The authors demonstrate a novel ultrahigh-resolution, minimally invasive technique for the detection of the domain-wall position within magnetic nanostructures. Based on the anomalous Nernst effect, the technique is suitable for a wide range of spintronic nanodevices implementing perpendicular-anisotropy materials. A thermal gradient generated on-chip is used to drive charge carriers, which are in turn deflected by the sample’s magnetization. Domain-wall positions within the device can be monitored with a resolution of 20 nm by measurement of the resulting Nernst voltages.

[Phys. Rev. B 95, 220410(R)] Published Tue Jun 27, 2017

]]>The interpretation of scanning tunneling microscopy (STM) data on poorly metallic materials is challenging: one needs to take into account the partial penetration of the electric field into the sample, the so-called tip-induced band bending (TIBB). This effect is well known from STM experiments on semiconductors, but rarely discussed for correlated-electron systems. The authors find that in the lightly doped Mott insulator (Sr${}_{1-x}$La${}_{x}$)${}_{2}$IrO${}_{4}$, TIBB is at the root of apparent discrepancies in the gap values measured by photoemission, optical measurements, and STM. They develop a model to extract the intrinsic Mott gap of the system, leading to agreement with results from other techniques.

[Phys. Rev. B 95, 235141] Published Fri Jun 23, 2017

]]>Large classes of matter have been systematized before by using a pair of groups to classify all spontaneous symmetry breaking phases or by using a pair of groups plus projective representations to classify all gapped quantum phases of one-dimensional boson and fermion systems with various symmetries. In this paper, the authors go beyond this and use a pair of groups plus three braided fusion categories and a chiral central charge in order to classify gapped quantum phases of two-dimensional boson and fermion systems with various finite symmetries. This allows, among other things, to generate many group tables of two-dimensional gapped phases.

[Phys. Rev. B 95, 235140] Published Thu Jun 22, 2017

]]>The Kitaev quantum spin liquid on a honeycomb lattice has received much interest in magnetism, mainly because of the possibility of emergence of novel excitations called Majorana quasiparticles. The quest is under way to realize this exotic state in actual materials. One of the promising materials is the layered honeycomb material RuCl${}_{3}$, for which the signatures of the Majorana quasiparticles have been reported. Here, the authors investigate the magnetic thermal conductivity of RuCl${}_{3}$ along with bulk thermodynamic measurements. They find that in the temperature range on the order of the Kitaev-coupling strength anomalous heat conduction takes place, which correlates with the growth of the magnetic specific heat. The result is consistent with the propagation of magnetic excitations, likely itinerant Majorana quasiparticles, driven by Kitaev coupling.

[Phys. Rev. B 95, 241112(R)] Published Thu Jun 22, 2017

]]>Organic light-emitting diodes convert electricity to light through the recombination of positive and negative charges. This process is fundamentally dependent on the spin of the carriers, and can therefore be influenced by a magnetic field. We show that magnetic-field-induced changes to the conductivity of a diode structure made of a conducting polymer can be resolved down to the scale of some 20 parts per billion. This extreme sensitivity to magnetic fields allows us to resolve field changes of less than 100 nanotesla, which is almost two orders of magnitude less than Earth’s magnetic field.

[Phys. Rev. B 95, 241407(R)] Published Thu Jun 22, 2017

]]>A team of experimentalists and theorists proposes a scalable protocol for quantum computation based on topological superconductors.

[Phys. Rev. B 95, 235305] Published Wed Jun 21, 2017

]]>The real benefit of antiferromagnets is that they can enable terahertz spintronic circuits. Spin pumping is a versatile tool for generating pure spin currents and probing spin dynamics. The high antiferromagnetic resonance frequencies represent a challenge for experimental detection, but magnetic fields can reduce these resonance frequencies. The authors compute the inverse spin Hall voltages resulting from dynamical spin excitations. They suggest practical opportunities that will significantly enhance the spin pumping and inverse spin Hall voltage for the uniaxial antiferromagnets MnF${}_{2}$ and FeF${}_{2}$.

