An ubiquitous feature of cuprate superconductors is the existence of a normal-state pseudogap for underdoped compositions. Several experiments suggest that there may be a phase transition to the pseudogap state in which a symmetry is broken. One proposed scenario is that the pseudogap state breaks time-reversal symmetry due to the formation of circulating currents within each unit cell. The authors study underdoped YBa${}_{2}$Cu${}_{3}$O${}_{6+x}$ using polarized neutron scattering and show that this scenario is unlikely.

[Phys. Rev. B 96, 214504] Published Fri Dec 15, 2017

]]>Chiral topological phases, like quantum Hall states, are characterized by anomalous unidirectional edge transport arising from a nontrivial insulating bulk. Recent developments suggest that they can be extended to periodically driven (Floquet) systems, where quantum information is pumped in lieu of charge or energy. The authors uncover a new chiral Floquet phase of quantum dynamics, whose edge pumps quantized units of fractionalized excitations. These phases are “radical,” in that the quantum dimension pumped is the square root of an integer. Unexpectedly, such edge borders a bulk with dynamical anyon transmutation, demonstrating a novel form of bulk-boundary correspondence.

[Phys. Rev. B 96, 245116] Published Tue Dec 12, 2017

]]>The Berry phase provides a modern formulation of electric polarization in crystals. Recently, this formulation was extended by the authors to higher electric multipole moments. In quantum mechanical crystalline insulators, such higher multipole moments manifest themselves by the presence of boundary-localized moments of lower dimension. In the presence of certain symmetries, these moments are quantized, and their boundary signatures are fractionalized. Here, the authors elaborate in detail on the theory of higher multipole moments and discuss associated topological pumping phenomena, as well as higher-order topological insulators derived from them.

[Phys. Rev. B 96, 245115] Published Mon Dec 11, 2017

]]>The recent discovery of spin and orbital textures in nonmagnetic crystals with inversion symmetry has broadened the scope for spintronics applications. These so-called hidden polarizations are however difficult to probe, in part because they average to zero within each unit cell. In this work, the authors show that a bulk detection of intra-unit cell spin and orbital textures can be achieved with nuclear magnetic resonance by splitting, with an electric current, the resonance peak of inversion partner nuclei. The proposal is illustrated with numerical results for Bi${}_{2}$Se${}_{3}$ and Bi${}_{2}$Te${}_{3}$, and other promising materials are identified on the basis of their crystal symmetries.

[Phys. Rev. B 96, 235201] Published Fri Dec 08, 2017

]]>The spin dynamics of localized electrons in solids is drastically different from that of mobile electrons. One of the main features characteristic for localized spins is the prominent difference between the longitudinal and transverse spin relaxation times ${T}_{1}$ and ${T}_{2}$${}^{\star}$ in magnetic fields. While up till now these times were believed to be of different nature, here the authors find the surprising relation ${T}_{1}$ $\times $ ${T}_{2}$${}^{\star}$ ≈ constant for electrons localized on donors in $n$-GaAs. This is explained by the impact of the magnetic field on spin diffusion. On the other hand, mobile electrons are characterized by a single spin lifetime in the motional-narrowing regime. Upon rising temperature or donor concentration, the authors observe a striking 100-fold increase of ${T}_{2}$${}^{*}$ due to the onset of motional narrowing.

[Phys. Rev. B 96, 241201(R)] Published Thu Dec 07, 2017

]]>Anderson localization of noninteracting particles has traditionally been viewed as being due to random potentials. However, random potentials are not needed, and localization due to nonrandom quasiperiodic potentials is well studied in one dimension. The authors extend this work to higher dimensions. A quantum particle in a quasiperiodic potential in three dimensions has three “eigenstate phases”: a localized phase at strong potential, a ballistic phase at weak potential, and a diffusive phase in between. They find that the universality class of the Anderson localization transitions in three dimensions is insensitive to whether or not the potential is random, which is not true in fewer dimensions.

[Phys. Rev. B 96, 214201] Published Wed Dec 06, 2017

]]>The phase diagram of 1$T$-TaS${}_{2}$ involves many different solid-state phases, including several different kinds of charge-density-wave (CDW) phase both commensurate and incommensurate with the underlying lattice. Recent experiments have shown that it is possible to drive transitions between these phases with intense femtosecond pulses of light. This work focuses on the light-driven transition from the nearly commensurate CDW phase to the incommensurate phase, using time-resolved x-ray diffraction. The authors find that the new incommensurate domains grow in a self-similar way. They also employ a double-pump method to study the dynamics and stability of the novel incommensurate phase during growth.

[Phys. Rev. B 96, 224101] Published Wed Dec 06, 2017

]]>The band crossings at the Fermi level of a Weyl semimetal are generally stable against disorder. The only way to open a gap in the spectrum is by annihilating pairs of Weyl nodes. Here, the authors show that coupling Weyl nodes by a superlattice does not always lead to gap opening, but does lead to a variety of different phases, including a nodal-line semimetal. The authors uncover a novel mechanism that protects the nodal line, a combination of a fractional lattice translation and charge-conjugation symmetry, and show that its in-gap surface states are not necessarily exponentially localized.

[Phys. Rev. B 96, 245101] Published Fri Dec 01, 2017

]]>Weyl semimetals, prototypical three dimensional topological metals, possess a unique band structure that has enriched the “standard model” of metallic behavior. This work unveils how the unusual topological surface states of Weyl semimetals radically alter their plasmonic characteristics, yielding intrinsic hyperbolic surface plasmons. These Fermi-arc plasmons have a rich phenomenology, and arise from the coupled dynamics of carriers both on the unusual open-segment surface Fermi arc and the bulk Fermi surface, clearly demonstrating the intricate interplay of bulk and surface. While Fermi-arc plasmons are fascinating in their own right, the essential role that the open-segment Fermi arcs play also indicates more broadly that Weyl semimetals may host new types of interacting phenomena not realizable in conventional metals − an exciting prospect for future developments.

[Phys. Rev. B 96, 205443] Published Wed Nov 29, 2017

]]>Spin currents in antiferromagnetic materials have recently attracted much interest in the field of spintronics. Although the thermal generation effect of spin currents, as in the spin Seebeck effect (SSE), is powerful for their study, the SSE in antiferromagnets has been experimentally studied only for Cr${}_{2}$O${}_{3}$ and MnF${}_{2}$. In this work, the authors experimentally observe the SSE in the polar antiferromagnet α-Cu${}_{2}$V${}_{2}$O${}_{7}$. Comparison of the experimental results with calculations using magnetic parameters determined by neutron scattering studies reveals that the magnon scattering plays an important role in the antiferromagnetic SSE observed in α-Cu${}_{2}$V${}_{2}$O${}_{7}$.

[Phys. Rev. B 96, 180414(R)] Published Tue Nov 28, 2017

]]>The phase diagram associated with cuprate superconductors is complicated by an array of different ground states. The parent material represents an antiferromagnetic insulator, but becomes a high-temperature superconductor upon doping. The underdoped region of the phase diagram is dominated by the so-called pseudogap phenomena, whose understanding presents one of the great challenges for the field. However, another aspect of these complex systems that is frequently overlooked is the tendency to phase separate into nanoscale regions upon hole doping. The overdoped region allowed the authors to examine the influence of the latter on the overall structure of the phase diagram.

[Phys. Rev. B 96, 195163] Published Tue Nov 28, 2017

]]>Hybrid organic-inorganic perovskites are a promising class of materials for low-temperature solution-processed thin-film optoelectronic technologies. Here, the authors assign the visible and near-infrared optical transitions in large-grain thin films, using variable temperature transient absorption measurements that provide a sensitive probe of the electronic energy levels. These studies address an ongoing controversy concerning the nature of higher-order subband transitions. The strong visible absorption band is assigned to nearly degenerate transitions originating from different momentum states within the multivalley electronic band structure using symmetry analysis and the response of the optical resonances during the low-temperature structural phase transition.

[Phys. Rev. B 96, 195308] Published Tue Nov 28, 2017

]]>Optical cooling of a mesoscopic nuclear spin ensemble in a semiconductor is a complex process that involves transferring angular momentum and energy between nuclear spins and optically oriented localized electrons. The established spin temperature is sensitive to the balance between pumping and losses as well as to the heat capacity of the nuclear spin ensemble. This property allows one to determine the zero-frequency value of the correlator of nuclear spin fluctuations and thus to estimate the spin-spin relaxation time ${T}_{2}$ at low magnetic fields from the frequency response of the electron-nuclear spin ensemble to a modulated optical pumping.

