Incommensurate compounds consist of two or more interpenetrating sublattices with lattice periods incommensurate along one or more crystal axes. Intriguing lattice dynamics have been predicted for these compounds and are relevant to not only the thermal properties but also electronic, optical, and magnetic properties via coupling between different energy excitations. Here, the authors report inelastic neutron scattering measurements of phonon and magnon dispersions in Sr${}_{14}$Cu${}_{24}$O${}_{41}$, which contains incommensurate chain and ladder substructures. Two distinct acoustic phononlike modes, corresponding to the sliding motion of one sublattice against the other, are observed for atomic motions polarized along the incommensurate axis. In the long-wavelength limit, it is found that the sliding mode shows a remarkably small energy gap, indicating very weak interactions between the two incommensurate sublattices. The measurements also reveal an appreciable contribution from gapped magnons to the low-temperature specific heat, as well as a gapped and steep linear magnon dispersion of the ladders. The high group velocity of this magnon branch and weak coupling with acoustic phonons can explain the large magnon thermal conductivity in Sr${}_{14}$Cu${}_{24}$O${}_{41}$ crystals. These findings offer new insights into the phonon and magnon dynamics and thermal transport properties of incommensurate magnetic crystals that contain low-dimensional substructures.

[Phys. Rev. B 94, 134309] Published Fri Oct 21, 2016

]]>Lattice models are widely used to represent the statistical mechanics of condensed phases for the study of alloy thermodynamics, phase transitions, magnetic properties, fluid mechanics, and other applications. A longstanding problem has been the identification of the true global ground states of a lattice model, which is challenging for general models, and only a limited number of solutions for highly simplified systems are known. Here, a practical and general algorithm is presented that allows to determine provable periodically constrained ground states of complex lattice models up to a given unit cell size and in most cases is able to prove global optimality over all possible choices of unit cells. This universal algorithm is developed by reformulating the ground-state problem in terms of maximum satisfiability (MAX-SAT) and convex optimization, which results in both provable accuracy and sufficient numerical efficiency to handle systems of physically relevant complexity. Considering that lattice-model ground states are conventionally solved using simulated annealing or genetic algorithms, techniques that are often unable to find the true global minimum and provide no handle to prove the optimality of their results, this work promises to resolve the longstanding uncertainties in lattice models of important physical phenomena.

[Phys. Rev. B 94, 134424] Published Fri Oct 21, 2016

]]>Topological insulators have been attracting much attention as potential candidates for spintronic applications. Spin-momentum-locked surface states can lead to spin current naturally accompanied by electric charge current. As a way to control the current, circularly polarized photon injection has been widely used as the photon can excite spin-polarized carriers selectively depending on the photon’s helicity. Here, the authors investigate the helicity-dependent photocarrier dynamics in an unbiased Bi${}_{2}$Se${}_{3}$ thin film by monitoring the terahertz pulse emitted from the sample after illumination by femtosecond laser pulses. Although all the measurements were performed on an unbiased sample with no electrical contacts, the authors observe three-fold periodic helicity-dependent terahertz emission, and attribute such circular anisotropy in the photocurrent to the circular photon drag effect, namely linear and angular momentum transfer from photons to photocarriers.

[Phys. Rev. B 94, 161405(R)] Published Thu Oct 20, 2016

]]>Classification of symmetry-protected topological phases such as the topological superconductor (TSC) is well known in the free fermion picture. Recently, it was observed that including many-body interactions that preserves symmetry gives rise to novel surface phases and breaks down the classification scheme. A complete understanding of the microscopic theory for such phases is missing. This paper is an attempt to understand the interaction that gives rise to these phases in the case of the three-dimensional TSC. The authors build a coupled wire model Hamiltonian for the gapless surface and show that the addition of a time-reversal symmetric two-body interaction, constructed in this paper, opens an energy gap in the spectrum. They also show this gapped surface phase is topologically nontrivial and is classified differently compared to the noninteracting case.