[Phys. Rev. B 95, 220408(R)] Published Tue Jun 20, 2017

]]>Topological crystalline materials are emergent topological phases due to crystalline space group symmetry. They are either gapful or gapless in the bulk, while hosting topological states at the boundary. Here, the authors define topological crystalline materials rigorously on the basis of a mathematical theory, known as twisted equivariant K-theory. Abstract mathematical ideas, such as the Mayer-Vietoris sequence and module structure, are explained in terms of band theory. The formulation is applicable to bulk gapful topological crystalline insulators/superconductors and their gapless boundary and defect states as well as to bulk gapless topological materials, such as Weyl and Dirac semimetals or nodal superconductors. The authors present a complete classification of topological crystalline surface states and band insulators protected by 17 wallpaper groups in the absence of time-reversal invariance, which may support topological states beyond simple Dirac fermions.

[Phys. Rev. B 95, 235425] Published Mon Jun 19, 2017

]]>Resonance effects lead to surprising and beautiful phenomena in many areas of physics. Here, the authors identify resonance points as key to understanding the breakdown and revival of strong zero modes in parafermionic clock models. Through analytic and numerical methods, they have been able to determine the behavior at and away from resonance points, proving many statements about the asymptotic behavior of these systems. In particular, they show that there exists special points in parameter space where strong zero modes can exist and, as such, topological degeneracy is preserved at all energies.

[Phys. Rev. B 95, 235127] Published Thu Jun 15, 2017

]]>In this paper, the author examines coherent transport of levitons through a quantum dot system in the Kondo regime. He demonstrates the repeated emergence of the Kondo resonance in the nonequilibrium regime where the Fermi sea is driven by optimal periodic Lorentzian pulses. This work suggests practical opportunities to design quasiparticle excitations in interacting electron systems by engineering time-dependent fields.

[Phys. Rev. B 95, 241302(R)] Published Thu Jun 15, 2017

]]>An individual Mn atom coupled to a heavy hole confined in a semiconductor quantum dot forms a hybrid spin with a large magnetic anisotropy. In this work, the authors study the dynamics of such hybrid spin in a charge tunable II-VI semiconductor quantum dot. The hole-Mn energy levels and the positively charged exciton form an ensemble of optical $\mathrm{\Lambda}$ systems that can be independently addressed. Analyzing the dynamics of the resonant photoluminescence of the $\mathrm{\Lambda}$ systems, they identify an efficient relaxation channel for the coupled hole-Mn spins driven by the interplay of the hole-Mn exchange interaction and the coupling to acoustic phonons.

[Phys. Rev. B 95, 245308] Published Tue Jun 13, 2017

]]>Topological phases are exotic states of matters which do not order locally yet exhibit a global ordering in their entanglement, making their characterization a challenging task. In this work, the authors construct an explicit holographic duality between topological phases in the bulk and the structure of the entanglement spectrum at its boundary, thereby establishing a one-to-one correspondence between different bulk topological orders and symmetry-breaking and symmetry-protected orderings of the entanglement. This allows classification and detection of topological phases holographically through “entanglement spectroscopy” at the boundary, and construction of order parameters that measure the condensation and confinement of anyons, and thus permits characterization of the nature of topological phase transitions.

[Phys. Rev. B 95, 235119] Published Mon Jun 12, 2017

]]>The detection of single microwave photons remains a difficult challenge because of their low energy. The authors demonstrate that an aluminum superconducting double dot can be tuned to a regime in which a photon creating a single quasiparticle pair changes the quantum capacitance of the device. By using high-bandwidth reflectometry techniques, they observe the absorption of photons in real time. They also exploit the band structure of the device to tune the frequencies to which it is sensitive and use this to carry out spectroscopy on the ambient black-body radiation environment in their dilution refrigerator.

[Phys. Rev. B 95, 235413] Published Fri Jun 09, 2017

]]>Thermoelectric effects can be surprisingly large in superconductor-ferromagnet tunnel junctions when the superconductor is subject to a spin-splitting field. The spin splitting can be provided by an applied magnetic field, but can also be created by exchange coupling to a ferromagnetic insulator. Here, it is shown that thermoelectric effects in superconductor-ferromagnet tunnel junctions can be enhanced by boosting the spin splitting with an intrinsic exchange field provided by a ferromagnetic insulator. These findings could lead to more precise local electron thermometry or efficient microrefrigerators.