[Phys. Rev. B 96, 205205] Published Tue Nov 28, 2017

]]>In a frustrated magnet with competing ferromagnetic and antiferromagnetic interactions, a spin-nematic state can be realized due to the condensation of two magnon bound stats. Volborthite Cu${}_{3}$V${}_{2}$O${}_{7}$(OH)${}_{2}\cdot $2H${}_{2}$O is a promising candidate. The authors study the spin dynamics in the high-field phases of volborthite. In the magnetization plateau state, the nuclear relaxation rates indicate an excitation gap with a large effective $g$ factor, pointing to magnon bound states. The relaxation rates also indicate unusual spin fluctuations with slow time scales. These findings suggest that an exotic spin state caused by the condensation of magnon bound states is realized below the magnetization plateau.

[Phys. Rev. B 96, 180413(R)] Published Mon Nov 27, 2017

]]>Our expectation of thermalization is one borne of experience, and it is interesting to understand how conventional statistical mechanics may fail. The so-called quantum disentangled liquid (QDL) is a paradigm outside of the (Generalized) Gibbs description, but also without spatial disorder, precluding many-body localization. In this paper, the authors provide evidence that this QDL behavior exists in the Hubbard model. The QDL diagnostic involves bipartite entanglement entropies after a partial measurement. Both integrability in 1D and strong-coupling techniques are used to argue that the QDL diagnostic is indeed satisfied.

[Phys. Rev. B 96, 195153] Published Mon Nov 27, 2017

]]>To date, most studies on electronic excitations in 2D materials have focused on the optically active excitations, but recent experiments have highlighted the existence of dark states, which are equally important in many respects. Here, the authors use $a\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ many-body calculations to unravel the nature of the dark excitations in monolayer MoSe${}_{2}$, MoS${}_{2}$, WSe${}_{2}$, and WS${}_{2}$. The lowest neutral and charged excitations are found to be dark. In accordance with experiment, dark trions are weaker bound than bright trions.

[Phys. Rev. B 96, 201113(R)] Published Mon Nov 27, 2017

]]>Terahertz spectroscopy of nanomaterials is one of the most active areas of terahertz physics thanks to its ability to reveal charge-carrier confinement on length scales of tens to hundreds of nanometers. Structural confinement modifies the experimental complex conductivity, such that it is Drude-like at high frequencies but suppressed at low frequencies. This response is often described as a consequence of carrier backscattering off nanoparticle boundaries and modeled using the sometimes controversial Drude-Smith formula. In this paper, it is demonstrated that the terahertz conductivity of a structurally confined Drude gas of electrons is actually suppressed at low frequencies due to carrier confinement on the diffusion length scale and not due to backscattering. A new conductivity formula is derived based on diffusion that is found to be very similar to the Drude-Smith conductivity formula, thereby explaining many of the previous successes of the Drude-Smith model.

[Phys. Rev. B 96, 205439] Published Mon Nov 27, 2017

]]>A key signature of a topological Majorana zero mode is the 2${e}^{2}$/$h$ quantized zero-temperature zero-bias conductance of a metal-superconductor tunnel junction. This theoretical work shows that the finite-temperature Majorana conductance obeys a scaling relation with respect to a single dimensionless parameter, given by the ratio of the temperature and tunnel coupling, but only in the limit of weak tunneling and low temperature. Unfortunately, exactly the same scaling also applies to the zero-bias conductance arising from trivial Andreev bound states. This implies that a clear distinction between topological and trivial physics is not feasible based on such scaling plots.

[Phys. Rev. B 96, 184520] Published Wed Nov 22, 2017

]]>Machine learning, the core of artificial intelligence and data science, is a very active field, with vast applications throughout science and technology. Recently, machine learning techniques have been adopted to tackle intricate quantum many-body problems and phase transitions. In this work, the authors construct exact mappings from exotic quantum states to machine learning network models. This work shows for the first time that the restricted Boltzmann machine can be used to study both symmetry-protected topological phases and intrinsic topological order. The exact results are expected to provide a substantial boost to the field of machine learning of phases of matter.

[Phys. Rev. B 96, 195145] Published Wed Nov 22, 2017

]]>The efficient simulation of many-body quantum systems is an important application of quantum computers. However, extracting useful information from a system of qubits is not straightforward. The quantum computing equivalent of the vast array of diagnostic tools that extract information from classical numerical simulation are still being developed. This paper addresses the efficient calculation of quantities that characterize entanglement between different parts of a quantum state using a quantum computer. Illustrative examples of applications as diverse as many-body localization and fractional quantum Hall effect are discussed.

[Phys. Rev. B 96, 195136] Published Mon Nov 20, 2017

]]>This paper is devoted to the analysis of the frequency dependence of the optical conductivity, ${\sigma}^{\prime}(\mathrm{\Omega})$, of a metal near a quantum critical point (QCP). According to general reasoning, the frequency dependence of ${\sigma}^{\prime}(\mathrm{\Omega})$ may come via the scattering time and the effective mass, ${m}^{\star}$, but not via the quasiparticle residue $Z$, because the conductivity probes quasiparticles rather than bare electrons. However, in local theories of QCPs, ${m}^{\star}/m$, coincides with $1/Z$, and it is not straightforward to separate the two quantities. The authors use a direct diagrammatic approach, in which ${m}^{\star}/m$ and $Z$ are treated separately. They compute the optical conductivity ${\sigma}^{\prime}(\mathrm{\Omega})$ near two-dimensional nematic and spin-density wave (SDW) QCPs. They explicitly demonstrate that the $Z$ factor is canceled out by series of vertex corrections, and ${m}^{\star}/m$ does not appear separately from $Z$. As a result, ${\sigma}^{\prime}(\mathrm{\Omega})$ diverges as $1/{\mathrm{\Omega}}^{2/3}$ in the nematic case and as $1/\mathrm{\Omega}$ (up to logarithmic factors) in the SDW case.

[Phys. Rev. B 96, 205136] Published Mon Nov 20, 2017

]]>The polar Kerr effect, or equivalently the ac anomalous Hall effect, is a signature of time-reversal symmetry breaking in chiral superconductors. Seeing it requires additional ingredients, such as disorder or multiple bands. The simplest model of chiral superconductivity in UPt${}_{3}$, which is thought to be $f$ wave, would not exhibit a Kerr effect. Here, it is shown that the “starfish” Fermi surface, where the bands stick together at the top and bottom of the Brillouin zone, can exhibit a Kerr effect due to multiband effects and to mixing of $f$- and $d$-wave order parameters that are respectively even and odd in the hcp sublattice index.

[Phys. Rev. B 96, 174511] Published Thu Nov 16, 2017

]]>The peculiar surface transport properties of topological insulators often require tuning of the Fermi level close to the Dirac point. This, however, reduces the screening capabilities of the Dirac-like surface carriers and thus enhances the sensitivity to Coulomb disorder present in compensated materials. Coulomb disorder drives the formation of puddles, local accumulations of charge carriers both in the bulk and on the surface. In the topological insulator BiSbTeSe${}_{2}$, the coexistence of surface puddles and bulk puddles is revealed by scanning tunneling spectroscopy and optical spectroscopy. Quantitative analysis shows that bulk puddles contribute to the screening of surface potential fluctuations in this undoped compound. Understanding and controlling the puddles will be a key to realizing novel phenomena in 3D topological materials.

[Phys. Rev. B 96, 195135] Published Thu Nov 16, 2017

]]>Lattices of magnetic adatoms deposited on the surface of $s$-wave superconductors (Shiba lattices) have been proposed as a new platform for topological superconductivity, allowing for the local manipulation and detection of Majorana modes via scanning-probe techniques. The authors demonstrate that the topological Majorana edge modes of nanoscopic Shiba islands display universal electronic and transport properties. Most remarkably, these Majorana modes possess a quantized charge conductance that is proportional to the topological Chern number ($C$), and carry a supercurrent whose chirality reflects the sign of $C$. These results establish nanoscopic Shiba islands as promising components in future topology-based devices.

