide bandwidth, high dynamic range, and low noise, making the device of particular use in quantum computing and photon detection. The authors present a theoretical framework in which signal gain may be calculated solely from loading design. This involves construction of a metamaterial band theory of the engineered dispersion, which is used as a basis to describe the mixing of nonlinear traveling waves.

[Phys. Rev. B 95, 104506] Published Mon Mar 06, 2017

]]>Topological phenomena in two-dimensional materials are characterized by an electrical conductivity that is restricted to the edges of a sample. A heterostructure of the semiconductors indium arsenide and gallium antimonide, InAs/GaSb, can be a quantum spin Hall insulator (QSHI), which exhibits spin-resolved channels at the edges but has an insulating bulk. The QSHI is linked to a nontrivial inverted band structure that emerges only for certain thicknesses of the InAs and GaSb layers. Here, the authors have exposed InAs/GaSb samples to variable compressive and tensile strain through piezo-electric elements. It is observed that the system’s electrical properties are very susceptible to even small values of such mechanical deformation. Band structure calculations reveal the impact of strain on the nontrivial electronic band structure and highlight the potential of strain-engineering for the observation of the quantum spin Hall insulating phase in this material system.

[Phys. Rev. B 95, 115108] Published Mon Mar 06, 2017

]]>Here, the authors show that in molecules there is an emergent spin molecular-orbital coupling (SMOC) that is not merely inherited from the atomic scale. This causes the spin of electrons to become entangled with currents running around the molecule. Molecules that are symmetric only under rotation by 2$\pi $/$N$, provide a striking example. For odd $N$ there is a ladder of orbital states and SMOC allows the system to climb up or down the ladder, much like spin-orbit coupling in atoms. However, for even $N$ there is a ring of states – SMOC can move the system around in either directions, without ever reaching a minimum or maximum. This is a consequence of the umklapp spin-orbit scattering present in even-$N$ molecules that is precluded in odd-$N$ molecules by time-reversal symmetry. The authors show that SMOC is large in organometallic complexes and explore how synthetic chemistry can be used to control SMOC. Such control has numerous potential applications.

[Phys. Rev. B 95, 115109] Published Mon Mar 06, 2017

]]>Bosonic symmetry protected topological (SPT) phases are bosonic analogs of free-fermion topological insulators and superconductors, but require interactions to be realized. Previously, a wide range of bosonic SPT phases protected by on-site symmetries has been systematically investigated, which are found to be related to group cohomology theory. However, a systematic understanding of SPT phases protected by spatial symmetries is still lacking. Here, the authors present systematic constructions of tensor-network wave functions for bosonic SPT phases protected by a general symmetry group SG involving both on-site and spatial symmetries. They find, in spatial dimension $d=1,2,3$, that a wide range of bosonic SPT phases are classified by the group cohomology ${H}^{d+1}$SG,U(1), in which the time-reversal symmetry and mirror symmetries should be treated as anti-unitary operations. They also provide generic tensor-network wave functions for these SPT phases that are useful for numerical simulations. As a by-product, the authors demonstrate a generic connection between rather conventional symmetry-enriched topological phases and SPT phases via an anyon condensation mechanism.

[Phys. Rev. B 95, 125107] Published Mon Mar 06, 2017

]]>Weyl semimetals (WSM) have quickly gained popularity since their discovery and they are now the subject of intense theoretical and experimental research. In general, the WSM is categorized as being either type I or type II. The type-I WSM is characterized by broken inversion symmetry, while the type II by broken Lorentz symmetry. Due to this characteristic difference in the Fermi surfaces of the two types, it is necessary to extend our understanding of WSMs to include the different physical properties that having a tilted spectrum offers. In this vein, disorder-induced phase transitions have been well established for type-I WSMs, while the converse is not the case for type-II WSMs. In this work, the authors demonstrate that the type-I WSM with a nonzero tilting term undergoes a quantum phase transition to the type-II WSM in the presence of random on-site electric and magnetic impurity disorder. They show that the Fermi velocity decreases with increasing disorder strength and, therefore, a type-I to type-II transition occurs before the insulating transition. The conclusion is that the type-I WSM with nonzero tilt will always undergo a disorder-induced transition to the type-II WSM phase.

[Phys. Rev. B 95, 094201] Published Thu Mar 02, 2017

]]>In topological insulators (TIs), spin-orbit coupling leads to the emergence of metallic topological surface states crossing the fundamental band gap. They exhibit a Dirac-cone-like dispersion with a helical spin structure. Bi${}_{2}$Se${}_{3}$(111) is the most prominent prototypical TI featuring a simple band structure with a single Dirac cone close to the Fermi level around the center of the surface Brillouin zone. Due to the interesting spin structure, TIs have emerged as promising materials in the field of spintronics and optospintronics. Ultrafast light pulses might pave a way to control spin currents. In this context, a profound knowledge about the dispersion and the spin polarization of both the occupied and the unoccupied electronic states is required. The authors present a joint experimental and theoretical study on the unoccupied electronic states of the topological insulator Bi${}_{2}$Se${}_{3}$. They discuss spin- and angle-resolved inverse photoemission results in comparison with calculations for both the intrinsic band structure and, within the one-step model of (inverse) photoemission, the expected spectral intensities. This allows them to unravel the intrinsic spin texture of the unoccupied bands at the surface of Bi${}_{2}$Se${}_{3}$.

[Phys. Rev. B 95, 115401] Published Wed Mar 01, 2017

]]>An invisibility cloak makes itself and the object inside undetectable from the outside. For a cloak designed using transformation optics, light is bent around the object by changing the sizes and shapes of the local dispersion surfaces of the metamaterials surrounding the object. Here, the authors have found another way to build a cloak by using a concept called pseudomagnetic field, which accelerates the photon in a direction perpendicular to its velocity. It corresponds to shifting local dispersion surfaces. The new cloak bends the light in an asymmetric fashion for light passing the left and right side of the object for a fixed polarization of pseudospin. A truncation of the inner materials of the cloak creates an additional asymmetric transmission behavior. Furthermore, the new method also enables one to design more optical devices, such as a retroreflector.

[Phys. Rev. B 95, 075157] Published Tue Feb 28, 2017

]]>The interplay of electronic correlations and spin-orbit coupling results in a rich phenomenology in semimetals with linear or quadratic band touching points. Remarkably, the associated low-energy physics and ground-state properties can often be described by effective field theories with enlarged symmetries such as rotation or Lorentz invariance. Realistic band structures of actual materials, however, are typically anisotropic and thus only allow for cubic rotation invariance. Here, the authors study the influence of anisotropy on the ground state of three-dimensional semimetals described by the Luttinger Hamiltonian, which captures the quadratic band touching present in HgTe, the pyrochlore iridates ${R}_{2}$Ir${}_{2}$O${}_{7}$, or strongly correlated half-Heusler compounds such as YPtBi. They highlight that a sufficient amount of anisotropy can induce qualitatively new ground states in these systems, namely a scale-invariant non-Fermi liquid state, or different octupolar tensor orders that break time-reversal invariance. The latter configurations share features of magnets and nematic order, so one could call them “nemagnetic”. The authors clarify their very natural interpretation as all-in-all-out or spin ice configurations for electrons in the pyrochlore iridates.