[Phys. Rev. B 95, 224505] Published Wed Jun 07, 2017

]]>Weyl semimetals are materials with topological defects in their band structure. They arise through mechanisms similar to vortex–anti-vortex pair creation, forming a hedgehog-structure in momentum space that is described by a four-dimensional energy-momentum Dirac cone. The authors show that, under pressure, the Dirac cone can be tilted in the same way as the relativistic light cone is tilted on approach to the black hole horizon. Following through with this analogy, at the defined horizon, time and space reversal may trigger unconventional phenomena, including the possibility for time travel. For Weyl semimetals under pressure, this means the creation of a new topological hyperbolic phase, which is anisotropic and highly active to electromagnetic radiation. Electrons may propagate across a multilayer structure made of Weyl semimetals analogous to light. Such a multilayer sandwich may have a negative refractive index for electron waves and, therefore, it may act as an electron-focusing Veselago lens. In this case, it would become possible to confine electrons to an extremely narrow beam. In the context of applications, this effect could be used to improve the resolution of the scanning electron microscopy technique, by constructing tunneling tips made up of Weyl semimetal multilayers.

[Phys. Rev. B 95, 214103] Published Mon Jun 05, 2017

]]>Low-temperature thermal conductivity measurements in the ferrimagnetic insulator Cu${}_{2}$OSeO${}_{3}$ reveal an unprecedentedly large magnonic contribution below T~10K, far exceeding (by nearly two orders of magnitude) that observed previously in any other ferromagnet or ferrimagnet. Features predicted by theory more than 50 years ago are identified, including ballistic behavior with ${\kappa}_{m}\propto {T}^{2}$, and, possibly, Poiseuille flow, wherein the magnon mean-free path exceeds the specimen dimensions as momentum conserving scattering occurs more frequently than scattering by resistive processes.

[Phys. Rev. B 95, 224407] Published Mon Jun 05, 2017

]]>The authors determine the fate of single-particle Weyl excitations in the presence of short-range disorder. Using analytic and numerical methods, the authors show that weakly disordered Weyl excitations are not ballistic and acquire an exponentially small but nonzero quasiparticle lifetime due to rare regions of the random disorder potential. As a result, the Green function near the Weyl quasiparticle peaks remains analytic and the avoided quantum critical point renormalizes the single-particle excitation spectrum at finite momentum.

[Phys. Rev. B 95, 235101] Published Thu Jun 01, 2017

]]>Defining thermal analogs of electrical components is required for the demanding task of managing heat in devices. The authors introduce a novel strategy to achieve a thermal transistor by utilizing thermal fluctuations. They identify the absorption of heat through inelastic transitions as the main limitation of existing proposals and solve this problem by partitioning the device so that the conductor is coupled to the heat source via an auxiliary system whose fluctuations act as a switch for heat flow. This can be achieved even with arbitrarily low heat injection, which results in extremely large amplification coefficients. This concept may open up a new avenue for the efficient manipulation of heat by heat and, more generally, for the field of noise-assisted thermal devices.

[Phys. Rev. B 95, 241401(R)] Published Thu Jun 01, 2017

]]>The Kitaev model is a spin model on a honeycomb lattice with bond-dependent anisotropic interactions whose ground state is an exactly solvable example of a quantum spin liquid. Interest in this model has been growing rapidly due to the potential for realization of this artificial model in insulating materials with strong spin-orbit interactions. One such example is the layered honeycomb material $\alpha $-RuCl${}_{3}$. Although this material orders magnetically rather than realizing a spin liquid ground state, there have been suggestions that suppression of the magnetic order through application of magnetic field could reveal a quantum spin liquid phase in this material. In this work, the authors examine the temperature-field phase diagram of $\alpha $-RuCl${}_{3}$ using neutron diffraction and bulk thermodynamic measurements. They found that the magnetic order disappears at magnetic fields above 8 Tesla, and a magnetic gap showing quantum critical scaling opens up in the high field phase.

[Phys. Rev. B 95, 180411(R)] Published Wed May 31, 2017

]]>The nature of the “hidden order” phase of URu${}_{2}$Si${}_{2}$ continues to defy understanding, despite three decades of intense research. Nonetheless, progress has been made in identifying key properties of the “hidden order” phase, including the observation of spin correlations indicative of Fermi surface nesting by neutron scattering. Here, the authors show that these spin correlations appear in the pressure-induced antiferromagnetic phase of URu${}_{2}$Si${}_{2}$, with remarkably similar properties. The persistence of these magnetic correlations suggests a significant kinship between these phases that had not been previously appreciated.