[Phys. Rev. B 96, 205131] Published Thu Nov 16, 2017

]]>Can one steer a material by its surface? A generalization of Bloch’s theorem to arbitrary boundary conditions reveals actionable links going from surface physics towards changing overall electronic properties. Furthermore, it predicts the existence, in short-range systems, of topological zero-energy modes with power-law corrections, and answers basic questions about system sizes, for which bulk and surface properties merge. The theorem yields exact inversion-free diagonalization algorithms, potentially of interest for large-scale band-structure calculations, by leveraging two new constructs: the analytic continuation of the Bloch Hamiltonian off the Brillouin zone and the boundary matrix that efficiently encodes the interplay between bulk and boundary conditions.

[Phys. Rev. B 96, 195133] Published Wed Nov 15, 2017

]]>Strong interaction with light may modify quantitatively the electronic properties of condensed matter systems. Here, the authors consider the classical example of topological electron states in a CdTe/HgTe/CdTe heterostructure and demonstrate that the coupling between electrons in a two-dimensional region and an external electromagnetic wave controls with high accuracy the normal and topological phases in the system. The electromagnetic radiation also modifies the properties of the edge states, which leads to the reversal of the edge spin currents. Importantly, all these effects can be realized in controllable and reversible way, and this has high potential impact for their integration of spintronics and optoelectronics.

[Phys. Rev. B 96, 205127] Published Wed Nov 15, 2017

]]>Weyl semimetals are bulk materials with extreme spin-orbit coupling giving rise to gapless Dirac-like excitations. They exhibit a number of peculiar electronic properties owing to the topological nature of the spectrum. While weak disorder is known to be irrelevant in Weyl semimetals, recent theoretical studies have suggested that scattering from rare regions can give rise to a finite density of states at charge neutrality. The authors present a self-consistent theory, taking into account coherent scattering on multiple resonant impurities. The theory predicts relatively high density of states and conductivity at the Weyl point with a nonanalytic dependence on the impurity concentration.

[Phys. Rev. B 96, 174205] Published Mon Nov 13, 2017

]]>The search for higher-T${}_{c}$ superconductivity is in the vanguard of contemporary physics. Most unconventional high-T${}_{c}$ superconductors exist near a correlated-insulator phase. Unexpectedly, no such phase has been discovered in the family of unconventional iron-based high-temperature superconductors despite almost a decade of intensive search. Here, the authors perform detailed optical spectroscopy and $a\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ studies of NaFe${}_{1-x}$Cu${}_{x}$As that reveal the existence of critically strong electronic correlations in this family of iron-based materials, culminating in a novel correlation-driven insulating parent phase with a large spectroscopic gap in the excitation spectrum. These results may lead to the discovery of new types of superconductivity in proximity to the novel correlated insulator.

[Phys. Rev. B 96, 195121] Published Thu Nov 09, 2017

]]>Here, the authors investigate the out-of-equilibrium properties of a simple quantum impurity model: the interacting resonant level model. They focus on the scaling regime, where the bandwidth of the fermions in the leads is larger than all the other energies, so that the lattice and the continuum versions of the model become equivalent. Using time-dependent density matrix renormalization group simulations, the new results cover a range of parameters, complementing previous analytical results that exist for only a few points in parameter space. The chief results are a demonstration of scaling collapse for $I$-$V$ curves and for entanglement in terms of the Kondo temperature.

[Phys. Rev. B 96, 195117] Published Wed Nov 08, 2017

]]>A current challenge in condensed matter physics is the search for novel quantum states of matter. In this paper, the authors provide a simple guiding principle for finding quantum spin liquids in real materials. Like in the case of unconventional superconductivity that involves pairing of two fermions into a bound state, it is shown here that chiral quantum liquids naturally emerge in the vicinity of a class of magnetic quantum critical points, due to pairing of two bosons into a bound state of zero total spin.

[Phys. Rev. B 96, 184409] Published Tue Nov 07, 2017

]]>Spin pumping describes the flow of a spin current from a ferromagnet into an adjacent conductor due to spin precession or thermal gradients in the ferromagnet. To measure its magnitude, often the damping of the ferromagnet in ferromagnetic resonance is investigated, which increases when spin pumping takes place. However, in the experiments shown here, it is shown that electromagnetic interactions between the ferromagnet and the conducting nonmagnet with the exciting magnetic radio-frequency field can also cause additional damping. This so-called radiation damping can mimic spin pumping where none exists or at least considerably falsify the results obtained from the standard evaluation of the experiment.

[Phys. Rev. B 96, 184405] Published Mon Nov 06, 2017

]]>The valley Hall effect and topological valley edge states are two fundamental properties in gapped valleytronic materials, such as MoS${}_{2}$ and biased bilayer graphene. Such properties have paved the way for applications in valleytronics. Here, the authors experimentally demonstrate a valley surface-wave photonic crystal on a single metal surface, as the photonic analog of the valley-Hall topological insulator phase. The photonic valley-Hall effect with valley-chirality locked beam splitting, and topological valley-polarized edge states, are demonstrated for the first time on a photonic platform.

[Phys. Rev. B 96, 201402(R)] Published Mon Nov 06, 2017

]]>Topological phases protected by the geometrical symmetries of crystal lattices turn out to be surprisingly simple. They can be built from simpler lower-dimensional states, arranged in a crystalline pattern.

[Phys. Rev. B 96, 205106] Published Mon Nov 06, 2017

]]>The modification of a circularly polarized vacuum field in three-dimensional chiral photonic crystals was measured by spontaneous emission from quantum dots in the structures. Due to the circularly polarized eigenmodes along the helical axis in the GaAs-based mirror-asymmetric structures involved, the authors observed highly circularly-polarized emission from the quantum dots. Both spectroscopic and time-resolved measurements confirmed that the obtained circularly-polarized light was influenced by a large difference in the photonic density of states between the orthogonal components of the circular polarization in the vacuum field.

[Phys. Rev. B 96, 195404] Published Fri Nov 03, 2017

]]>The quantum anomalous Hall (QAH) effect involves spin-polarized, dissipation-free chiral edge state transport in the absence of an external magnetic field. This effect has been realized in magnetically doped topological insulators, such as Cr- and V-doped (Bi,Sb)${}_{2}$Te${}_{3}$ thin films. To date, the observation of the QAH effect in these materials is limited to millikelvin temperatures deep below their ferromagnetic transition temperature. In this report, the authors provide complementary magnetic and electronic study of such systems using muon spin and photoemission spectroscopies. They show that the topological insulator layers become fully ferromagnetic only at much lower temperature than the onset temperature for ferromagnetism and demonstrate the presence of a magnetic impurity band with a finite density of states at the Fermi level. Both properties may be responsible for the low temperature needed for observing the QAH effect.

[Phys. Rev. B 96, 184402] Published Thu Nov 02, 2017

]]>A smooth heterojunction between a topological (left panel of figure) and a trivial (right panel) hosts not only chiral Dirac states but also massive Volkov-Pankratov states (VPSs). The authors show that VPSs behave in a relativistic manner in a strong electric field: not only are they populated in increasing chemical potential, but their energy is lowered in a Lorentz boost by the electric field thus enhancing their visibility. Signatures of this relativistic field effect are reported in HgTe/CdHgTe heterojunctions using high-frequency transport measurements. These include anomalous charge screening (Dirac screening), its breakdown at the VPS band edge, and evidence of multiple VPSs.

[Phys. Rev. B 96, 195104] Published Wed Nov 01, 2017

]]>Hydrodynamics is a powerful theory for the description of transport in materials where the electron-electron mean-free path is much smaller than all the other length scales of the problem. Such a nonperturbative theory relies on a small number of coefficients, such as bulk and shear viscosities and thermal conductivity. When time-reversal symmetry is broken, a new dissipationless term, controlled by the Hall viscosity, appears in the Navier-Stokes equation. Here, the authors demonstrate that measurements of nonlocal resistances in multiterminal Hall-bar devices allow one to access the Hall viscosity of a two-dimensional electron liquid.

[Phys. Rev. B 96, 195401] Published Wed Nov 01, 2017

]]>Majorana zero modes are localized zero-energy degrees of freedom in topological superconductors that can store quantum information in a protected manner. Finding a direct proof of the Majorana identity of zero-energy states has been a challenge. The authors show that, in a Coulomb blockaded superconductor island coupled to normal metal leads, Majorana zero modes on the island bind with electrons in the leads to form a charge-$e$ boson, thereby converting the quantum statistics of charge carriers. This statistical transmutation leads to unusual temperature and voltage dependence of the tunneling current as a unique signature of the Majorana zero modes.