[Phys. Rev. B 95, 075149] Published Mon Feb 27, 2017

]]>There have long been considerable efforts to study the many-body quantum phase diagram and quantum phase transitions of interacting bosonic particles in solid-state systems. The particular aim is the realization and identification of Bose-Einstein condensation, a coherent ground state with all particles condensed into a macroscopic many-body ground-state wave function. The authors probe photogenerated excitonic ensembles in coexistence with a two-dimensional hole system confined in GaAs double quantum well structures and electrostatically trapped by local gate electrodes using photoluminescence and resonant inelastic light scattering experiments. In this work, the authors observe a collective excitation mode at the transferred in-plane momentum and an energy of only 0.44 meV. This mode is interpreted as a plasma excitation of the two-dimensional excess hole subsystem coupled to the excitonic system. In this respect, the two subsystems behave like coupled oscillators. The plasmon energy is determined by the many-body interaction between the photogenerated exciton and hole subsystems. The low-energy excitation spectrum particularly of the excitonic system can play a key role in identifying Bose-Einstein condensation of excitonic ensembles, where, for example, roton excitations are expected.

[Phys. Rev. B 95, 085312] Published Mon Feb 27, 2017

]]>The author investigates in a nonperturbative way the effects of Rashba interaction and electromagnetic field on the edge states of a two-dimensional topological insulator. He shows that the electron dynamics is equivalent to a problem of massless Dirac fermions propagating with an inhomogeneous velocity, unveiling interesting consequences. The Rashba coupling always enhances the electron velocity and, despite the inelastic and time-reversal breaking processes induced by the electromagnetic field, no backscattering occurs without interaction. Furthermore, when the photoexcitation occurs far from the Rashba region, the latter effectively acts as a “superluminal gate”, boosting the photoexcited wavepacket outside the light-cone determined by the bare Fermi velocity ${v}_{F}$. In contrast, for an electric pulse overlapping the Rashba region, the emerging wave packets are squeezed in a manner that depends on the overlap area. The author also discusses the effects of electron-electron interaction, for both intraspin and interspin density-density coupling. The results suggest that Rashba interaction, often considered as an unwanted disorder effect, may be exploited to tailor the shape and the propagation time of photoexcited spin-polarized wave packets.

[Phys. Rev. B 95, 085434] Published Fri Feb 24, 2017

]]>The authors present a method for characterizing the hyperuniformity (the suppression of density fluctuations at long wavelengths) for quasicrystals (and other structures whose diffraction pattern includes a dense set of Bragg peaks) based on determining the behavior of the integrated spectral intensity $Z$($k$) for small wavenumber $k$. Surprisingly, we find that quasicrystals with peaks at the same wavenumbers $k$ can have different behavior for $Z$($k$) and, hence, qualitatively different degrees of hyperuniformity. This effect has never been explored in the laboratory. The figure shows a one-dimensional example: two “sidewalks” with different widths (pitched at an incommensurate angle) built on a lawn with a square crystal pattern of spots. The construction rule is that, every time the sidewalk crosses a spot, the sidewalk slab is cut and a new one begins. This rule guarantees both sidewalks are quasicrystalline. But the upper one (a Fibonacci sidewalk) has a different degree of hyperuniformity than the lower one.

[Phys. Rev. B 95, 054119] Published Thu Feb 23, 2017

]]>The time evolution of a quantum system after a sudden change of one of its parameters may show characteristic nonanalyticities, provided that this quench crosses a quantum phase transition. The authors investigate the relation between such dynamical quantum phase transitions and the universal properties of the corresponding quantum critical point.

[Phys. Rev. B 95, 075143] Published Thu Feb 23, 2017

]]>It has been argued that the itinerant magnetism in the simplest of all iron arsenide systems, the monoarsenide FeAs, may be related to the magnetic ground states of iron-based superconductors. Here, the authors provide new insight into the magnetism of FeAs through a combination of resonant x-ray scattering measurements and electronic structure calculations. They report measurements of the magnetic anisotropy of the spin helices by using polarized x-rays tuned to the resonant absorption edge of iron. They elucidate details of the magnetic structure, showing that the ratio of ellipticity of the spin helix is significantly larger than previously thought, and discover both a right-handed chirality and an out-of-plane component of the magnetic moments in the spin helix. The electronic structure calculations demonstrate how the origin of this spin canting effect may be accounted for by considering the spin-orbit coupling at work in the system.

[Phys. Rev. B 95, 064424] Published Wed Feb 22, 2017

]]>Understanding the spread of quantum entanglement and scrambling of information across a quantum many-body system is a fundamental problem in quantum dynamics. A necessary part of the spread of the entanglement is the growth in time of Heisenberg operators of initially localized operators. This operator growth can be diagnosed by studying the space-time structure of commutators of well-separated operators. Previous studies in this area focused on translation invariant models, but the effects of disorder, which can radically alter the motion of heat and charge, had not been studied. Here, the authors study the growth of operators in interacting systems with quenched disorder. In the localized phase, they find that operator sizes grow logarithmically in time similar to other measures of entanglement spreading. In the disordered metal, they find that operator sizes grow linearly with time, i.e., ballistically, in contrast to the diffusive motion of charge and heat. The ballistic growth of operators is quantified by the butterfly velocity which the authors relate to the charge diffusion constant and the interaction-induced inelastic scattering rate. When the diffusion of charge is slow, the resulting butterfly velocity is much smaller than the maximum speed allowed by microscopic causality constraints.

[Phys. Rev. B 95, 060201(R)] Published Tue Feb 21, 2017

]]>Recently, there have been extensive efforts to extend the physics of the two-dimensional (2D) graphene to three-dimensional (3D) semimetals with point/line nodes. Although it has been known that certain crystalline symmetries play an important role in protecting band degeneracy, a general recipe for stabilizing the degeneracy, especially in the presence of spin-orbit coupling, is still lacking. Here, the authors show that a class of novel topological semimetals with point/line nodes can emerge in the presence of an off-centered rotation/mirror symmetry whose symmetry line/plane is displaced from the center of other symmorphic symmetries in nonsymmorphic crystals. Due to the partial translation perpendicular to the rotation axis/mirror plane, an off-centered rotation/mirror symmetry always forces two energy bands to stick together and form a doublet pair in the relevant invariant line/plane in momentum space. Such a doublet pair provides a basic building block for emerging topological semimetals with point/line nodes in systems with strong spin-orbit coupling. When an external magnetic field is applied to these semimetals, a Dirac-type point/line node with four-fold degeneracy splits into two Weyl-type point/line nodes with two-fold degeneracy, with emergent surface states connecting the split nodes.