[Phys. Rev. B 95, 195171] Published Wed May 31, 2017

]]>Orbital degrees of freedom are a key ingredient in unconventional physics, including colossal magnetoresistance (CMR). When ordered, orbital arrangements can be characterized using conventional crystallographic approaches. Yet CMR emerges from states of orbital disorder, for which the experimental signature is much more ambiguous. Here, the authors study the CMR parent compound LaMnO${}_{3}$, using total scattering to understand its orbital order/disorder transition. They find a discontinuous change in local structure that indicates a fundamental change in the type of orbital arrangement at the transition. The analysis highlights the difficulty of discriminating between local structural models when static and dynamic disorder are strongly coupled.

[Phys. Rev. B 95, 174107] Published Tue May 30, 2017

]]>Nonequilibrium systems under periodic driving realize novel topological phases that cannot be achieved in equilibrium systems. One unique feature of periodically driven systems is that they can host a purely dynamical symmetry that involves time translation. This work explores a new class of Floquet topological phases protected by one realization of such dynamical symmetry, i.e., “time glide symmetry”, which is defined by a combination of reflection and time translation. Lattice models with time glide symmetric driving that are introduced show stable gapless surface states along with nontrivial topological numbers defined with time glide symmetry. In addition, a general classification theory of time glide symmetric topological phases is obtained by using a Clifford algebra approach.

[Phys. Rev. B 95, 195155] Published Fri May 26, 2017

]]>Finding suitable nonlocal order parameters that distinguish various symmetry-protected topological (SPT) phases is an important subject in view of experimental and numerical detection of SPT phases. By “simulating” the generating manifold of cobordism group in the operator formalism, the authors here propose nonlocal operations as diagnoses for SPT phases protected by point group symmetries. The nonlocal operations involve “partial point group transformations”, which are obtained by point group transformations restricted to a spatial subregion on a given ground-state wave function. Through analytical and numerical calculations, the authors show that the complex $U(1)$ phase of the ground state expectation value of such partial point group transformations may serve as an order parameter for those SPT phases. The examples in the paper include the ${Z}_{8}$ and ${Z}_{16}$ invariants of topological superconductors protected by inversion symmetry in $(1+1)$ and $(3+1)$ dimensions, respectively, as well as the lens space topological invariants in $(2+1)$-dimensional fermionic topological phases.

[Phys. Rev. B 95, 205139] Published Thu May 25, 2017

]]>As one of the most common waves in daily life, scalar sound is not easy to control by external fields since it lacks the intrinsic degrees of freedom such as the charge and spin in electrons. Here, the authors present an experimental observation of acoustic vortex states in sonic crystals, where the vortex chirality forms a good carrier of information for sound. By selectively exciting such novel states, they have fabricated a lattice array of sound vortices, and controlled the vortex chirality according to the operating frequency or incident direction of external sound. Furthermore, a peculiar beam-splitting behavior is observed, where the two spatially separated sound beams carry opposite vortex chirality, as manifested in the image here.

[Phys. Rev. B 95, 174106] Published Tue May 23, 2017

]]>Symmetry-protected topological (SPT) phases (e.g., the well known topological insulators) are a class of energetically gapped condensed matter systems that exhibit interesting topological properties only in the presence of certain global symmetries. While a great understanding of noninteracting fermionic SPT phases has been achieved recently, interacting SPT phases, in particular those that exist only in strongly interacting systems, are much less understood in general. Here, the authors study strongly interacting fermionic SPT phases in two spatial dimensions with finite Abelian unitary symmetries and provide a potentially complete classification of them.

[Phys. Rev. B 95, 195147] Published Mon May 22, 2017

]]>Dielectric antennas possess a complex multipolar response comprising both electric and magnetic resonances. These various modes can be selectively enhanced or suppressed using illumination beams of varying symmetries. However, such demonstrations are currently limited to single-particle antennas. Here, the authors address this issue with a theoretical study of dielectric dimer antennas illuminated by cylindrical vector beams (CVBs). They excite and study hybridized multipolar resonances in both horizontal and vertical dimer systems and show how the different beam symmetries of CVBs can selectively excite electric and magnetic modes in vertical systems, as well as couple to dark multipolar modes. The authors design an antenna system that yields unprecedented magnetic field enhancement and establishes the dielectric dimer antenna as a useful test bed for further experimental investigation.