[Phys. Rev. B 96, 205403] Published Wed Nov 01, 2017

]]>Quantum dots in photonic crystal membrane structures are an attractive platform for on-chip quantum photonics. However, charge control of quantum dots in thin membranes is challenging. The widely used p-i-n diodes operate typically at high bias, resulting in high leakage currents. Here, a modified diode design is presented, fully compatible with the fabrication of photonic crystals, that enables deterministic charging of embedded quantum dots at small biases and, hence, small currents. Narrow optical linewidths and optical spin pumping are demonstrated on quantum dots in this ultrathin diode structure.

[Phys. Rev. B 96, 165440] Published Mon Oct 30, 2017

]]>A new type of a topological quantum structure was identified recently in PtSn${}_{4}$ as a Dirac node arc feature on the surface. More importantly, PtSn${}_{4}$ shows extremely large magnetoresistance (XMR). PdSn${}_{4}$ is isostructural with PtSn${}_{4}$ and has the same electron count. By comparing these similar compounds, the authors address the problem about the origin of the XMR in them and establish that neither the carrier compensation nor the Dirac node arc surface state can be the primary reason for it. On the other hand, Kohler’s rule scaling is fulfilled over the entire range of temperatures and field strengths that are explored here.

[Phys. Rev. B 96, 165145] Published Fri Oct 27, 2017

]]>Nanometer-scale ferroelectric vortex that is characterized by a curling polarization around a core is regarded as a new state of matter. The control of vortex chirality by homogeneous electric field is a key to the utilization of vortices in technological applications, but remains elusive since the toroidal moment of a ferroelectric vortex is conjugate to a curled electric field rather than to a homogeneous one. Here, the authors design free-standing nanodots with rational nanostructures and demonstrate using phase-field modelling that the chirality of the polarization vortex in such designed nanodots is deterministically switched by a homogeneous electric field.

[Phys. Rev. B 96, 134119] Published Wed Oct 25, 2017

]]>There is an increasing fascination centered on superconductivity and topological properties with bands that are flat due to quantum mechanical interference. An important question remains: can we actually control the energy of the flat band against the dispersive ones? Here, the authors have constructed classes of models in two (or higher) dimensions in a systematic extension of the known flat bands, where the controllability provides a flat band with a tunable gap, or a flat band that pierces right through the dispersive one. While the former may favor ferromagnetism, the latter is expected to favor high-${T}_{c}$ superconductivity through virtual pair hopping from dispersive to flat bands.

[Phys. Rev. B 96, 155137] Published Wed Oct 25, 2017

]]>Tensor network states have proven to be powerful tools for investigating quantum many-body systems. Here, the authors propose a tensor network method, based on an adaptation of entanglement renormalization, for investigating strongly disordered systems. The method makes use of the strong disorder renormalization group to determine the order in which lattice sites are coarse grained, which sets the overall structure of the corresponding tensor network ansatz, before optimization using variational energy minimization. This approach leads to a new class of efficiently contractible tensor network ansatz for 1D systems, which may be understood as a generalization of the multiscale entanglement renormalization ansatz for disordered systems.

[Phys. Rev. B 96, 155136] Published Tue Oct 24, 2017

]]>Order-disorder phase transitions are common in a wide range of materials and are exploited in many technological applications. A key challenge in developing predictive mesoscale models for these materials lies in constructing rigorous first-principles based thermodynamic descriptions. In this study, the authors develop a general formalism to generate order parameters for any crystal structure and alloy ordering. Furthermore, they describe a thermodynamic and statistical mechanics framework to determine the free energy as a function of order parameters. The methods described here are broadly applicable to any multicomponent crystalline solid and provide the first step in generating truly $a\phantom{\rule{0}{0ex}}b\phantom{\rule{0.333em}{0ex}}i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ multiscale models.

[Phys. Rev. B 96, 134204] Published Mon Oct 23, 2017

]]>Planar photonic technology provides a highly promising route of creating an on-chip and on-demand source of single photons, which can be scaled into photonic circuitry. While self-assembled quantum dots have been proven to be excellent coherent single-photon emitters in many microscale structures, in nanoscale devices the photon coherence can be compromised due to the nearby surfaces. Here, the authors achieve 94% indistinguishability of photons emitted from a nanoscale photonic waveguide under $p$-shell excitation.

[Phys. Rev. B 96, 165306] Published Thu Oct 19, 2017

]]>The electronic structure of WTe${}_{2}$ and of orthorhombic $\gamma $-MoTe${}_{2}$ are theoretically claimed to contain pairs of Weyl type-II points associated with linearly touching electron and hole pockets. A series of recent angle-resolved photoemission spectroscopy (ARPES) experiments claims to observe general agreement with these predictions. Here, the authors report a de Haas–van Alphen study on $\gamma $-MoTe${}_{2}$ which, in contrast to ARPES and predictions, finds a simpler and more isotropic Fermi surface. Upon the guidance of ARPES, they show here that it is possible to explain the observed Fermi surface geometry by an independent displacement of the theoretical electron and hole bands relative to the Fermi level. However, this $a\phantom{\rule{0}{0ex}}d$ $h\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}c$ procedure, previously used by a number of groups, suppresses the crossings between electron and hole bands and hence the predicted Weyl type-II points.

[Phys. Rev. B 96, 165134] Published Tue Oct 17, 2017

]]>Ferromagnets perturbed by time-dependent fields are technologically important examples of nonequilibrium physics. Using kinetic Monte Carlo simulations and droplet theory of magnetization reversal, the authors explain recent experiments on ordered uniaxial ferromagnetic films driven by oscillating fields. The associated dynamic phase transition at a critical field period is known to be in the equilibrium Ising universality class. Consistent with experiments, the authors find that the response to a constant bias field in the supercritical period limit differs from an equilibrium ferromagnet above criticality in that, instead of a wide central susceptibility maximum, side bands are observed, which are shown to result from stochastic resonance.

[Phys. Rev. B 96, 134306] Published Fri Oct 13, 2017

]]>Periodically driven systems have recently been shown to host topological phases that are inherently dynamical in character, opening up a new arena in which to explore topological physics. One important group of such phases, known as Floquet topological insulators, arise in systems of free fermions and exhibit protected topological edge modes analogous to the edge modes of static topological insulators. In this work, the authors use methods from K theory to provide a complete topological classification of Floquet topological phases of this kind. The main result is a periodic table for Floquet topological insulators, which may be viewed as a time-dependent extension of the periodic table of topological insulators and superconductors originally introduced by Alexei Kitaev.

[Phys. Rev. B 96, 155118] Published Fri Oct 13, 2017

]]>$\alpha $-RuCl${}_{3}$ is the most promising candidate for the long-sought materialization of the Kitaev model, where compasslike exchange interactions lead to a gapless spin liquid with emergent Majorana fermion excitations. Prerequisite for these expectations is that the symmetry-imposed noncubic crystal field is smaller compared to the spin-orbit coupling and does not perturb significantly the description of the electronic ground state in terms of the ${J}_{\text{eff}}$ = 1/2 state. Here, the authors combine polarization-dependent x-ray absorption spectroscopy, a technique that gives direct access to the crystal field of a material, and full multiplet calculations to determine the size and sign of the noncubic crystal-field splitting. The results demonstrate that the “electronically cubic conditions” and the ${J}_{\text{eff}}$ = 1/2 ground state for Ru in $\alpha $-RuCl${}_{3}$ are fulfilled.

[Phys. Rev. B 96, 161107(R)] Published Thu Oct 12, 2017

]]>The authors present a unified perspective on a large class of one-dimensional symmetry-protected topological phases. Characterizing critical points between such phases leads to a conjecture of a topological lower bound on the central charge in terms of the phase-specific edge mode dimension.

[Phys. Rev. B 96, 165124] Published Thu Oct 12, 2017

]]>Multiband $\mathbf{k}\cdot \mathbf{p}$ Hamiltonians near the high symmetric points in the Brillouin zones of two-dimensional (2D) materials are widely used to predict and understand the unique transport and optical properties of 2D materials. Based on the crystal symmetry of 2D materials, the authors develop a multiband $\mathbf{k}\cdot \mathbf{p}$ Hamiltonian for monolayer GaSe and InSe using the theory of invariants. The authors establish the relationship between the crystal lattice symmetry and the Hamiltonian matrix elements, accounting for spin-orbit interaction and strain. The work provides a systematic way to construct the relevant multiband $\mathbf{k}\cdot \mathbf{p}$ Hamiltonian for various monolayer and few-layer materials.