[Phys. Rev. B 95, 075135] Published Tue Feb 21, 2017

]]>In recent years, physicists have made great strides toward better understanding isolated many-body quantum systems. Already, two distinct phases are well known: systems that are ergodic and thermalize completely, and, by contrast, systems that exhibit many-body localization (MBL) due to a strong disorder potential, which prevents thermalization. It is tempting to wonder whether a phase of matter could exist between the extremes of full thermalization and MBL in a generic, i.e., nonintegrable, isolated many-body quantum system. Here, the authors provide evidence that such a nonergodic phase can be realized in a translationally invariant itinerant electron model, with both charge and spin degrees of freedom. Exact diagonalization calculations on finite system sizes reveal that there exists a “spin band” of nonthermal eigenstates. These states have entanglement entropy that scales as an overall “volume law,” but after a partial measurement of the spin on each site the scaling is modified to an “area law,” thus suggesting the states are not ergodic. These properties and others suggest that this model realizes a new nonthermal phase of matter, known as a quantum disentangled liquid (QDL). The putative existence of this phase has striking implications for the foundations of quantum statistical mechanics.

[Phys. Rev. B 95, 054204] Published Fri Feb 17, 2017

]]>Laser beams carrying orbital angular momentum (OAM), first proposed by Allen et al. in 1992, is one of hottest research subjects in modern optics. Such a beam, called an optical vortex, is used to realize superresolution microscopes, chiral optical ablation, optical tweezers, and so on. More recently, electron beams with nonzero OAM are also proposed and experimentally realized. In spite of the growing attention towards such “vortex beams”, their applications for condensed matter physics are almost untapped so far, especially for controlling microscopic electronic and magnetic degrees of freedom in solids. Here, the authors show theoretically that vortex beams can be useful for controlling magnetic properties of solids. In particular, they focus on chiral ferromagnets and antiferromagnets where topologically stable magnetic defects (skyrmions) appear. On the basis of numerical simulations using the stochastic Landau-Lifshitz-Gilbert equation, they show that via the laser-beam-induced spatially nonuniform temperature changes, the characteristic donut-shaped spatial profile of vortex beams can be faithfully transferred to magnets as a family of magnetic defects (n$\pi $ vortices), which include skyrmions and skyrmioniums.

[Phys. Rev. B 95, 054421] Published Thu Feb 16, 2017

]]>The rare-earth nickelate perovskites ($R$NiO${}_{3}$, $R$ = rare-earth element) are an archetypal class of correlated electron materials. They order antiferromagnetically at a composition-dependent “Néel” temperature. While it is known that the magnetic structure has a period of four Ni spins along each (pseudo)cubic crystal axis, the arrangement of these spins is still under debate, especially in view of the conjecture of ferroelectricity in the antiferromagnetic phase. Here, the authors report the experimental discovery of an unexpected second magnetic phase transition in TlNiO${}_{3}$, a material closely resembling the analogous rare-earth compounds. Results of nuclear magnetic resonance and muon spin rotation experiments reveal the presence of two distinct magnetic phase transitions. The new phase is suppressed by magnetic fields on the order of at most 1 T.

[Phys. Rev. B 95, 060411(R)] Published Thu Feb 16, 2017

]]>Helium nanodroplets are considered ideal model systems to explore quantum hydrodynamics in self-contained, isolated superfluids. Rotation of a superfluid droplet is manifested as a collection of quantum vortices. Here, the authors use single-shot femtosecond X-ray coherent diffractive imaging to investigate the shapes and vorticity of rotating superfluid ${}^{4}$He droplets. They find that most of the diffraction contours exhibit noticeable ellipticity. A smaller but still significant number of distorted diffraction contours are marked by regions of high intensity concentrated along the direction of the long axis in the diffraction image, indicating the presence of extremely deformed ${}^{4}$He droplets. Forward modeling of the diffraction images shows that the shapes of superfluid ${}^{4}$He droplets are very similar to their classical counterparts. In particular, the existence of both axially symmetric (oblate) and, for higher angular momenta, two-lobed (triaxial prolate) droplets is revealed. The latter shapes are strongly deformed compared to the spheroidal bodies that are expected for smaller rotational speeds. The observed shapes provide direct access to the droplet angular momenta, angular velocities, and estimated number of quantum vortices inside the droplets.

[Phys. Rev. B 95, 064510] Published Thu Feb 16, 2017

]]>The simultaneous presence of sizable spin-orbit interactions and electron correlations in iridates has led to predictions of novel electronic ground states, including Dirac semimetals, Kitaev spin liquids, and superconductivity. Electron and hole-doping studies of Sr${}_{2}$IrO${}_{4}$ are one of the venues being pursued to attain such novel quantum states, driven in part by the extensive parallels with the La${}_{2}$CuO${}_{4}$ parent compound of high-${T}_{c}$ cuprates. In particular, Rh doping has been intensively pursued but the mechanism of electronic doping and its impact on the evolution of magnetic and transport properties has been under scrutiny. Using x-ray absorption spectroscopy at the Rh $K$, $L$, and Ir $L$ edges, the authors reveal unusual charge partitioning between Rh and Ir sites. The resultant anomalous doping introduces holes into the ${J}_{\text{eff}}$=1/2 band at low Rh content only for these holes to be removed at higher Rh content. This anomalous doping provides a natural explanation for the reentrant insulating phase in the phase diagram of Rh-doped Sr${}_{2}$IrO${}_{4}$ and should be taken into account when searching for novel electronic phases in 5$d$ iridates doped with 4$d$ or 3$d$ elements where anomalous charge partitioning is expected.

[Phys. Rev. B 95, 060407(R)] Published Wed Feb 15, 2017

]]>Solitons are self-preserving traveling waves of great interest in nonlinear physics, offering many interesting application such as high-bandwidth optical fiber communication. Solitons can also appear in ultrafast acoustics, and several observations suggest acoustic solitons as short as 200 fs. Here, the authors designed an experimental setup to observe and characterize acoustic solitons traveling through a GaAs(001) substrate. The experimental signal is explained well with the help of the Korteweg–de Vries equation, demonstrating the particlelike nature and the unique properties of the solitons. Moreover, the temporal distribution of the solitons is also analyzed with the help of the inverse scattering method. Such investigations provide a new tool to probe transient properties of highly excited matter through the study of the solitons due to the acoustic pulse emitted after laser excitation.