[Phys. Rev. B 94, 165142] Published Wed Oct 19, 2016

]]>Weyl semimetals make up a novel three-dimensional topological phase of matter. Among their most fascinating properties is the existence of a very peculiar type of surface bound state. Contrary to common topological materials, the surface Fermi surface of a Weyl system consists of disconnected arcs – the so-called Fermi arcs – instead of a closed surface. These Fermi arcs connect the projected position of band crossings in their bulk band structure. The Weyl phase typically arises from a symmetry-protected topological insulator upon spontaneously breaking one of the protecting symmetries. In the case of the pyrochlores, the magnetic momenta inside the constituting Ir tetrahedra break time-reversal symmetry, so the pyrochlores are considered magnetic Weyl semimetals. Here, the authors show theoretically that the nontrivial spin polarization of the surface Fermi arcs of a magnetic Weyl semimetal provide characteristic feature observable via spin-resolved scanning tunneling microscopy (STM). They analyze the Fourier transform (FT) of Friedel oscillations in the local density of states occurring due to scattering by localized magnetic and nonmagnetic impurities. They note that the FT features inherit traces of the disconnected nature of the Fermi surface and the spin rotation happening on the Fermi arcs.

[Phys. Rev. B 94, 165146] Published Wed Oct 19, 2016

]]>Magnetic insulators are widely used as prototypes for the study of quantum criticality. Among the more exotic experimental realizations is the $z=1$ Heisenberg quantum critical point (QCP). It hosts asymptotic freedom of quasiparticles, unconventional scaling properties, and excitations analogous to the Higgs boson in high-energy physics. An interesting question is how this quantum phase transition is affected by disorder. Theory predicts a destruction of the quantum critical point and the emergence of a quantum Griffiths phase. In the present work, this issue is addressed experimentally by studying one of the precious few materials that host a pressure-induced Heisenberg QCP, namely the organometallic quantum magnet piperazine hexachlorodicuprate (PHCC). Disorder is introduced by chemical substitution of bromine for chlorine on a nonmagnetic site. Magnetic ordering at low temperatures is then studied as a function of applied hydrostatic pressure for a range of impurity concentrations. The tool of choice is the highly sensitive muon-spin rotation technique. The authors uncover a dramatic effect of disorder on the magnetic phase diagram. In agreement with theoretical expectations, they detect strong magnetic inhomogeneities already for small impurity concentrations, followed by the destruction of pressure-induced magnetic order at high concentrations of impurities.

[Phys. Rev. B 94, 144418] Published Fri Oct 14, 2016

]]>The authors present a new method for variationally optimizing projected entangled-pair states (PEPS) in the thermodynamic limit based on the global energy functional and novel tensor contraction strategies. It is successfully benchmarked using Ising and Heisenberg Hamiltonians. The method is completely independent from previous PEPS implementations and improves on the variational energies and order parameters as compared to other state-of-the-art PEPS simulations. The algorithm presented here should prove valuable in simulating ground-state properties of strongly correlated quantum spin systems, can be used with other variational ansatzes, and can be applied to tensor network states in general.

[Phys. Rev. B 94, 155123] Published Fri Oct 14, 2016

]]>The electronic structure of crystalline materials reflects the influence of underlying discrete symmetries of the atomic lattice. When these symmetries are broken, associated degeneracies in the spectrum of states often split, with important consequences for potential electronic and spintronic applications. The mathematical language of group theory can be used to capture the effect of these symmetries and provide insight into the origins of spectrum features obtained from essentially empirical ab initio numerical calculations such as density functional theory. A salient example comes from the unusual electronic structure of atomically thin two-dimensional “four-six-ene” semiconductors such as tin sulfide, germanium telluride, etc. (related to group-V phosphorene but formed from group IV and VI). In this case, group theoretic methods provide a straightforward framework for understanding the consequences of inversion symmetry breaking due to inequivalent sublattice atomic identity. In particular, the quantum states at the edge of the fundamental band gap – relevant to all transport, optoelectronic, and spintronic properties – are shown to directly inherit their character from nearby points of high symmetry in the reciprocal lattice, where the form of allowable energetic interactions is constrained.

[Phys. Rev. B 94, 155124] Published Fri Oct 14, 2016

]]>Double quantum dots play an important role in the field of semiconductor and carbon nanotube quantum dots. They have resulted in, for example, correlated electron physics beyond the single spin-1/2 Kondo system and the realization of qubits for quantum computation. Using organic molecules to create a double quantum dot can be beneficial due to the larger energy scales present as compared to its more extended counterparts. Furthermore, they open up the possibility to investigate interactions between spins and perform coherent experiments on electrons in organic molecules. In this article, the authors experimentally investigate charge transport through a three-terminal single-molecule junction. The data shows features in transport that arise from the broken conjugation in dihydro-anthracene, which localizes π electrons and creates a set of nearly degenerate orbitals. The ferromagnetic and antiferromagnetic interactions are of the same order of magnitude, so that the singlet and triplet states are close in energy. These findings thus demonstrate a double quantum dot programmed in a single molecule due to its broken conjugation. This research is a starting point for further experiments, whereby with chemical means the interaction between the two dots can be lowered to observe Pauli spin-blockade and investigate spin coherent effects in single molecules.