[Phys. Rev. B 96, 155430] Published Wed Oct 11, 2017

]]>Phonon-assisted tunneling plays a crucial role in sub-10-nanometer gate-length devices. Here, the authors present a first-principles special thermal displacement method that bridges the models used independently in molecular electronics and in device simulations. The method is conceptually simple and much more efficient than any existing first-principles transport method, enabling modeling of realistic rectifiers and transistors with account of electron-phonon coupling. The results are in excellent agreement with both experiments and state-of-the-art perturbation theory calculations discussed in the paper, demonstrating that the method is an appealing design tool for next-generation devices and nanomaterials.

[Phys. Rev. B 96, 161404(R)] Published Wed Oct 11, 2017

]]>Monocrystalline SnSe, which is a binary semiconductor, has recently been demonstrated to be a high-performance thermoelectric material with a record-high figure of merit of 2.6 at high temperature (923 K). Using angle-resolved photoemission spectroscopy, the authors probed the full electronic structure of SnSe. They observed three convergent hole bands with small energy differences in their band tops and relatively small in-plane effective masses, in agreement with $a\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ calculations. These bands are critical for the enhancement of the Seebeck coefficient, while preserving the high electrical conductivity of the material. Thus, they represent a key factor contributing to the high figure of merit in SnSe.

[Phys. Rev. B 96, 165118] Published Tue Oct 10, 2017

]]>Quantum liquid crystals are strongly correlated quantum many-body systems with broken rotational symmetry but with partial or complete translational symmetry. Inspired by the classical theory for hexatic liquid crystals, which deals with the thermal unbinding of dislocations in 2D solids, the present work advances the idea that 3D solids can melt quantum mechanically via unbinding, or rather Bose-Einstein condensation, of dislocation worldsheets, resulting in quantum analogs of nematic, smectic, and columnar liquid crystals. In these phases, the phonons, as represented by two-form dual gauge fields, develop a mass gap reflecting the loss of shear rigidity.

[Phys. Rev. B 96, 165115] Published Mon Oct 09, 2017

]]>The development of nanoscale optical devices requires high-quality nanocavities to mediate the optical feedback. Any fabrication method will generate imperfections that may induce light loss, limiting the device performance. However, in some cases such disorder may enable new functionalities as, for example, in state-of-the art photonic-crystal waveguides where localization originates from the random multiple scattering of light. Understanding the different mechanisms leading to this type of localization is crucial to exploit disorder as a resource as well as to design structures which are more robust against disorder.

[Phys. Rev. B 96, 144201] Published Fri Oct 06, 2017

]]>Three-dimensional Kitaev models – generalizations of the well known exactly solvable Kitaev honeycomb model – exhibit a rich variety of quantum spin liquid phases. Their fractionalized excitations are Majorana fermions that may form topological band structures. These can be enriched by lattice symmetries, as shown in the present work. In particular, the authors propose two new instances of Kitaev spin liquids that exhibit nodal chains and 3D Dirac nodes in the bulk. Such crystalline Kitaev spin liquids are direct generalizations of crystalline topological insulators, albeit in an interacting spin system. A potential material realization in metal-organic frameworks is also discussed.

[Phys. Rev. B 96, 155107] Published Fri Oct 06, 2017

]]>Orthorhombic perovskites provide a perfect playground for investigations of novel magnetic phenomena, such as ferroelectricity, ultrafast spin reorientation, colossal magnetoresistance, and many others. Here, the authors combined magnetization and neutron scattering measurements with crystalline electrical field calculations to characterize the magnetic properties of a DyScO${}_{3}$ single crystal. They establish that Dy moments have a strong Ising-like single-ion anisotropy with a doublet ground state and an enormous value of effective moment. This results in a significant dipolar interaction, which strongly suppresses conventional transverse fluctuations and makes magnetic excitations invisible to neutron spectroscopy.

[Phys. Rev. B 96, 144407] Published Thu Oct 05, 2017

]]>The electronic edge states of MoS${}_{2}$ nanoparticles are important for the catalytic activity of the particles and could also have potential for photocatalysis. Here, the authors explore computationally optical properties of MoS${}_{2}$ monolayer nanoflakes. These nanostructures show a rich size-dependent optical response below the absorption onset of the infinite monolayer. The authors present a detailed analysis of the emerging optical absorption resonances revealing their edge-state origins. In particular, metallic edge states support localized edge plasmons, and overall, resonances are sensitive to changes in the edge-atom configuration.

[Phys. Rev. B 96, 155407] Published Thu Oct 05, 2017

]]>Fracton phases are three dimensional topological phases in which quasiparticle excitations are restricted to move along planes, lines, or not to move at all. In this work, we show how such phases can be understood in terms of parton constructions, and in the process we find a new type-II model with fractal logical operators.

[Phys. Rev. B 96, 165105] Published Thu Oct 05, 2017

]]>The self-learning Monte Carlo (SLMC) method speeds up the Monte Carlo simulation by designing and training an effective model to propose efficient global updates. We implement the continuous-time quantum Monte Carlo algorithm for quantum impurity models in the framework of SLMC. We introduce and train a diagram generating function (DGF) to model the probability distribution of field configurations in the continuous imaginary time at all orders of diagrammatic expansion. By using the DGF to propose global updates, we show that the self-learning continuous-time Monte Carlo method can significantly reduce the computational complexity of the simulation.

[Phys. Rev. B 96, 161102(R)] Published Tue Oct 03, 2017

]]>Magnon-polarons are coherently mixed quasiparticles – half magnon, half phonon – and are generated by magnetoelasticity. Though predicted by Kittel long ago, their effect on spin transport was discovered only recently in yttrium iron garnet (YIG) using optical and local spin Seebeck effect (SSE) experiments. In the latter, magnon-polarons are manifest as resonant peaks in the SSE signal as a function of magnetic field. Here, the authors show that they also show up in the $n\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l$ SSE, with a surprising twist: measured nonlocally, the magnon-polaron peak turns into a dip. The authors show that this crossover is a consequence of the magnon physics underlying the SSE: thermal generation and diffusive backflow of magnons in YIG compete, which can generate any sign for the magnon-polaron anomaly in nonlocal experiments.

[Phys. Rev. B 96, 104441] Published Fri Sep 29, 2017

]]>Randomly interacting Majorana fermions governed by the Sachdev-Ye-Kitaev model have recently been shown to exhibit profound links to quantum chaos and black holes. Here, the authors propose a solid-state implementation of the model that exploits hardware components that are now widely pursued for topological quantum computation – namely, an array of Majorana wires interfaced with a disordered quantum dot that mediates the desired interactions. This work highlights a potential pathway for using table-top condensed-matter systems to probe novel concepts from high-energy physics and quantum information.

[Phys. Rev. B 96, 121119(R)] Published Fri Sep 29, 2017

]]>The ferrimagnetic insulator yttrium iron garnet (YIG) has been widely used in microwave and spintronic devices. Anomalous features in spin Seeback effect voltages have been observed in Pt/YIG and attributed to magnon-phonon coupling. Using inelastic neutron scattering to map out low-energy spin waves and acoustic phonons in YIG as a function of increasing magnetic field, the authors find that the magnon-phonon interaction enhances the hybridized scattering intensity at the magnon-phonon dispersion crossing points in zero field. These results go against the expectations of conventional spin-lattice coupling, calling for a different explanation of the magnon-phonon interactions and the resulting magnon polarons in YIG.

[Phys. Rev. B 96, 100406(R)] Published Wed Sep 27, 2017

]]>In addition to the familiar Maxwell terms, the Lagrangian for a three-dimensional U(1) gauge theory can have an extra “theta” term which has two effects: attaching electric charge to magnetic monopoles in the bulk, and generating a Chern-Simons term on the boundary. This work studies theta terms in the context of higher rank tensor U(1) gauge theories, which host gapless higher-spin gauge modes and subdimensional charge excitations. There are multiple different varieties of theta terms, enforcing different types of charge attachment in the bulk. These terms also lead to generalized tensor Chern-Simons theories on the boundary, bringing about the existence of two-dimensional fracton phases.