[Phys. Rev. B 95, 064306] Published Tue Feb 14, 2017

]]>The spin Hall effect is of great technological interest because it can produce efficient spin-transfer torques with the potential to advance magnetic memory technologies. Here, the authors demonstrate that the spin Hall effect can be enhanced by the presence of $f$-orbital states near the Fermi level. The authors compare measurements of spin Hall torques generated by the rare-earth metals Gd, Dy, Ho, and Lu with the results of first-principles calculations of intrinsic spin Hall conductivity with and without $f$-orbital participation. They find that contributions from the $f$-orbital states are required to explain the experimental trend in spin Hall conductivity as a function of $f$-orbital filling. The observed trend is consistent in both sign and magnitude with a simple Hund’s rules picture of $f$-orbital filling, similar to the trend observed in the 4$d$ and 5$d$ transition metals as a function of $d$-orbital filling. These results suggest that rare earths are a promising research direction for creating and tailoring materials with large spin-orbit torques.

[Phys. Rev. B 95, 064412] Published Tue Feb 14, 2017

]]>The investigation of topological transitions in quantum systems is a thriving area of modern research. The authors recently predicted that multiterminal Josephson junctions realize a novel type of topological matter. Weyl singularities may appear in the Andreev bound-state spectrum of junctions with four superconducting terminals, giving rise to topological transitions as the superconducting phases are tuned. These transitions manifest themselves in a quantization of the transconductance between two voltage-biased terminals in fundamental units of 4${e}^{2}$2/$h$, where $e$ is the electric charge and $h$ is the Planck constant. The present work addresses the observability of this effect. The quantized transconductance is associated with adiabatic transport at fixed occupations of the Andreev states. On the other hand, a bias voltage leads to multiple Andreev reflections, where a quasiparticle is dissipatively transferred from the occupied states below the superconducting gap to the empty states above the superconducting gap. By computing the currents in the presence of weak inelastic relaxation, the authors establish the voltage threshold below which the equilibrium occupations of the Andreev states are restored, and the transconductances reach their quantized values. The results are an important step towards experimental verification of the topological properties of multiterminal Josephson junctions.

[Phys. Rev. B 95, 075417] Published Tue Feb 14, 2017

]]>The out-of-time-ordered (OTO) correlation is unconventional. It is a key quantity for quantifying quantum chaoticity and has been recently used in the investigation of quantum holography. On the other hand, it is difficult to reveal and understand the special features of many-body localization (MBL) by conventional quantities and methods. Here, the authors demonstrate that OTO correlation can provide a pertinent and comprehensive description for MBL. It readily reveals the special logarithmic dynamics of MBL, distinguishes MBL sharply from Anderson localization as well as ergodic states, and allows accurate extraction of localization lengths for MBL. The divergence of the localization length at a finite interaction predicts an ergodic-MBL transition. Furthermore, the infinite-time “thermal” fluctuation of the OTO correlation is zero (finite) in the ergodic (MBL) phase and thus can be considered as an order parameter for the ergodic-MBL transition, through which the transition can be identified and characterized. Specifically, the critical point and related critical exponents can be calculated.

[Phys. Rev. B 95, 054201] Published Fri Feb 10, 2017

]]>Many semiconductor nanostructures are made from planar two-dimensional heterojunction systems. With a view to merging semiconductor physics, spintronics, and quantum computing, strongly spin-orbit coupled systems such as semiconductor holes have generated burgeoning research interest. While recent experiments in various low-dimensional hole systems have shown great promise in achieving electrical spin control, a thorough understanding of spin-orbit interactions in holes is lacking. Owing to the intricate interactions between holes and their environment, calculations on two-dimensional holes in semiconductor heterojunctions have hitherto always been numerical and material specific, and thus are not easily generalizable to other material systems. In this work, the authors exploit the variational method, k.p perturbation theory, and the theory of invariants to derive general, semianalytical expressions for the spin-orbit interaction in semiconductor heterojunctions. In particular, the authors systematically evaluate the dependence of the Rashba and Dresselhaus spin-orbit interactions on experimental parameters such as material, hole density, and background dopant type. The authors show that the semianalytical approach is easily generalizable to various materials, and present results for common semiconductors GaAs, Ge, InSb, InAs, and Si. Furthermore, the semianalytical results yield good agreement with existing numerical as well as experimental findings.

[Phys. Rev. B 95, 075305] Published Fri Feb 10, 2017

]]>The dynamics in the vicinity of quantum phase transitions entails a variety of important and universal phenomena, such as critical slowing down or the Kibble-Zurek mechanism, which are associated with the low-energy and long-time properties at criticality. Here, the author reveals that the opposite regime of transient nonequilibrium time evolution at criticality can similarly host universal temporal behavior. By strongly perturbing a quantum critical state through quenching the associated order parameter, the system experiences a dynamical quantum phase transition with physical quantities becoming nonanalytic at critical times. These dynamical quantum phase transitions are caused by superextensive energy fluctuations induced by the particular protocol, which turn out to have profound consequences for quantum speed limits and potential restricted thermalization despite the nonintegrability of the system.

[Phys. Rev. B 95, 060504(R)] Published Thu Feb 09, 2017

]]>In the conventional theory of superconductivity the critical temperature Tc is determined by the electron-phonon coupling constant and the phonon cut-off frequency. The hallmark experiments of McMillan and Rowell demonstrated that bosons (phonons) responsible for pairing can be observed through the frequency dependence of the gap parameter. Determination of the electron-boson coupling strength in high-${T}_{c}$ cuprates is, however, not an easy task. One of the promising ways is to measure the energy relaxation rate of photoexcited carriers by using femtosecond real-time techniques. Here, considering the electron relaxation process within the conduction band, it is commonly assumed that the underlying Eliashberg electron-boson coupling function is independent of electron excess energy. Conversely, studies of light-induced suppression of superconductivity in Pr${}_{1.85}$Ce${}_{0.15}$CuO${}_{4}$ reported here imply a strong variation of the electron-boson coupling function on electron energy. Considering the competing scenarios of superconductivity being mediated by either phonons or magnetic excitations, the results suggest that high-energy electrons strongly couple either to phonons or magnetic modes, while the situation is reversed when considering pairing of low-energy electrons.

[Phys. Rev. B 95, 085106] Published Fri Feb 03, 2017

]]>Topological crystalline insulators are insulating in the bulk, but exhibit conducting surface states protected by crystal symmetries. Here, the authors show that the surface states of crystalline topological insulators come in two different varieties: (i) as standard Dirac cones with pointlike Fermi surfaces (type-I) and (ii) as tilted Dirac cones that appear at the contact of electron and hole pockets. They call these new tilted surface states “type-II Dirac states” in analogy to the three-dimensional type-II Weyl points that have been recently discovered in WTe${}_{2}$. These type-II Dirac states can exist only at the surface of topological crystalline insulators, but are absent in ordinary topological insulators, where they are forbidden by symmetry. The two types of Dirac surface states have very different physical properties, in particular with regards to their thermodynamics and magnetotransport. The authors predict that the antiperovskites ${A}_{3}\phantom{\rule{0}{0ex}}E$O are an example of a crystalline topological insulator that hosts the type-II Dirac surface states.