[Phys. Rev. B 94, 165414] Published Fri Oct 14, 2016

]]>In the helical edge states of a quantum spin Hall (QSH) bar, the phenomenon of electron photoexcitation is quite different from systems commonly used in optoelectronics. The vertical electric dipole transitions typically occurring between valence and conduction bands of conventional semiconductor-based devices are forbidden in QSH edge states due to a selection rule. Also, electrons in QSH edge states are described by a massless Dirac fermion theory rather than the conventional Schrödinger-like parabolic band, so their response to an electromagnetic field is intrinsically affected by a peculiar effect known as the chiral anomaly. Here, the authors analyze this problem in the mesoscopic regime. They show that, when a spatially localized electric pulse is applied at the edge of a QSH system, helical electron wave packets can be photoexcited by purely intrabranch electrical transitions, without invoking the magnetic Zeeman coupling or transitions to bulk states. At the end of the pulse, the wave packets counter propagate with opposite spin orientations, keeping their space profile unaltered. Notably, the authors find that the space profile of the photoexcited electron wave packets is independent of the temperature and chemical potential of the initial equilibrium state. Instead, the profile depends purely on the applied electric pulse, in a linear manner, as a signature of the chiral anomaly. Then authors discuss how the wave packet profile can be tailored by the electric pulse parameters, for both low and finite frequencies.

[Phys. Rev. B 94, 165412] Published Thu Oct 13, 2016

]]>The well known spin-orbit coupling is a peculiar type of interaction that links the direction of electron motion to its intrinsic magnetic moment, called spin. The role of the spin orbit is believed to be particularly important in giving rise to exotic states of matter, such as topological insulators and topological superconductors, that could host, for example, Majorana fermions. Despite several methods of measuring the spin-orbit constant having already been developed, given the rapid progress in the discovery and possible applications of new materials with strong spin-orbit coupling, it is important to provide experimentalists with novel and general approaches to unequivocally measure the value of this coupling. Here, the authors show theoretically that the spin-orbit coupling in two-dimensional and one-dimensional systems, both superconducting and metallic, can be read off directly and unambiguously via spin-resolved scanning tunneling microscopy (STM). The authors analyze the Friedel oscillations in the local density of state occurring due to scattering by localized magnetic impurities. In particular, they focus on the Fourier transform (FT) of the Friedel oscillations and note that the FT features observed in non-spin-polarized measurements split when using spin-polarized STM, and the splitting is exactly equal to twice the inverse spin-orbit length. Moreover, the superconducting systems exhibit extra high-intensity FT features with respect to the metallic systems, at a wavevector given by twice the inverse spin-orbit length. Such features provide a direct measure of the spin-orbit coupling in these materials.

[Phys. Rev. B 94, 134511] Published Wed Oct 12, 2016

]]>Type-II multiferroics –- materials where magnetic order induces ferroelectricty -– offer great potential for novel electronic devices due to the strong intrinsic coupling between order parameters. Such coupling is found in CuFeO${}_{2}$, where ferroelectricity is driven by proper-screw “helical” magnetic ordering for applied magnetic fields between 7.5 and 13 T at low temperatures. Here, the authors report a polarization memory effect in CuFeO${}_{2}$, whereby the nonpolar antiferromagnetic ground state retains a strong memory of the high-magnetic-field ferroelectric phase. The dependence of the polarization on the magnetothermal history is investigated, and it is found that the direction and almost the full polarization magnitude is “remembered” after cycling through the zero-field nonpolar phase. This polarization memory persists in samples in which multiple crystallographic domains are suppressed by applied stress, ruling out crystallographic boundaries as the cause of the observed memory. Instead, the authors propose that the memory is carried by Bloch-type antiferromagnetic domain walls with definite helicity. As similar memory effects have been also been reported for other multiferroics, such as MnWO${}_{4}$ and CuO, the proposed mechanism could be a general feature of phase transitions between collinear-antiferromagnetic and incommensurate magnetic phases.