[Phys. Rev. B 96, 125151] Published Wed Sep 27, 2017

]]>The half-Heusler superconductors such as YPtBi are multiorbital systems with both strong spin-orbit coupling and broken inversion symmetry. The spin-orbit coupling allows for the formation of Cooper pairs with effective spins larger than unity, which introduces novel pairing states. Some of these states display Fermi surfaces of Bogoliubov quasiparticles. On the other hand, the absence of inversion symmetry leads to topologically protected nodes associated with specific surface features as, for example, flat bands. The authors consider two likely pairing symmetries for this material, one that breaks time-reversal symmetry and one that does not, and find a rich variety of bulk nodes and surface states.

[Phys. Rev. B 96, 094526] Published Tue Sep 26, 2017

]]>In the tunnel magneto-Seebeck effect, a temperature gradient generates an electric voltage over a nonmagnetic tunnel barrier between two magnetic films that depends on the relative orientation of the magnetizations. This effect has been modeled in the past by the single-electron picture as implemented in band structure calculations. Here, the authors depart from this interpretation and investigate the thermoelectric response induced by electron-magnon interactions. The importance of collective magnetic effects found in the present study challenges the conventional single-electron model for the thermopower of magnetic tunneling junctions.

[Phys. Rev. B 96, 094429] Published Mon Sep 25, 2017

]]>Here, the authors discover the unexpected important role of intrinsic atomic donor spin-orbit coupling (SOC) in driving the relaxation of coupled-donor electron states in silicon-28, which is a promising host material for semiconductor qubits, despite the commonly perceived weak strength of atomic SOC in this material. The only existing spin-relaxation mechanism considered in this context before was the relaxation of two-electron states induced by hyperfine interaction with the donor nuclear spins. The new intrinsic SOC mechanism becomes important when the donors are closely spaced as is the case in silicon quantum computing architectures. This mechanism may also shed important new light on the well-studied problem of electron spin resonance in highly doped silicon.

[Phys. Rev. B 96, 115444] Published Mon Sep 25, 2017

]]>The recent observation of excitons in highly excited states has allowed the study of Rydberg physics in cuprous oxide crystals. This opens the unique possibility to elaborate on similarities and differences between Rydberg atoms and Rydberg excitons, as done here experimentally and theoretically for quantities related to the splitting of multiplets with particular principal quantum number $n$ in zero and finite electric or magnetic fields. The authors find characteristic scaling laws with $n$ with some discrepancies between excitons and atoms, but surprisingly, they find mostly similarities, even though the underlying physics, such as the complex valence band structure, are quite different.

[Phys. Rev. B 96, 125142] Published Fri Sep 22, 2017

]]>Thermal phase transitions are successfully described by fluctuations of an appropriate order parameter, which further allows classification of the transition into first or second order. However, some quantum analogs of thermal phase transitions defy this description and identifying the mechanisms behind these unconventional processes will fundamentally improve our understanding of matter. Here, the authors provide a thorough study of such a transition, which has been experimentally accessed in graphene: the emergence of a Kekulé valence bond solid. Additional gapless quantum fluctuations of Dirac electrons change the nature of this transition from first to second order. Using a nonperturbative functional renormalization group approach, the authors advance the state-of-the-art estimates for the corresponding critical exponents and predict measurable corrections to scaling.

[Phys. Rev. B 96, 115132] Published Wed Sep 20, 2017

]]>Sr${}_{2}$IrO${}_{4}$ is considered as a strong spin-orbit coupled equivalent of high-temperature superconducting cuprates. Recent experiments have most notably shown that upon electron doping a pseudogap emerges in Sr${}_{2}$IrO${}_{4}$ and that magnonlike excitations survive the rapid disappearance of long-range magnetic order. However, due to the relatively small crystal sizes and the large Ir neutron cross section, information on charge excitations and magnetism has mostly been gathered using synchrotron-based spectroscopic methods. Here, the authors use Raman scattering on electron-doped Sr${}_{2-x}$La${}_{x}$IrO${}_{4}$ single crystals to carry out a temperature and doping study of the lattice and magnetic excitations, and observe the emergence upon doping of an electron-hole continuum that couples to the phonons. Even in metallic samples, fluctuating magnetic moments and mobile carriers therefore coexist, again in close analogy with the doped cuprates.

[Phys. Rev. B 96, 115138] Published Wed Sep 20, 2017

]]>An elegant way to probe the mysterious “pseudogap” state in cuprates is to suppress superconductivity with strong magnetic fields and work at low temperature. Then, impurity scattering dominates, which simplifies calculations. A number of theories thus successfully explain the sudden change in carrier number observed experimentally. To tell these theories apart, the work here provides new predictions. First, it shows how electron pockets cause strong signatures in the specific heat and Seebeck coefficient. Second, it addresses inconsistencies in scattering rate approximations for the Hall and the Seebeck effects. Third, it shows the influence of the Van Hove singularity on these quantities.

[Phys. Rev. B 96, 125139] Published Wed Sep 20, 2017

]]>Nanoscale magnetic phase separation in dysprosium has been revealed by aberration-corrected Lorentz microscopy. The in-plane magnetization distribution of the coexisting magnetic phases has been visualized successfully by combining high-resolution Lorentz microscopy with the transport-of-intensity equation method. The phase separation of the ferromagnetic and the helical antiferromagnetic (HAFM) phases results in the formation of static magnetic solitons at around 130 K in the absence of external magnetic field. External fields of up to about 11 kOe induce nanoscale phase separation of the HAFM, the distorted HAFM, and the fan phases at 142 K. The developed technique for the high-resolution observation of fine magnetic structures is a valuable addition to the modern toolbox of magnetic materials research.

[Phys. Rev. B 96, 100405(R)] Published Tue Sep 19, 2017

]]>In addition to the celebrated Affleck-Ludwig entropy originating from the open boundaries of the path-integral manifold, recent research has shown that the entropy correction on nonorientable manifolds such as the Klein bottle is also a universal characterization of critical systems with an emergent conformal field theory (CFT). Here, the authors show that the Klein bottle entropy can be interpreted as a boundary effect by transforming the Klein bottle into an orientable manifold, whereby the Klein bottle entropy bears a resemblance to the Affleck-Ludwig entropy. The authors then propose a generic scheme to extract these universal boundary entropies from lattice models. Numerical results for the $q$-state Potts model are compared with the CFT predictions.

[Phys. Rev. B 96, 115136] Published Tue Sep 19, 2017

]]>To address the long-standing question of double phase transitions in the superconducting state of U${}_{1-x}$Th${}_{x}$Be${}_{13}$, the authors study the quasiparticle excitations and magnetic response using a monocrystalline sample. The high-resolution heat-capacity and magnetization data demonstrate that the lower-temperature transition is between two superconducting states with different gap symmetries. Quite surprisingly, the $C(T,H,p\phantom{\rule{0}{0ex}}h)$ data strongly suggest that the superconducting gap is fully open over the Fermi surface in U${}_{0.97}$Th${}_{0.03}$Be${}_{13}$. The present thermodynamic results entirely overturn a widely believed idea that nodal quasiparticle excitations occur in uranium heavy-fermion superconductivity with broken time-reversal-symmetry.

[Phys. Rev. B 96, 100505(R)] Published Mon Sep 18, 2017

]]>Time crystals are proposed states of matter that spontaneously break time translation symmetry. Despite much recent interest, there is no settled definition for such states and existing definitions only tangentially refer to the well-established foundations of spontaneous symmetry breaking. We offer a definition of time crystals, which treats time translation much like any other symmetry and follows the traditional recipe for defining Wigner symmetries and order parameters. Using our definition, we find: (i) systems with time independent Hamiltonians should not exhibit time translation symmetry breaking, and (ii) the many-body localized $\pi $ spin glass/Floquet time crystal can be viewed as breaking both a global internal symmetry and the time translation symmetry, as befits the two aspects of its name.

[Phys. Rev. B 96, 115127] Published Mon Sep 18, 2017

]]>Materials with Rashba spin-orbit coupling have long been known to exhibit interesting transport properties due to their unique spin-momentum locking. However, the topology of the dispersion itself has an even more striking impact on electron-impurity scattering, which is only observable at ultralow energies. The authors investigate the universal properties of 2D electron scattering off of circularly symmetric potentials in this regime by deriving nonperturbative approximations for the low-energy scattering quantities, including the $T$-matrix, $S$-matrix, and scattering cross section. As the energy of the electron approaches the bottom of the lowest spin-split band, a number of unusual features appear: the low-energy $S$ and $T$ matrices become independent of the potential, all partial waves contribute equally, and the total scattering cross section exhibits quantized plateaus.