[Phys. Rev. B 95, 035151] Published Mon Jan 30, 2017

]]>Light waves propagating in optical fibers or other guides are usually vulnerable to defects or channel deformations, such as twisting and bending. These perturbations invariably generate undesired reflections and scattering and typically imply a power penalty. Here, building on an analogy with electronics and with the spin Hall effect, the author unveils a general theoretical solution for this dilemma. It is shown that there is a wide class of three-dimensional metamaterial platforms protected by a particular symmetry – a combination of a geometrical operation with other more subtle symmetries of the materials response – that guarantees bidirectional transport of light totally free of reflections, independent of the specific geometry of the propagation channel and other imperfections. Crucially, this scattering anomaly only requires the symmetry protection in the frequency range of interest, and hence relies on much weaker assumptions than topological theories.

[Phys. Rev. B 95, 035153] Published Mon Jan 30, 2017

]]>Tamm resonators are an alternate type of cavities, in which the light-matter interaction may be significantly enhanced, as required for many applications in photonics or optoelectronics. A Tamm resonator comprises at least one metal layer as mirror, in the present case combined with a distributed Bragg reflector. The metal layer may turn out to be useful, for example, for injecting carriers or applying an electric field. Here, the authors use photon echoes in the exciton four-wave mixing signal to demonstrate that the studied Tamm resonator, which contains quantum dots as the optically active medium, allows one to perform coherent manipulations on quantum-dot excitons at power levels more than one order of magnitude lower than those required for bare quantum dots. This occurs despite the moderate quality factor of about 100. Moreover, it speeds up the radiative decay of the excitons by a factor of two (the so-called “Purcell effect”). On the other hand, the coherence of the quantum dot exciton, which may become compromised by plasmon excitations in the metal, is hardly shortened. These results demonstrate that Tamm resonators are indeed prospective candidates for performing coherent optical manipulations.

[Phys. Rev. B 95, 035312] Published Mon Jan 30, 2017

]]>Here, the authors extend the study of semimetallic WTe${}_{2}$ to the 2D limit, utilizing new fabrication schemes that preserve sample quality to create high-mobility electronic transport devices from few-layer thick crystals, previously found to be electrically insulating. The large magnetoresistance persists in these samples and can be turned off by electrostatically tuning the system to a simple-metal (noncompensated) regime. Furthermore, the semimetallic magnetoresistance is found to follow a density-independent subquadratic power law, a topic for future study. Finally, quantum oscillations are also analyzed as a function of total carrier density, providing the first insights into the band structure of ultrathin WTe${}_{2}$.

[Phys. Rev. B 95, 041410(R)] Published Mon Jan 30, 2017

]]>What happens to the edge states when two edges intersect? The authors answer this question for the case in which the topologically protected edge states are the quasi-one-dimensional bound states that propagate unidirectionally under the two-dimensional Dirac equation, if it includes an inhomogeneous potential term that changes sign across a zero line. These “zero line modes” are bound to the line along which the potential vanishes, and so if the potential has a “checkerboard” form, such that two zero lines cross, then the question becomes: what happens to the zero line modes at the intersection? By finding an exact integral representation for the two-dimensional Dirac energy eigenstates, the authors show that the eigenstates take surprisingly narrow “T-junction” forms, so that incoming wave packets split coherently at the intersection into two outgoing packets, while remaining tightly bound to the zero lines. The intersection thereby acts as a beam splitter for zero line modes.

[Phys. Rev. B 95, 045430] Published Mon Jan 30, 2017

]]>Phase transitions to absorbing states are among the simplest examples of critical phenomena out of equilibrium. The characteristic feature of these models is the presence of a fluctuationless configuration that the dynamics cannot leave, which has proved a rather stringent requirement in experiments. Recently, a proposal to seek such transitions in highly tunable systems of cold atomic gases offers to probe this physics and, at the same time, to investigate the robustness of these transitions to quantum coherent effects. Here, the authors specifically focus on the interplay between classical and quantum fluctuations in a simple driven open quantum model which, in the classical limit, reproduces a contact process, which is known to undergo a continuous transition in the “directed percolation” universality class. The authors derive an effective long-wavelength field theory for the present class of open spin systems and show that, due to quantum fluctuations, the nature of the transition changes from second to first order, passing through a critical point which appears to belong instead to the “tricritical directed percolation” class.

[Phys. Rev. B 95, 014308] Published Fri Jan 27, 2017

]]>In the field of geometrically frustrated magnetism, the very nature of the ground state of Tb${}_{2}$Ti${}_{2}$O${}_{7}$ has remained a long-standing conundrum. In this pyrochlore material, no conventional spin-ice or long-range magnetic order is stabilized, even at very low temperatures. Quantum fluctuations are suspected of being at the origin of such an exotic quantum phase, yet so far there is no conclusive evidence. Here, high-resolution synchrotron-based terahertz spectroscopy probes the lowest energy excitations of Tb${}_{2}$Ti${}_{2}$O${}_{7}$. By exciting transitions within the crystal electric field and crystal lattice, it is revealed that a double hybridization of crystal-field phonon modes is present across a broad temperature range. This unique vibronic process triggers coupling between several degrees of freedom that provides a crucial path for quantum spin-flip fluctuations to inhibit the stabilization of conventional magnetic states. While providing evidence for the unique quantum phase in Tb${}_{2}$Ti${}_{2}$O${}_{7}$, the result also highlights the powerful and complementary nature of terahertz spectroscopy as a probe in the study of exotic magnetic and frustrated phases.

[Phys. Rev. B 95, 020415(R)] Published Fri Jan 27, 2017

]]>The Dzyaloshinskii-Moriya interaction in the cubic chiral magnet MnSi stabilizes a magnetic helix - a periodic one-dimensional modulation of the magnetization. The orientation of this helix is determined by weak magnetocrystalline anisotropies, but it can be reoriented by applying a magnetic field. Here, the authors have studied this reorientation process by means of small-angle neutron scattering and susceptibility measurements. Their results are in excellent agreement with predictions of an effective mean-field theory taking into account the precise symmetries of the crystal structure. Measurements of the magnetization and ac susceptibility provide evidence that the reorientation of helimagnetic domains is associated with large relaxation times exceeding seconds. In addition, hysteresis at the Ising transitions indicates, within the same theoretical framework, the formation of an abundance of plastic deformations of the helical spin order. These deformations comprise topologically nontrivial disclinations, reminiscent of the skyrmions discovered recently in the same class of materials.