[Phys. Rev. B 94, 144411] Published Wed Oct 12, 2016

]]>Recent developments in density matrix renormalization group/matrix product state techniques allow today for a quantitative computation of dynamical correlations at finite temperature in low-dimensional quantum spin systems. By combining numerical results and field theoretical asymptotic methods, e.g., bosonization, it is now possible to give a full description of the dynamical quantity one is interested in for the range of temperatures experimentally accessible. This enhances of course the possibility of one-to-one comparisons with experiments without having to resort to uncontrolled approximations. Here, the authors focus on the temperature dependence of the NMR spin-lattice relaxation rate $1/{T}_{1}$ in spin-$\frac{1}{2}$ chain systems. This quantity is directly related to local, dynamical spin-spin correlation functions. The results agree in the low-temperature regime with bosonization predictions, and show interesting deviations from this behavior at intermediate temperatures.

[Phys. Rev. B 94, 144408] Published Tue Oct 11, 2016

]]>Quasi-one-dimensional (1D) antiferromagnets are good candidates to realize a Tomonaga-Luttinger liquid (TLL), describing 1D interacting quantum systems, provided 3D coupling remains small as compared to temperature. In systems such as NiCl${}_{2}$-4SC(NH${}_{2}$)${}_{2}$ (DTN), made of weakly coupled spin $S=1$ chains, interchain coupling plays an important role at low temperature as it leads to antiferromagnetic long-range order below $1$ K, corresponding to a Bose-Einstein condensate of magnons. Using time-dependent numerical simulations at finite temperature, the authors compute here the nuclear magnetic resonance (NMR) relaxation rate $1/{T}_{1}$, a key experimental quantity, and address the question of the low-temperature crossover between TLL predictions and the higher-temperature regime in purely $1$D quantum spins chains. They also comment on the experimental observation of the TLL regime, possibly masked by $3$D long-range order in materials such as DTN.

[Phys. Rev. B 94, 144409] Published Tue Oct 11, 2016

]]>Layered molecular-intercalated iron selenides (FeSe) show great promise as a class of high-${T}_{\mathrm{c}}$ unconventional superconductors, with a ${T}_{\mathrm{c}}$ as high as 45 K reported so far. The FeSe layers in these materials also show structural and electronic similarities to monolayer FeSe on SrTiO${}_{3}$, where ${T}_{\mathrm{c}}$ values up to 65 K have been observed. Information on the superconducting properties of these materials may provide vital insight into the unconventional mechanism of high-temperature superconductivity in iron-based compounds. One property of particular interest is the symmetry of the superconducting gap function on the Fermi surface, which is a key prediction for any theory of superconductivity. However, a precise experimental determination of this has proven elusive in the iron-based superconductors. In this paper, the authors study the LiOD-intercalated FeSe derivative Li${}_{1-x}$Fe${}_{x}$ODFe${}_{1-y}$Se via inelastic neutron scattering, detecting a spin resonance which appears in the superconducting state below the pair breaking energy 2$\mathrm{\Delta}$. This shows that superconductivity in this compound is unconventional and, in conjunction with previous ARPES and STS results, strongly constrains the possible superconducting gap symmetries in this material.

[Phys. Rev. B 94, 144503] Published Mon Oct 10, 2016

]]>Since the discovery of graphene, there has been great interest in finding other nodal semimetals in two and three dimensions. Over the past few years, numerous examples of these materials have been proposed or observed. However, the explanations for the symmetry and topology mechanisms protecting nodal features in these materials have generally been provided on a case-by-case basis. In this paper, we use a mathematical consideration of crystal lattices on flat manifolds like Mquotobius strips and Klein bottles, most recently revived by Watanabe, Po, Vishwanath, and Zaletel, to develop a framework for discussing all possible two-dimensional and quasi-two-dimensional semimetals in systems with strong spin-orbit interactions. Using this framework, we draw connections to known nodal features in two- and three-dimensional semimetals like SrIrO${}_{3}$, and we predict new ones, including an exotic “Cat’s Cradle” Weyl fermion feature.

[Phys. Rev. B 94, 155108] Published Thu Oct 06, 2016

]]>The magnetic ratchet effect is optical rectification in two-dimensional systems – such as bilayer graphene – whereby a steady in-plane magnetic field and the alternating electric field of a laser produce a dc electric current. It occurs in systems with broken inversion symmetry due, say, to an inhomogeneous distribution of impurities, leading to increased electronic scattering on the upper layer of the bilayer. For a given direction of electric field, electrons are driven downwards by the Lorentz force towards the lower layer where scattering is low whereas, when the electric field switches direction, electrons are driven towards the upper layer where scattering is high. Such asymmetry in scattering leads to a nonzero dc current. By deriving linear-in-magnetic-field terms in the low-energy electronic Hamiltonian, the authors predict a very large ratchet effect in bilayer graphene. They compare symmetry breaking due to impurities with interlayer asymmetry due to an external gate, the latter allowing for a tuneable magnetic ratchet.