[Phys. Rev. B 95, 201111(R)] Published Mon May 22, 2017

]]>Despite two decades of intense research, the origin of ultrafast demagnetization remains disputed. Here, the authors employ resonant magnetic x-ray reflectivity to follow simultaneously magnetization and structural dynamics at the BESSY femtoslicing source. They find that significant changes in nonmagnetic x-ray reflectivity accompany the subpicosecond demagnetization, which can be modeled as a variation of film thickness. Their simulations further show that the higher photon flux and energy resolution provided by x-ray free electron lasers will yield decisive data to differentiate between different mechanisms proposed to govern ultrafast demagnetization dynamics.

[Phys. Rev. B 95, 184422] Published Thu May 18, 2017

]]>The theoretical suggestion and the subsequent experimental evidence for the existence of discrete time crystals (DTCs) constitutes one of the most exciting recent fundamental advances in out-of-equilibrium (OOE) quantum physics. These are OOE phases in which time translation symmetry is spontaneously broken alongside the symmetry breaking familiar from static equilibrium systems. In this work, the authors study the fragility of this phenomenon by considering the effect of an external environment on the $\pi $ spin glass, the paradigmatic model of a disordered DTC, which had hitherto only been considered in completely isolated systems.

[Phys. Rev. B 95, 195135] Published Wed May 17, 2017

]]>Interfaces between solids play a key role in heat conduction by phonons in applications ranging from heat dissipation in light-emitting diodes to waste heat recovery with thermoelectric devices. Despite their importance, our understanding of thermal phonon transport across interfaces lags far behind that of electrons and photons, with basic parameters such as transmission coefficients being largely unknown. In this work, the authors demonstrate an experimental approach that enables the measurement of phonon transmission coefficients at solid interfaces using experiments and $a\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ computation. This work provides a unique microscopic window into the microscopic processes governing interfacial transport of terahertz-frequency thermal phonons that could impact numerous applications.

[Phys. Rev. B 95, 205423] Published Wed May 17, 2017

]]>The subgap excitations known as Andreev levels govern the low-energy physics of hybrid superconductor-semiconductor nanostructures, and lie at the origin of topological superconductivity. A quantitative understanding of their scaling with respect to relevant parameters is, however, still limited. Here, the authors address this issue for the prototypical case of a quantum dot coupled to a superconductor. Experimentally, the authors demonstrate the ability to controllably tune the hybridization of the dot and the superconductor. In addition, they develop a methodology to reliably extract the relevant parameters by numerically adjusting the Anderson model to the experimental data. As a result, they carefully study the scaling of Andreev levels within the parameter space and obtain an experimental phase diagram of the system. Their results show an impressive quantitative agreement with the theory.

[Phys. Rev. B 95, 180502(R)] Published Mon May 15, 2017

]]>Quantum annealers are computing machines that utilize quantum effects to solve hard optimization problems. Understanding the circumstances under which quantum annealers are more efficient than other optimization methods is a challenging open problem. A clue to this conundrum comes from studying the complexity of quantum annealers. In general, the complexity of a physical system relates to how efficiently the system can be simulated by classical algorithms. One then asks: if some class of quantum annealers are hard to simulate classically, are they also more powerful optimization machines? Here, the authors study quantum annealers belonging to two different complexity classes: the so-called “stoquastic” systems, which can be efficiently simulated with classical algorithms, and “nonstoquastic” systems, currently with no efficient classical treatment. Applied to a prototypical optimization problem, the authors observe that, on average, the stoquastic systems perform better on easier instances of the problem, while the more complex nonstoquastic annealers show significant advantage when applied to the hardest instances. The authors conjecture that the performance of the nonstoquastic Hamiltonians is closely related to their internal structure during the annealing process and elucidate how properties such as magnetic frustration can play a key role in devising more powerful quantum annealers.