[Phys. Rev. B 95, 024429] Published Wed Jan 25, 2017

]]>Engineering topological degeneracy within an experimentally accessible platform has been an active area of research with applications towards the pursuit of universal topological quantum computation. In this work, the authors consider Abelian fractional quantum spin Hall states with multiply connected gapped boundaries. Such a system manifests topological degeneracy (also dubbed “boundary degeneracy”) and in principle can realize universal quantum computation with no superconducting proximity required. The authors construct an exactly soluble model manifesting this topological degeneracy and solve it within Hamiltonian formalism, which is a natural framework to quantify the Hilbert space structure associated with this degenerate subspace. Furthermore, this platform is accessible within experiments and can be created by applying a depletion gate to the quantum spin Hall material and using a spin-mixing process realized by generic magnetic impurities that gap out the edge modes.

[Phys. Rev. B 95, 045309] Published Wed Jan 25, 2017

]]>Measuring the magnetization vector field of magnetically ordered materials is the ultimate goal of magnetic imaging. The authors propose a technical realization of quantitative magneto-optical Kerr microscopy, which makes it possible to obtain the continuously changing vector field of complete magnetization processes on arbitrarily oriented surfaces. It is based on a pulsed light emitting diode (LED) light source that allows for a computer controlled, real-time contrast selection of all three magnetization components of the domain patterns. As parasitic Faraday contributions in the microscope optics are eliminated and by using monochrome LED light, the accuracy and signal-to-noise ratio of the analysis is significantly enhanced compared to preceding solutions. The approach to vector imaging thus exceeds previous limits of the experimental instrumentation for magneto-optical domain investigation. The potential of the technique was demonstrated by the quantitative analysis of a remagnetization cycle of a Permalloy film element, and by the vectorial analysis of a domain structure in a GaMnAs thin semiconducting film with extremely low Kerr response.

[Phys. Rev. B 95, 014426] Published Tue Jan 24, 2017

]]>While it is mostly well-known for its excellent thermoelectric properties at high temperatures, Yb${}_{14}$MnSb${}_{11}$ has also come to be characterized as a ferromagnetic semiconductor. Such systems support ferromagnetic correlations similar to metallic compounds, although with semiconducting electronic configurations. As such, they are promising materials for use in spintronic devices. The authors here use neutron scattering techniques to examine the distribution of the magnetization density and the energy scale of the magnetic exchange interactions in the Yb${}_{14}$MnSb${}_{11}$ compound. They find that there is a large ferromagnetic moment localized to the Mn site in the crystal structure with a ‘Kondo cloud’ of negative moments distributed over the other atomic sites. The results provide long sought answers to questions regarding the valence, magnetization distribution, and energy scales of the magnetic interactions in this compound. The ferromagnetic spin-wave spectrum supports this premise and is well characterized by inclusion of nearest and next-nearest neighbor exchange interactions.

[Phys. Rev. B 95, 020412(R)] Published Mon Jan 23, 2017

]]>The three-dimensional (3D) Dirac materials represent a dimensional extension of the two-dimensional physics seen in graphene. Recently, two types of 3D Dirac materials have been proposed. The first type are the symmetry protected 3D Dirac materials with the Cd${}_{3}$As${}_{2}$ as the representative material. The second type are materials with accidental touching of the conduction and valence bands in a single point. In this paper, the authors are reporting bulk quantum oscillations in Pb${}_{1-x}$Sn${}_{x}$Se, a latter type of material, which has also been recently identified as a topological crystalline insulator with a topological phase transition predicted to take place at $x$=0.17. The authors have used the quantum oscillations in resistivity and magnetization to identify the phase of the oscillation and prove that this material has the 3D Dirac linear dispersion for $x$=0.17. Combining the oscillations in the resistivity and magnetization, the authors provide the consistent indexing method for drawing the Landau level diagrams that are used in identifying the Berry phase.

[Phys. Rev. B 95, 035208] Published Fri Jan 20, 2017

]]>In the presence of strong disorder and weak interactions, closed quantum systems can enter a many-body localized phase wherein the system does not conduct heat or charge, does not equilibrate even for arbitrarily long times, and robustly violates quantum statistical mechanics. While this is well established in closed quantum systems, the interplay of such systems with delocalized degrees of freedom is much less understood. Here, the authors consider a model for which, in the noninteracting limit, some degrees of freedom are localized while others remain delocalized. Such a system can be viewed as a model for a many-body localized system brought into contact with a small bath of a comparable number of degrees of freedom. The authors numerically and analytically study the effect of interactions on this system and find that in certain parameter regimes, results are consistent with interaction-induced localization of the entire system.

[Phys. Rev. B 95, 035132] Published Thu Jan 19, 2017

]]>The authors identify a new class of optically controllable, semiconductor-based defect spin that is formed from the $d$-orbital electrons of chromium ions in silicon carbide and gallium nitride. These ions possess a simple lambda optical structure that couples only weakly to phonons and lattice strain. Therefore, even though they probe an ensemble of many ions at once with varying strain environments, the optical transitions they observe are exceptionally narrow and possess a high radiative efficiency. These properties allow the authors to individually interrogate the magnetic sublevels of the ground-state spin using resonant optical excitation, enabling ensemble optical spin polarization as well as optically detected magnetic resonance in the time domain. Each ion species emits the majority of its luminescence within a near-infrared zero-phonon line, suggesting a capacity for efficient photonic integration. Additionally, as magnetically active $d$-orbital states, the spins of these ions possess a number of degrees of design freedom not available to other common defect spin species such as those based on vacancy complexes. The authors therefore expect that these studies will broaden the range of opportunities available to semiconductor-based quantum device engineering, and will motivate further explorations into the use of transition metal ions as optically active qubit states.

[Phys. Rev. B 95, 035207] Published Thu Jan 19, 2017

]]>Due to the one-dimensional nature of single-wall carbon nanotubes (SWCNTs), electron-electron interactions are of great importance because the lack of screening enhances the effective Coulomb interactions between electrons, and individual SWCNTs have been considered as an ideal realization of a Tomonaga-Luttinger liquid (TLL). In an actual SWCNT sample, SWCNTs usually pack together closely and form a bundle structure, in which intertube effects may alter the TLL state significantly. Here, the authors study the TLL effect in bundles of metallic SWCNTs using ${}^{13}$C nuclear magnetic resonance techniques to understand how the intertube effects alter the electron-electron interaction in the bundle form. They find a modified charge Luttinger parameter for the bundled metallic SWCNTs. Their findings give direct evidence that bundling reduces the effective Coulomb interactions via intertube interactions within bundled metallic SWCNTs.