[Phys. Rev. B 94, 165404] Published Thu Oct 06, 2016

]]>Magnetic resonance spectroscopy is one of the most important tools used across the physical and life sciences, but typically requires macroscopic samples. A key challenge is therefore to develop methods suitable for the nanometric regime. This paper investigates a new approach whereby quantum relaxation of a single nitrogen-vacancy center spin in diamond is monitored to construct a spectrum of the surrounding electron and nuclear spin environment within a nanometric volume. In contrast to existing decoherence-based techniques, the technique is all optical, requiring no complex quantum control, and intrinsically broadband, which means it can probe both electron and nuclear spin resonances. As an illustration, the authors report broadband nanoscale magnetic resonance spectroscopy of nitrogen impurities located within the diamond. This work provides a new route towards magnetic resonance spectroscopy of electrons and nuclear spins at the single molecule level.

[Phys. Rev. B 94, 155402] Published Wed Oct 05, 2016

]]>The $G\phantom{\rule{0}{0ex}}W$ method is the standard method for predicting quasiparticle energies as measured in experimental photoemission spectroscopy. Until recently, routine $G\phantom{\rule{0}{0ex}}W$ calculations have been restricted to fairly small systems and few $k$ points for sampling the Brillouin zone, since the computational demand usually scales quartic with the number of atoms and quadratic with the number of $k$ points. The original work of Lars Hedin suggests that one can do better by calculating the self-energy as $\mathrm{\Sigma}$(1,2)=$i\phantom{\rule{0}{0ex}}G$(1,2)$W$(1,2) – the equation that coined the phrase ”$G\phantom{\rule{0}{0ex}}W$”. Here, $G$ is the one-particle Green’s function and $W$ the screened interaction between the electrons. Using this relation directly in computations brings the scaling down to linear in the number of $k$ points and cubic in the number of atoms. Although this approach has been used in the past by others, in this work its full potential is unveiled by implementing it in a massively parallel code and adopting it to the projector augmented wave method. This makes $G\phantom{\rule{0}{0ex}}W$ calculations as convenient as calculations based on local density functionals and allows for calculations for hundred atoms in few hours.

[Phys. Rev. B 94, 165109] Published Wed Oct 05, 2016

]]>Good progress has been made in electronic structure calculations for real materials as well as for model Hamiltonians. However with an eye on $a\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ calculations, it is still very challenging to accurately calculate, e.g., the dynamical response of small molecules or atom clusters. At present the method of choice is the GW-BSE method. Its notorious drawback is that solving a computationally demanding self-consistency equation is required. Thus motivated, the author explore an alternative idea replacing the self-consistency problem by the functional renormalization group (FRG). The traditional FRG was formulated for systems with translational symmetry. We investigate an alternative ($\epsilon $FRG) that is also applicable to inhomogeneous matter, such as disordered metals or molecules. It accounts for Fermi-liquid corrections to quasiparticle energies and particle-hole excitations. In principle it can go beyond the state of the art approximation, GW-BSE, by solving the Bethe-Salpeter equation (BSE) self-consistently We present an efficient implementation of $\epsilon $FRG together with performance tests on the spinless disordered Hubbard model.

[Phys. Rev. B 94, 155102] Published Mon Oct 03, 2016

]]>Among others, pyrochlore iridates with strong spin-orbit coupling have received attention recently due to the prediction of Weyl semimetal states in these materials. Despite tremendous efforts, there exists no direct measurement of the spin structure at the iridium site by means of neutron diffraction. This might be caused by the small ordered Ir moment and also by the neutron absorbing properties of the Ir ions. Up to now, only basic indirect evidence for the spin structure at the iridium site exists. From the observation here of the full magnetic structure of Nd${}_{2}$Ir${}_{2}$O${}_{7}$ via neutron diffraction experiments, the authors obtain direct insight into unknown aspects of the physics of the iridium ions in pyrochlore iridates.