[Phys. Rev. B 95, 184416] Published Mon May 15, 2017

]]>Transparent conducting oxides are used in a wide variety of applications, including solar cells and displays. Recently, the perovskite oxide BaSnO${}_{3}$ was demonstrated to be a superior material, with a carrier mobility that is 30 times larger than that of the prototypical perovskite oxide, SrTiO${}_{3}$. This outstanding value has opened up prospects for applications in advanced electronic devices. Using accurate first-principles calculations and Boltzmann transport theory, the authors perform a careful and detailed numerical analysis of the LO phonon and ionized impurity scattering mechanisms to elucidate their impact on mobility. They also compute the Hall factor explicitly, enabling a direct comparison to experimental reports of Hall mobilities for bulk and thin films. The analysis provides insights into the nature of the dominant mechanisms that limit mobility in state-of-the-art samples, and will aid the design of perovskite oxides with targeted transport properties.

[Phys. Rev. B 95, 205202] Published Mon May 15, 2017

]]>Periodically driven Floquet systems can host dynamical phases, including a range of exotic topological phases that have no static analogs. This work presents a homotopy approach to the study of driven systems, which treats unitary evolutions as paths in the space of unitary operators. By considering loop evolutions in this space, the authors obtain a topological classification, free of uncertainties and ambiguities about the long-time robustness of of behavior in specific physical models. Two classes of Floquet symmetry-protected topological phase are identified, characterized by unitary evolutions that act trivially in the bulk but nontrivially at the boundary of an open system. The first class is captured by an explicit group cohomology construction; it has the remarkable property of converting a trivial boundary state into a nontrivial symmetry-protected topological state. The second class exhibits anomalous counter propagating Hilbert-space transport at the boundary; it lies beyond the cohomology construction.

[Phys. Rev. B 95, 195128] Published Fri May 12, 2017

]]>In laser ultrasonics, ultrashort light pulses generate coherent acoustic pulses of picosecond duration via multiple possible physical mechanisms, involving optoacoustic conversion processes. These wide-band GHz acoustic signals are optically detected at the sample surfaces by ultrafast time-delayed probe light pulses. When the coherent acoustic pulses in GaAS are detected via the Brillouin scattering of probe light pulses of 400 nm wavelength, certain spectral components of the acoustic pulses remain invisible. The theoretical analysis relates this observation to the existence of zeros in the spectral transformation function of acousto-optic conversion. The phenomenon of zero sensitivity of the optical reflectivity to coherent acoustic phonons of particular frequencies is rather general and depends on both the probe light wavelength and the evaluated material. These findings substantially contribute to picosecond ultrasonics and laser-based nondestructive testing.

[Phys. Rev. B 95, 184302] Published Thu May 11, 2017

]]>In the past decade, topological materials have been continuously attracting the interest of the condensed-matter physics community because of their unique band structures and transport properties. Recently, ZrTe${}_{5}$ is becoming a promising platform to study topological phase transitions, as it could possibly be a 3D Dirac semimetal, a 3D weak topological insulator (TI), or a 3D strong TI, which are distinguished by whether there is a finite band gap and whether there is a topological surface state (TSS). This paper performs a systematic high-momentum-resolution photoemission study on ZrTe${}_{5}$ using 6 eV photon energy. The conduction and valence bands near $\mathrm{\Gamma}$ are measured with a gap between 18 and 29 meV. The gap size is smaller than former studies. This work also examines the spectral difference at different photon energies, which is attributed to final-state effects and the 3D nature of the band structure. By doing so, the authors confirm the gap is not due to some specific out-of-plane momentum, and conclude that there is no TSS. The final-state interpretation also reconciles the discrepancies of previous studies regarding the existence of the TSS. Hence, the results are consistent with ZrTe${}_{5}$ being a 3D weak topological insulator.

[Phys. Rev. B 95, 195119] Published Wed May 10, 2017

]]>Here, the authors investigate the possibility of building up nonequilibrium spin accumulation in a ferromagnetic thin film composed of a metallic alloy of nickel and iron, commonly known as permalloy. Analogously to thermoelectric generators, a temperature gradient in a ferromagnet is known to generate a distribution of accumulated electron spins over macroscopic distances, which may be used as an efficient source of spin-polarized electrons in future spintronic devices. To enable the detection and quantitative determination of this imbalance of electron spins, the authors have developed an entirely optical technique, based on the magneto-optical Kerr effect, using a Sagnac interferometer microscope with a spatial resolution of one micrometer and an absolute polarization angle resolution of ten nanoradians. This new technique is not only robust against electrical artifacts, but allows the user to measure previously inaccessible experimental geometries where the magnetization is not restricted to lying in the plane of the film.