[Phys. Rev. B 95, 035128] Published Wed Jan 18, 2017

]]>Fraunhofer interference is a paradigmatic phenomena arising due to phase coherence in diverse systems from optics to superconducting junctions. In Josephson junctions, coupling two superconductors through a weak link, such patterns arise in the maximal dissipationless current the system can sustain, oscillating as function of a perpendicular applied magnetic field. By investigating this effect in a recently realized material system epitaxially coupling a thin superconductor to a semiconducting region, the authors here discover novel effects arising due to a combination of magnetic field screening, spin physics, and disorder. In an appropriately aligned in-plane magnetic field, they find that due to screening by the superconducting leads, a flux dipole develops in the semiconducting region leading to an effective confinement of the superconducting states to edges of the intervening semiconducting region. When the out-of-plane field is swept in the presence of an in-plane field, striking asymmetries in the Fraunhofer pattern are observed. By analyzing the underlying theoretical symmetries of the system, they demonstrate that such an effect arises as a result of an intricate interplay between disorder in the junction, splitting of the spin states in the applied field, and coupling between the momentum of the electrons and their spin.

[Phys. Rev. B 95, 035307] Published Wed Jan 18, 2017

]]>Quantum dots are promising building blocks for future optoelectronics or quantum information applications. Many of their properties derive from the quantum state of the hole trapped in the dot. “Light” and “heavy” hole states differ by the anisotropic character of their spin and of the electric dipole they form with a trapped electron. Light holes are of particular interest as they offer extended opportunities for optical manipulation of single carriers, or the electrical manipulation of magnetic objects. Here, the authors demonstrate that the hole ground state can be engineered through a proper design of the strain built in the dot when inserted inside a nanowire of a different lattice parameter. Two complementary techniques provide evidence of a light hole state: polarization-resolved Fourier imaging, which is sensitive to the nature of the electron-hole electric dipole, and magneto-optical spectroscopy, which probes the hole spin state through its coupling to magnetic atoms.

[Phys. Rev. B 95, 035305] Published Tue Jan 17, 2017

]]>Monolayer two-dimensional transition metal dichalcogenide (TMDCs) exhibit exceptionally strong spin-orbit and electron-electron interaction effects and have provided a rich new playground for the exploration of exciton physics. Recent experiments have demonstrated that in the presence of excess charge carriers the prominent excitonic features in optical absorption split into two separate peaks. The appearance of the additional peak is usually attributed to the presence of trions, charged fermionic quasiparticles formed by binding two electrons to one hole or two holes to one electron. The authors here argue that in the density range for which amplitudes of two peaks are comparable three-particle physics is of importance, and the appropriate picture is one of excitons interacting with the Fermi sea formed by additional charge carriers. These interactions result in the dressing of excitons into exciton-polarons. The exciton spectrum splits into a lower energy attractive exciton-polaron branch, normally identified as a trion branch, and a higher energy repulsive exciton-polaron branch, normally identified as an exciton branch. The authors provide a complete theory of absorption, which incorporates both static and dynamic effects of Fermi sea, and analyze frequency and doping dependence of optical conductivity in detail. The calculated density dependence of peak splitting and their amplitudes and widths are in agreement with recent experiments.

[Phys. Rev. B 95, 035417] Published Tue Jan 17, 2017

]]>The interplay between symmetry and topology is one of the most exciting avenues of modern condensed matter research. Here, the authors propose a new and unified approach for describing the low-energy physics of two-dimensional spin-singlet superconductors, placing them in a class of topologically ordered states akin to quantum Hall fluids and spin liquids. Starting from a microscopic model of two-dimensional paired fermions with a dynamical electromagnetism (that is also confined to two spatial dimensions), the authors derive Chern-Simons theories for all spin-singlet gapped superconductors, including $s$-wave and chiral states. The topological field theories constructed here thus pave the way towards understanding superconductors as symmetry-enriched topological phases of matter.

[Phys. Rev. B 95, 014508] Published Fri Jan 13, 2017

]]>The band-unfolding method is now widely used to calculate the effective band structures of a disordered system from its supercell model. The unfolded band structures are obtained as if the disordered system really has the crystallographic symmetry of its underlying structure. However, it has still been difficult to decompose the unfolded band structures into the modes based on the crystallographic symmetry of the underlying structure. This has prohibited detailed analyses of, e.g., selection rules and avoided band crossings for the unfolded band structures. The authors here develop a procedure to decompose the unfolded band structures according to the small representations (SRs) of the little groups. The decomposition is performed using the projection operators for SRs derived from the group representation theory. Using this procedure, the authors investigate the phonon band structure of disordered face-centered-cubic Cu${}_{0.75}$Au${}_{0.25}$, which has large variations of atomic masses and force constants among the atomic sites due to the chemical disorder. In the unfolded phonon band structure, several peculiar behaviors such as discontinuous and split branches are found in the decomposed modes corresponding to specific SRs.

[Phys. Rev. B 95, 024305] Published Fri Jan 13, 2017

]]>The discovery of topological matter should be an opportunity for revolutionizing electronic devices. Nevertheless, for all their unique properties, to date there are limited feasible devices or applications based on topological materials. The work here recognizes that topological Weyl semimetal materials, through their protected gap closing and spin-orbit locking, could be used to detect low-frequency infrared radiation. Detecting infrared light electrically beyond a ten-micron wavelength is rather difficult with few ready made commercial solutions. The authors find that Weyl semimetals could provide an ideal solution for detecting mid- and far-infrared light even at room temperature. In this work, they describe the conditions of the materials to be effective photodetectors (noncentrosymmetry, doping levels etc.) and provide a detailed analysis of this photovoltaic effect.

[Phys. Rev. B 95, 041104(R)] Published Fri Jan 13, 2017

]]>The authors present a systematic density functional theory GGA characterization of the ${R}_{4}$Fe${}_{2}$As${}_{2}$Te${}_{1-x}$O${}_{4-y}$F${}_{y}$ family of compounds (called “42214”) as a function of a set of key tuning parameters: rare earth ($R$ = Pr, Sm, and Gd), Te content, oxygen to fluorine substitutional doping, and external pressure. The analysis is based on an unfolding procedure that allows observation of the behavior of hole and electron pockets of the Fermi surface and of the nesting function. The present study includes characterization of a related hypothetical compound having Se substituting for Te. Results show that this peculiar compound family offers very good opportunities to properly harness material properties. Nesting, Fermi surface (topology and size), and magnetism can be easily modified by changing chemical composition: such tuning may lead to improved superconducting properties.

[Phys. Rev. B 95, 014415] Published Thu Jan 12, 2017

]]>As the temperature lowers in the gapless region, quantum quasi-1D spin systems show one-dimensional (1D) short-range ordering (SRO) corresponding to a Tomonaga-Luttinger liquid (TLL) by the interaction within chains, and then at lower temperatures where the small interchain interaction cannot be ignored, they have a three-dimensional (3D) long- range ordered (LRO, BEC) phase. This generally accepted scenario for the dimensional crossover of quasi-1D spin ordering, however, is debatable around the critical field, because the 1D SRO (TLL) regime has a linear boundary on the $H$-$T$ phase diagram, while the 3D LRO transition temperature has a convex shape. To this dimensionality paradox around the critical field, the authors have given the solution, experimentally by detailed analyses of the specific heat and ac susceptibility for bond-alternating antiferromagnetic chains in pentafluorophenyl nitronyl nitroxide (F${}_{5}$PNN). They reveal the existence of a definite field region near a critical field, where 3D LRO (BEC) directly occurs without passing through 1D SRO region at higher temperatures, even if the interchain interaction is sufficiently small compared to the intrachain one.