[Phys. Rev. B 94, 161102(R)] Published Mon Oct 03, 2016

]]>Wannier functions provide a useful description of the electronic states in a crystal, providing a chemical picture for the bonding structure of a material, in addition to applications in computational materials science. Topological insulators are an interesting class of materials with spin-polarized conducting surface states alongside an insulating bulk. To date, constructing Wannier functions for topological insulators has proven difficult, relying on an educated guess for the trial orbitals or requiring the system to be parameterized in order to tune between the normal and topological phase. The authors have developed an automated approach within the optimized projection functions method for constructing localized Wannier functions for topologically nontrivial bands. They demonstrate the method on a two-dimensional tight-binding model, a strong three-dimensional topological insulator, and finally, the prototypical topological insulating material Bi${}_{2}$Se${}_{3}$. They find that the Wannier functions contain large imaginary components and are more extended than those in the topologically trivial phase.

[Phys. Rev. B 94, 125151] Published Fri Sep 30, 2016

]]>Superfluidity in two dimensions relies on the stability of a vortex imposed by external rotation of the fluid. It is lost if the flow around the test vortex is screened by spontaneously generated free vortices. In thermal equilibrium, this occurs through a Kosterlitz-Thouless phase transition, where vortex-antivortex pairs bound by Coulomb-like forces unbind only above the critical temperature. Recent experiments with exciton-polariton fluids and other driven dissipative systems raise the question of how this physics changes away from thermal equilibrium. In this paper, the authors generalize the electrostatic duality, which represents vortices as Coulomb charges, to a full electrodynamic description of the nonequilibrium system. The unbinding of vortices is analyzed within this theory using a renormalization group framework. In contrast to the equilibrium case, it is found that vortices always unbind beyond a large emergent length scale due to nonlinearities in the field dynamics. Hence, there is no superfluidity in a truly infinite driven system, while a finite system may appear as a superfluid. The heuristic derivation of the dual electrodynamics presented in this paper is supplemented by a systematic one starting from a microscopic lattice theory in a companion paper.

[Phys. Rev. B 94, 104520] Published Tue Sep 27, 2016

]]>Duality transformations have a long history in physics, and recently saw a surge of renewed interest in the context of strongly correlated fermion systems. A prime example for such a transformation was established to describe the phase transition in two-dimensional superfluids in thermal equilibrium. Here, Kosterlitz and Thouless developed a dual representation of the superfluid, which maps the vortices in the latter to charges in a Coulomb gas. In this framework, the dissociation of vortex-antivortex pairs at the critical temperature corresponds to the formation of a plasma of free charges. How can such a framework be leveraged over to nonequilibrium situations, relevant to the understanding of driven open fluids of light such as exciton-polariton systems? In this work, the authors make a crucial step in this direction by deriving a transformation that maps the stochastic equation of motion for the phase of a driven open condensate — the compact Kardar-Parisi-Zhang (cKPZ) equation — to a dual electrodynamic theory. This results in modified, and in particular, nonlinear Maxwell equations for the electromagnetic fields, and a diffusion equation for the charges representing vortices in the cKPZ equation. In a companion paper, the authors apply this theoretical framework to the study of nonequilibrium vortex unbinding.

[Phys. Rev. B 94, 104521] Published Tue Sep 27, 2016

]]>The nematic state, where a system is translationally invariant but breaks rotational symmetry, has drawn great attention recently due to the experimental observations of such a state in high-${T}_{c}$ superconductors. For iron-based superconductors, the nematic state intertwines strongly with the antiferromagnetic order. Hence, clear experimental delineation of the nematic state has always been challenging. Here, the authors study the electronic structure of a multilayer FeSe film using angle-resolved photoemission spectroscopy. The band reconstruction in the nematic state is clearly delineated. They find that the energy splitting between ${d}_{x\phantom{\rule{0}{0ex}}z}$ and ${d}_{y\phantom{\rule{0}{0ex}}z}$ bands shows a nonmonotonic distribution in momentum space. The energy splitting was also observed on the ${d}_{x\phantom{\rule{0}{0ex}}y}$ bands with an energy scale around 45 meV. The momentum dependence of the ${d}_{x\phantom{\rule{0}{0ex}}z}$ and ${d}_{y\phantom{\rule{0}{0ex}}z}$ energy splitting and the reconstruction of ${d}_{x\phantom{\rule{0}{0ex}}y}$ exclude the simple on-site ferro-orbital ordering as a driving force of nematicity. Instead, strong anisotropy exists in the hopping of all ${d}_{x\phantom{\rule{0}{0ex}}z}$, ${d}_{y\phantom{\rule{0}{0ex}}z}$, and ${d}_{x\phantom{\rule{0}{0ex}}y}$ orbitals, the origin of which holds the key to a microscopic understanding of the nematicity in iron-based superconductors.