[Phys. Rev. B 95, 180401(R)] Published Mon May 08, 2017

]]>Here, the authors argue that the recent experimental discovery of Majorana zero modes in topological superconductors may be the key to realizing an elusive state of matter, the odd-frequency superconductor, in which Cooper pairing is entirely dynamical in nature. Via an exactly soluble model, it is shown that when Majorana zero modes couple to a spin-polarized metallic system, equal-spin $s$-wave odd-frequency pairing is generically induced in the spin-polarized metal. The key properties of odd-frequency superconductivity, such as a vanishing static superconducting order parameter and a paramagnetic Meissner effect, are demonstrated in this system by explicit calculations.

[Phys. Rev. B 95, 184506] Published Mon May 08, 2017

]]>Due to its highly tunable unconventional superconductivity, FeSe is one of the most intriguing materials among the iron-based superconductors. In addition, it exhibits a tetragonal to orthorhombic “nematic” phase transition in the absence of static magnetic order. This article presents a set of tight-binding parameters, optimized directly with respect to experimental angle-resolved photoemission spectroscopy (ARPES) data obtained at 100 K. It thus provides a quantitatively accurate model of the electronic structure of the tetragonal phase of FeSe and, hence, a coherent foundation for the understanding of the unique physical properties of this system. The authors go on to predict a feature that has so far remained unnoticed in simpler DFT-based tight-binding-model analyses, namely, a significant temperature dependence of the chemical potential in FeSe. This is confirmed by performing ARPES on FeSe single crystals, whereby a 25-meV rigid chemical potential shift is detected across the entire Brillouin zone over the temperature range between 100 K and 300 K. The finding has important implications for any future theoretical models of nematicity and superconductivity in FeSe and related materials.

[Phys. Rev. B 95, 195111] Published Mon May 08, 2017

]]>Antiferromagnets are rapidly gaining importance as crucial ingredients in many applications. They are abundant in nature and they are robust against externally applied fields. Rather usefully, their spin resonances often lie within the THz regime, which makes them ideal candidates for optical studies. In this paper, the authors present a thorough experimental and theoretical exploration of the optical spin excitation in antiferromagnetic NiO. Based on a phenomenological theory, they derive expressions for the optically induced magnetization via the inverse Faraday effect and the inverse Cotton-Mouton effect. Light polarization is conserved by pumping and probing along the optical axis of the material, facilitating the comparison between theory and experiment. Those agree amazingly well, making possible the identification of the driving mechanism behind the ultrafast magnon excitations. Moreover, the authors succeed in obtaining information about the otherwise elusive spin-domain distribution in NiO and in showing that the energy transfer into the magnon mode is about three orders of magnitude more efficient via the inverse Cotton-Mouton effect than via the inverse Faraday effect.

[Phys. Rev. B 95, 174407] Published Fri May 05, 2017

]]>Tunneling of an electron from one composite fermion liquid into another in a bilayer system offers a unique spectroscopic probe into the short-distance high-energy physics of this highly nontrivial strongly correlated state. The authors identify the interlayer exciton responsible for the maximum current, and find excellent quantitative agreement with the experimentally measured energy as well as its dependence on an in-plane magnetic field. They also predict that the spin-polarization transitions of the fractional quantum Hall states as a function of the Zeeman energy will be marked by discontinuous jumps in the energy of this exciton.

[Phys. Rev. B 95, 195105] Published Wed May 03, 2017

]]>The topological phase of a one-dimensional triplet superconductor support fractionalized edge states described by Majorana fermions. Generically, these edge states are not completely decoupled. Their wave functions decay exponentially, and the tails induce a coupling that decays exponentially with the length of the system. This is not the whole story, however. In an appropriate parameter range (if the pairing is smaller than the hopping in the simplest model, the Kitaev model), the Majorana wave functions oscillate on top of decaying, and these oscillations induce exact zero modes, at which the Majorana edge states are rigorously decoupled. In the present paper, motivated by the observation of ground-state level crossings in chains of cobalt adatoms, which, up to a Jordan-Wigner transformation, are described by the same model, the authors explicitly calculate the Majorana edge-state wave functions, and they fully characterize the number and positions of the level crossings induced by the wave-function oscillations.

[Phys. Rev. B 95, 174404] Published Tue May 02, 2017

]]>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

]]>