[Phys. Rev. B 95, 020408(R)] Published Thu Jan 12, 2017

]]>The authors investigate the properties of 378 half-Heusler compounds using density functional theory with the goal of identifying promising candidates for spintronic applications, e.g. half-metals. Although DFT has often been applied to the search for half-metals, this study may be the most comprehensive attempt to identify which of the compounds predicted by DFT to be half-metals are likely to be fabricated. The calculated formation energy of each of the 378 potential half Heuslers was compared to that of all competing phases and combination of phases in the Open Quantum Materials Database. Those semiconductors, half-metals, and near half-metals within an empirically determined 0.1 eV/atom hull distance margin for neglected effects were deemed of interest for further experimental investigation.

[Phys. Rev. B 95, 024411] Published Wed Jan 11, 2017

]]>Materials physicists are increasingly coming to understand that domain walls in complex materials often have a life of their own. Far from being unwanted “defects”, they may have rich and interesting physics not available in the bulk material. Here, the authors use first-principles theoretical methods to consider ferroelectric domain walls in a promising family of corundum-derivative materials. Beyond studying issues of domain wall mobility and clarifying the factors that allow polarization reversal, the authors also identify an unusual set of domain-wall-specific couplings between polarization, magnetization, and chirality that they suggest may lead to new functionalities.

[Phys. Rev. B 95, 014105] Published Tue Jan 10, 2017

]]>A Cr atom in a semiconductor host carries a localized spin with an intrinsic large spin to strain coupling particularly promising for the development of hybrid spin-mechanical systems. The authors demonstrate here that the spin of an individual Cr atom can be controlled optically. To optically access to the spin of an individual atom, they insert it in a cadmium telluride quantum dot. With a single Cr atom introduced in the quantum dot, the energy and polarization of the photon emitted or absorbed by the dot depends on the spin state of the magnetic atom. The authors first show that excitation with a laser beam tuned to the wavelength of one of these optical transitions can be used to initialize the state of the Cr spin and to probe its dynamics optically: the Cr behaves like an optically addressable spin-based memory. Under optical excitation exactly resonant with an absorption transition one can also enter the strong coupling regime where hybrid states of matter and light are created. The spin-dependent strong coupling with the laser field is exploited to optically tune the Cr atom’s energy levels.

[Phys. Rev. B 95, 035303] Published Tue Jan 10, 2017

]]>Yttrium iron garnet (YIG) holds a special place among all magnetic materials since it shows an unprecedentedly small magnetic damping, resulting in the narrowest known line in ferromagnetic resonance and enabling propagation of spin waves over long distances. Due to these unique characteristics, YIG films have been recently considered as a promising material for spintronic and magnonic applications. By combining YIG and a single layer of graphene in a bilayer, one obtains a unique model system for investigations of magnetic interfacial effects. Here, the authors demonstrate experimentally that the system shows a sizable chiral charge pumping, which could arise from an imbalance of screw dislocations. The effect, which cannot be attributed to the ordinary spin pumping, reveals itself in the creation of a dc electric field/voltage in graphene as a response to the dynamic magnetic excitations (spin waves) in an adjacent out-of-plane magnetized YIG film. The authors show that the induced voltage changes its sign when the orientation of the static magnetization is reversed, clearly indicating the broken mirror reflection symmetry about the planes normal to the graphene/YIG interface. The strength of effect shows a nonmonotonic dependence on the spin-wave frequency, in agreement with the proposed theoretical model.

[Phys. Rev. B 95, 024408] Published Mon Jan 09, 2017

]]>Quantum oscillations are the fundamental measurement made when studying metals and semimetals. They provide information about the Fermi surface by measuring interference of particles drifting around it in a magnetic field. So far it was only possible to measure them by applying an external magnetic field. Here, the authors argue that the deformation of the Dirac/Weyl semimetal can produce a strong enough pseudo-magnetic field to measure quantum oscillations in the strength of the strain. They argue that the effect of the strain is identical to the effect of the real magnetic field at energies close to the Dirac/Weyl points. They propose a setup in which the strain can be tuned continuously, and in which the quantum oscillations can be first observed in the complete absence of a magnetic field.

[Phys. Rev. B 95, 041201(R)] Published Mon Jan 09, 2017

]]>Can one investigate more efficiently quantum critical behavior by employing a local order-parameter pinning field, which explicitly breaks the symmetry of the model under investigation? To answer this question, the authors consider the two-dimensional square-lattice bilayer quantum Heisenberg model using a world-line quantum Monte Carlo method. The pinning-field approach is found to accurately locate the quantum critical point over a wide range of pinning-field strengths. However, the identification of the quantum critical scaling behavior is found to be complicated by the fact that the pinning field introduces strong corrections to scaling. A renormalization group analysis exhibits important analogies to surface critical phenomena, and a crossover effect to an infinite pinning-field fixed point, which the authors study in detail by simulations of an improved classical lattice model in the three-dimensional Ising universality class. At the infinite pinning-field fixed point, the short-distance expansion of the order-parameter profile exhibits a new universal critical exponent, which characterizes the experimentally relevant critical adsorption on a line defect. This result also implies the presence of slowly decaying scaling corrections, which are analyzed here in detail.

[Phys. Rev. B 95, 014401] Published Tue Jan 03, 2017

]]>The NiCl${}_{2}$-4SC(NH${}_{2}$)${}_{2}$ spin-chain compound, also called “DTN”, is one of the most studied archetype materials that at low temperature presents the magnetic-field-induced ordered phase that is described as a Bose-Einstein condensate (BEC). The paper here presents an NMR study of the static properties of this phase, where, for the first time, the exact absolute value of the order parameter has been determined and compared to theoretical predictions. The precise phase boundary of the BEC phase close to its upper critical field is found to be in perfect agreement with theory, and used to define and discuss the range of validity of the Hartree-Fock-Popov approximation. DTN is a unique quasi-1D BEC-type material for which the sub-dominant 3D exchange couplings are known from independent measurements, so that theoretical descriptions are completely constrained. Therefore, precise state-of-the-art numerical methods could be used to critically discuss the applicability of approximate 1D-based descriptions (e.g., Tomonaga-Luttinger liquid $+$ mean field) to the case when the quasi-1D character is of intermediate strength. The NMR results presented here and the corresponding theoretical descriptions can be considered as reference data and analysis for BEC-type compounds, which have been missing for a long time in this field of research.

[Phys. Rev. B 95, 020404(R)] Published Tue Jan 03, 2017

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