[Phys. Rev. B 94, 115153] Published Mon Sep 26, 2016

]]>Materials with strong spin-orbit coupling often host a unique electronic structure, both in the bulk and on the surface. In particular, in systems that break inversion symmetry, spin-orbit coupling facilitates the Rashba-Dresselhaus effect, leading to a lifting of spin degeneracy in the bulk and intricate spin textures of the Bloch wave functions. Such materials could see potential applications in spintronics and quantum information technology, but these applications would benefit from a broader suite of materials, with a greater range of properties, and in particularly from very strong spin-orbit splitting. This paper’s angular-resolved photoemission and low-temperature scanning tunneling microscopy experiments, together with relativistic first-principles band structure calculations, identify several Dirac surface states in BiPd, one of which exhibits an extremely large spin splitting. Unlike the surface states in inversion-symmetric systems, the Dirac surface states of BiPd have completely different properties at opposite faces of the crystal and are not trivially linked by symmetry. The spin splitting of the surface states exhibits a strong anisotropy by itself, which can be linked to the low in-plane symmetry of the surface termination. Both may be useful to implement new functionalities for spintronic applications.

[Phys. Rev. B 94, 121407(R)] Published Thu Sep 22, 2016

]]>Although NaFe(WO${}_{4}$)${}_{2}$ is not multiferroic itself, the magnetic order in this $S$=5/2 chain material is a reference for many multiferroic materials, such as MnWO${}_{4}$ and $R$MnO${}_{3}$, because NaFe(WO${}_{4}$)${}_{2}$ exhibits a transition from an incommensurate spiral order to a commensurate up-up-down-down structure. Here, the authors perform a very extensive study of the magnetic properties in the this double tungstate compound at low temperatures and high magnetic fields. A complex $B$-$T$ phase diagram is unveiled. Strong magnetoelastic anomalies with a relative contraction of the $b$ lattice parameter of up to 2.6 $\times $ 10${}^{-4}$ are associated with the up-up-down-down ordering, which appears in the commensurate and also in the anharmonic incommensurate structure. In contrast, the transition to the magnetically ordered phase, with a purely harmonic incommensurate spiral modulation, does not cause resolvable anomalies in the thermal expansion. Thus, the strong magnetoelastic anomaly coupling can be attributed to the lifting of magnetic frustration in the up-up-down-down structure, where neighboring moments align either parallel or antiparallel, although all these bonds are equivalent in the paramagnetic state.

[Phys. Rev. B 94, 104423] Published Wed Sep 21, 2016

]]>Superconducting qubits are among the most promising elements for the implementation of the concept of quantum computing. Quasiparticles are an intrinsic sources of qubit decoherence, and are more generally detrimental to the operation of superconducting devices, e.g., Cooper pair pumps. Experiments reveal that quasiparticles fail to equilibrate and their density remains high even at low temperatures. Planting normal-metal traps on a superconducting device offers a way to reduce the quasiparticle density: once a quasiparticle tunnels into the normal metal and relaxes to subgap energy via inelastic processes, it cannot return to the superconductor. This paper presents a theoretical model for the time-resolved dynamics of quasiparticles injected into a qubit, and experiments with transmon qubits validating the model. The authors show that, contrary to expectations, the effective trapping rate depends on temperature, which is a consequence of the strong energy dependence of the quasiparticle density of states in the superconductor. At low temperatures, the relaxation process in the normal metal is the bottleneck limiting the effectiveness of traps. The authors also show that the trapping rate saturates for larger traps. At saturation, the rate is limited by the inverse of the time it takes for quasiparticles to diffuse across the device.