[Phys. Rev. B 96, 125304] Published Mon Sep 18, 2017

]]>In ordinary superconductors the two collective modes of the complex order parameter are usually strongly resilient to the spectroscopic observation. The amplitude or Higgs mode is weakly coupled to the electromagnetic field and it overlaps with the continuum of single-particle excitations at 2$\mathrm{\Delta}$, which buries its signal. The phase or Goldstone mode is optically inert, and it is boosted to the plasma frequency by Coulomb interactions. In the present work, terahertz experiments and theoretical calculations are combined in order to demonstrate the existence of an alternative route for the optical visibility of the Goldstone mode in a nanostructured granular Al thin film. The Coulomb screening due to the grains self-capacitance and the intrinsic inhomogeneity of the local superfluid stiffness due to shell effects lead to finite-frequency subgap absorption of the phase modes, showing that nanograins arrays are a promising setting to control the Goldstone mode via optical means.

[Phys. Rev. B 96, 094514] Published Fri Sep 15, 2017

]]>Magnetoelectric materials are characterized by a coupling of electric and magnetic degrees of freedom, which can be brought about when magnetic ordering breaks inversion symmetry. This is the case in LiCoPO${}_{4}$, which displays one of the largest magnetoelectric effects among transition metal compounds. Using neutron diffraction in combination with state-of-the-art high-field magnet technology, the authors of this paper study the magnetic structures and phase diagram of LiCoPO${}_{4}$ in fields up to 25.9 T. It is shown that the known magnetoelectric response is tied to commensurate magnetic order, while an incommensurate cycloid phase allows electric polarization along a direction yet to be probed experimentally.

[Phys. Rev. B 96, 104420] Published Fri Sep 15, 2017

]]>Na${}_{2}$IrO${}_{3}$ is a promising material to realize Kitaev physics in nature. Even though it shows long-range magnetic order of antiferromagnetic zigzag type at low temperature, the Kitaev interaction has been predicted to be significant. The origin of zigzag order, however, is currently still debated. Here, the authors suggest that adding charge fluctuations to the Kitaev-Heisenberg (KH) model represents an interesting explanation of zigzag antiferromagnetism. Moreover, their phenomenological three-band Hubbard model puts this material into a larger context as it combines topological insulator physics for weak electron interactions and KH physics for strong ones.

[Phys. Rev. B 96, 121110(R)] Published Thu Sep 14, 2017

]]>The ferromagnetic quantum phase transition (FMQPT) in clean metals has attracted much interest since a first-order transition is commonly observed. The orthorhombic Ising ferromagnet URhGe provides an excellent opportunity to study the FMQPT, because its Curie temperature can be tuned to zero by applying a magnetic field $H$ parallel to the $b$ axis, perpendicular to the spontaneous moment that aligns along the $c$ axis. Here, the authors perform high-precision angle-resolved magnetization measurements on URhGe to investigate the FMQPT with $H$ applied near the $b$ axis. They find a clear first-order transition below a tricritical point (TCP) located above 4 K, and determine the detailed profiles of the wing structure of the first-order transition in the $T$-${H}_{b}$-${H}_{c}$ phase diagram. The obtained wing structure is consistent with the theoretical expectation that three second-order transition lines merge tangentially at TCP.

[Phys. Rev. B 96, 094411] Published Mon Sep 11, 2017

]]>In addition to conventional acoustic properties (mass density and compressibility), recent research has shown that inhomogeneous acoustic media may require coupling tensors between effective volume strain and momentum fields. These coupling tensors imply conversion between monopolar and dipolar motion in the effective material response. Intrinsic coupling between strain and momentum is known as Willis coupling in elastodynamics, and it is analogous to bianisotropy in electromagnetism. This paper investigates the microscale and mesoscale effects that lead to Willis coupling, studies the proper homogenization of acoustic metamaterials with non-negligible Willis coupling, and demonstrates the impact of Willis coupling on defining physically meaningful effective material parameters.

[Phys. Rev. B 96, 104303] Published Mon Sep 11, 2017

]]>In the thermal Hall and Nernst effects, electric and heat currents, respectively, are induced perpendicular to an applied temperature gradient. Usually, such a response requires the application of a magnetic field. In contrast, here the authors establish that in Weyl semimetals – semimetals with a linear conical energy dispersion around a set of points called Weyl nodes – the Nernst effect can be realized without magnetic field, provided that the Weyl cones are tilted. This is the anomalous Nernst effect. It is significantly altered by the tilt of the Weyl cones, with Fermi surface contributions becoming important. Two experimental setups that would allow measuring both responses are discussed.

[Phys. Rev. B 96, 115202] Published Fri Sep 08, 2017

]]>The authors demonstrate room-temperature multiferroic (ferroelectric-ferromagnetic) behavior in lead titanate, in which about 30% of the titanium is substituted by palladium. Palladium is an unusual element with an instability to ferromagnetism under some special conditions. Typically, it has not been easy to incorporate it into perovskite oxides – the most common structure for ferroelectric device materials. Although palladium is expensive, commercial devices like ferroelectric memories often use very thin films, for which the added cost of palladium is negligible. Thus, the present study combines basic new physics of the first multiferroic palladium compounds with potential commercial devices.

[Phys. Rev. B 96, 104104] Published Thu Sep 07, 2017

]]>Symmetry-enriched topological phases (SET) exhibit both long-range entanglement and intricate interplay with global symmetries, such as anyon permutation symmetry or fractionalization of quantum numbers, going beyond the Landau paradigm of symmetry breaking. Here, the authors study two-dimensional SET phases based on the novel concept of symmetric local unitary transformations. Using the fixed-point structure of wave functions, they construct systematically exactly solvable models that can describe generic nonchiral SET phases. The technique applies to on-site as well as spatial symmetries and, furthermore, to anomalous symmetries that can only be realized on the surface of three-dimensional symmetry-protected phases.

[Phys. Rev. B 96, 115107] Published Wed Sep 06, 2017

]]>In high-temperature superconductors, the superconductivity appears on the verge of Mott localization and is mediated by collective electronic excitations. Here, the authors present a general theoretical framework (TRILEX) that combines the ideas of DMFT and spin-fluctuation theory, and is shown to capture both the $d$-wave superconductivity and the Mott insulating phase in the Hubbard model. The emphasis is laid on the local scattering between the electrons and the effective long-range bosonic degrees of freedom. The renormalized electron-boson vertex is obtained from a quantum Monte Carlo solution of a self-consistent single-impurity effective model. Therefore, the calculation is of significantly lighter weight compared to the minimal cluster DMFT required for the same purpose. Additionally, both the level and flavor of the approximation are variable, in a fully controlled manner.

[Phys. Rev. B 96, 104504] Published Tue Sep 05, 2017

]]>In iron-based superconductors, the interplay between nematicity, magnetism, and superconductivity is still not fully understood. Experimentally, the isoelectronic series FeSe${}_{1-x}$S${}_{x}$ provides an ideal playground to tune different electronic ground states and to selectively investigate their mutual interplay. Here, the authors use high-resolution angle-resolved photoemission spectroscopy to compare the electronic structure of different single crystals in the tetragonal phase. They find that in the absence of nematicity and magnetism, the substantial reduction of electronic correlations correlates with the decrease of the superconducting transition temperature from FeSe to FeS.

[Phys. Rev. B 96, 121103(R)] Published Tue Sep 05, 2017

]]>In the search for exciting new physics and the design of next-generation devices, gated two-dimensional heterostructures are becoming omnipresent. As the fabrication and characterization techniques in this field improve, first-principles methods must follow suit. The authors present here an implementation of density functional perturbation theory, tailored to simulate the electronic and vibrational properties of two-dimensional heterostructures in the field-effect configuration. They apply the method to gated graphene and show that while the field effect activates the coupling between electrons and flexural phonons, this coupling is strongly screened by the electrons of doped graphene.

[Phys. Rev. B 96, 075448] Published Thu Aug 31, 2017

]]>The family of transition-metal dichalcogenides provides a rich platform for studying the interplay between the crystal structure and electronic properties in strongly correlated electron systems. Applying external pressure as a tuning parameter, the authors observed a striking correlation between the critical temperature ${T}_{c}$ for superconductivity and the strength of the Se-Se bonds in the high-pressure pyrite phase of PdSe${}_{2}$. The bond length of the Se${}_{2}$ dimer was identified as the main parameter for controlling the superconductivity in the pyrite structure. Strong pressure-induced enhancement of ${T}_{c}$ up to its maximum value of 13.1 K cannot, however, be explained solely by phonon softening, and this implies the relevance of additional factors influencing ${T}_{c}$. Indeed, electronic band calculations reveal the presence of topologically nontrivial states in the pyrite structure.

[Phys. Rev. B 96, 060509(R)] Published Wed Aug 30, 2017

]]>Magnetic tunnel junctions have seen active development for spin-transfer-torque based magnetic random access memory (STT-MRAM), a new technology attractive for low-power applications, especially when embedded in semiconductor logic processors. Such efforts open new frontiers for materials and device physics involving interface magnetic states and materials properties, such as an unexpected dependence of spin-torque switching efficiency on a tunnel junction’s resistance-area product. This paper examines the observation and some possible causes. An experiment-based understanding will improve device performance for wider adaptation, while at the same time deepening the knowledge about magnetic excitations and materials physics.

[Phys. Rev. B 96, 064437] Published Wed Aug 30, 2017

]]>In frustrated magnets, competing interactions give rise to many nearly degenerate low-energy states. For classical Heisenberg antiferromagnets the spin spiral ground states form a degenerate manifold in momentum space that, somewhat unexpectedly, looks like a Fermi surface. Here, the authors demonstrate an exact classical-to-quantum correspondence that allows one to map the spin spiral surfaces of classical spin systems to the Fermi surfaces of free-fermion models.

[Phys. Rev. B 96, 085145] Published Wed Aug 30, 2017

]]>Odd-parity magnetic multipole order, where time-reversal and space-inversion symmetries are spontaneously broken, may open a new paradigm in the fields of multipole physics, multiferroics, and spintronics. BaMn${}_{2}$As${}_{2}$ is a new candidate material. Here, the authors theoretically identify that the seemingly simple antiferromagnetic order in BaMn${}_{2}$As${}_{2}$ is actually magnetic hexadecapole order mixed with magnetic quadrupole order. Interestingly, doping hole carriers realize an itinerant odd-parity magnetic multipole order. Emergent electromagnetic responses, such as magnetoelectric effect, antiferromagnetic Edelstein effect, and electric current-induced nematicity, are clarified. In particular, the current-induced nematicity is a characteristic response of an itinerant magnetic hexadecapole state and indicates a novel phase, the “magnetopiezoelectric metal”.

[Phys. Rev. B 96, 064432] Published Mon Aug 28, 2017

]]>Recently, a reevaluation of particle-hole symmetry in fermionic quantum Hall systems has led to a wealth of new insights about quantum Hall phases and connections between quantum Hall physics and other topological phases as well as high-energy physics. Similar progress may be possible in the bosonic case if such a symmetry exists, unlike the fermionic case such a symmetry is not exact. Here, the authors provide the first evidence for the existence of an emergent bosonic particle-hole symmetry.

[Phys. Rev. B 96, 075148] Published Wed Aug 23, 2017

]]>In one-dimensional electron systems, cooperative effects are expected to cause very specific electronic instabilities. Atomic nanowires, formed via self-organized growth on semiconductor surfaces, represent viable physical realizations for the study of such fascinating one-dimensional quantum states. The system in the focus of this work, Si(553)-Au, is created by Au adsorption on a stepped silicon substrate. It features two different chain types, interspersed with each other: Au chains on the terraces, and Si step edges that are subject to the formation of spin chains. By combining high-resolution scanning tunneling microscopy and local tunneling spectroscopy, the authors reveal the complex interplay between the two distinct wire architectures. The interaction is effectively “one-way” in that the Si step edges respond to the Au chains, but not vice versa. As a consequence, the symmetry of the system is lowered as the parity of the Si chains is broken. This fundamental effect creates two different configurations of chains with opposite directionality.

[Phys. Rev. B 96, 081406(R)] Published Tue Aug 22, 2017

]]>Rare events can have major effects on quantum matter. Extremely unlikely events cause certain physical properties to diverge to infinity near the quantum phase transition of the disordered Ising antiferromagnet in a transverse field, but destroy criticality of the clean system completely when a longitudinal component of the field is present. Using a tree tensor network renormalization group method combined with a novel matrix product operator representation, the authors detect signatures of rare events and determine the zero-temperature phase diagram of the disordered antiferromagnetic Ising chain in the presence of both longitudinal and transverse magnetic fields. The numerical technique used in this paper is generalizable to more complicated many-body systems and higher dimensions.

[Phys. Rev. B 96, 064427] Published Mon Aug 21, 2017

]]>Novel solid-state qubits have a lot to offer for quantum information processing because of the potential simplicity of engineering quantum computational hardware similar to modern silicon-based electronics. This work presents optical and electron-spin properties of a novel germanium-vacancy defect in diamond. The defect center combines high brightness and exceptional spectral stability with microwave and optical access to electron spin. The combination of a spin-½ qubit and an optical interface makes possible of the germanium-vacancy defect in scalable quantum networks.

[Phys. Rev. B 96, 081201(R)] Published Fri Aug 18, 2017

]]>Confinement – the process by which particles with fractional quantum numbers bind to form quasiparticles with integer quantum numbers – is as important to condensed matter as to particle physics. By a combination of inelastic neutron scattering experiments and theoretical calculations, the authors establish the compound SrCo${}_{2}$V${}_{2}$O${}_{8}$ as a beautiful paradigm for spinon confinement in a quasi-one-dimensional quantum magnet. In particular, using state-of-the-art theoretical techniques, the authors provide a quantitative and macroscopic explanation for the existence of free spinons above the ordering temperature. They also explain how those spinons get confined into pairs with transverse and longitudinal polarizations by interchain interactions as well as how these bound pairs evolve with temperature.

[Phys. Rev. B 96, 054423] Published Thu Aug 17, 2017

]]>In most semiconductors and insulators the presence of a small density of charged impurities cannot be avoided, but their effect can be reduced by introducing defects of opposite charge. Screening in such a system leads to the formation of electron-hole puddles, which dominate bulk transport, as first recognized by Efros and Shklovskii. This paper investigates the typical length scale, which determines the distance between puddles and the suppression of puddle formation close to metallic surface states. The authors find that this length scale is much smaller than previously anticipated, with profound consequences for the sample quality of thin films of topological insulators.

[Phys. Rev. B 96, 075204] Published Thu Aug 17, 2017

]]>Periodically driven system may provide a route for realizing novel phases of matter. Particularly, Floquet topological insulators could be induced by resonantly driving a transition in a spin-orbit coupled quantum well. In this work, disorder, dynamics, and topology are all considered on the same footing within the realm of such phases. First, it is found that Floquet topological phases are robust to disorder. Next, it is found that disorder actually induces a transition into a Floquet topological phase, when the pure driven system is trivial. The resulting driven, disordered, topological phase is a realization of the Floquet topological Anderson insulator.

[Phys. Rev. B 96, 054207] Published Wed Aug 16, 2017

]]>Using the recently conjectured fermion particle-vortex duality, the authors demonstrate that the theory of a single (2+1)-dimensional Dirac fermion coupled to an Abelian gauge field in a (3+1)-dimensional bulk exhibits a strong-weak duality. The duality is a manifestation of the electromagnetic S-duality in the bulk. At the self-dual point, they find several exact relations between transport coefficients, such as conductivity, Hall conductivity, and thermoelectric coefficients. Consequently, the duality provides new constraints on the physics of a strongly interacting system.

[Phys. Rev. B 96, 075127] Published Mon Aug 14, 2017

]]>The authors succeeded in growing high-quality crystals of binary FeAs, a spin density wave antiferromagnet with similarities to two families of unconventional superconductors. Quantum oscillations were measured in magnetic torque at high fields, making it possible to compare experimental electronic structure to prior density functional theory calculations. The two agree on the multicarrier nature of FeAs in the magnetic state, but diverge in some aspects of the Fermi-surface geometry. The theoretical results have so far been unable to describe the magnetic structure, and the presented experimental data now indicate that that could be due to an incorrect Fermi surface picture. Oscillations analysis also shows enhanced effective masses compared to theory, indicating that electron correlations may have previously been underappreciated in this material.

[Phys. Rev. B 96, 075120] Published Thu Aug 10, 2017

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