[Phys. Rev. B 94, 104516] Published Tue Sep 20, 2016

]]>Spin transport at interfaces between nonmagnets and ferromagnets plays an important role in spintronic devices. Lately, there is an increasing suspicion that spin-orbit coupling, which couples the spin and momentum of carriers, might contribute significantly to this process. Unfortunately, the existing description of spin transport at such interfaces, magnetoelectronic circuit theory, is not valid when spin-orbit coupling is present at the interface. This paper presents a generalization of magnetoelectronic circuit theory to interfaces with spin-orbit coupling. Like the original theory, this generalization describes spin transport in terms of drops in spin and charge accumulations across the interface, but also includes responses to in-plane electric fields and offsets in spin accumulations. The most important result is a description of the way in-plane electric fields generate spin accumulations, spin currents, and torques at the interface. The effects described by this generalized circuit theory impact the interpretation of experiments involving spin-orbit torques, spin pumping, spin memory loss, the Rashba-Edelstein effect, and spin Hall magnetoresistance.

[Phys. Rev. B 94, 104419] Published Fri Sep 16, 2016

]]>Most spintronic devices share two features: they utilize spin-orbit coupling and they contain interfaces. While bulk spin-orbit effects are thought to be well described by phenomenological theories, interfacial spin-orbit effects are not. A theory that could describe interfacial spin-orbit effects would be useful in analyzing experiments on heavy-metal ferromagnet bilayers, which are a key feature of potential energy-efficient implementations of MRAM. To develop such a theory, the authors present the boundary conditions needed for drift-diffusion models to treat interfaces with spin-orbit coupling. Together with the drift-diffusion equations, these boundary conditions give an analytical model of spin-orbit torques caused by both the spin Hall and Rashba-Edelstein effects. A key feature of these boundary conditions is that they capture spin currents generated by interfacial spin-orbit scattering. The authors validate this phenomenological approach by comparing the results with those obtained by solving the spin-dependent Boltzmann equation. They discuss the interpretation of current experiments, and describe in particular how interfacial effects give rise to torques on a nearby ferromagnetic layer even through a nonmagnetic spacer layer.

[Phys. Rev. B 94, 104420] Published Fri Sep 16, 2016

]]>Landau theory of Fermi liquids is the foundational concept behind our understanding of conventional metals. Its basic postulate is that the low-energy physics can be described by weakly interacting quasiparticles — a dressed but effectively free version of electrons formally represented by poles of the single-particle Green’s function. However, this concept notoriously fails for strongly interacting systems such as high-${T}_{c}$ cuprate superconductors, and our understanding of properties of these “non-Fermi” quantum liquids is far from complete. One well-known candidate theory for non-Fermi liquids is Hertz-Millis theory: a finite-density fermion system coupled to a massless critical boson. In (2+1) dimensions, this theory is strongly interacting at low energies, but as of yet there is no controlled insight in this regime; conventional perturbation theory fails. In this article, the authors show that in the so-called quenched approximation the fermionic spectrum can be computed exactly to all orders in the coupling constant. They show extensive detailed analysis that, in this quenched limit, the nonperturbative low-energy dynamics are indeed that of a non-Fermi liquid. Instead of coherent quasiparticles one finds the emergence of critical fermionic excitations described by branch cuts in the single particle Green’s function.

[Phys. Rev. B 94, 115134] Published Wed Sep 14, 2016

]]>The authors present here several advances towards a comprehensive understanding of WTe${}_{2}$. Using microfocus laser ARPES, they resolve for the first time the distinct electronic structure of both inequivalent top and bottom (001) surfaces of WTe${}_{2}$. Moreover, they demonstrate for the first time the presence of large surface state Fermi arcs on both surfaces. Using surface electronic structure calculations, they further demonstrate that these Fermi arcs are topologically trivial and that their existence is independent of the presence of type-II Weyl points in the bulk band structure. Contrary to common belief, the observation of surface state Fermi arcs is thus not suitable to robustly identify a type-II Weyl semimetal. Finally, the authors use the observation of Fermi arcs and distinct top and bottom surfaces to clarify the controversial bulk electronic structure of WTe${}_{2}$. They show that the bulk Fermi surface is formed by three-dimensional electron and hole pockets with areas that are found to be in good agreement with transport experiments, with the exception of small hole pockets that have not been observed in quantum oscillation experiments.

[Phys. Rev. B 94, 121112(R)] Published Wed Sep 14, 2016

]]>In recently predicted type-II Weyl semimetals, the Weyl states connect hole and electron bands that would otherwise be separated by an indirect gap. The set of points in the momentum space at which both bands touch are called Weyl points and are connected by Fermi arcs at the surface of the sample. Experimental evidence confirmed existence of these exotic fermions in MoTe${}_{2}$ and Mo${}_{x}$W${}_{1-x}$Te${}_{2}$. Here, the authors show that such states also exist in pure WTe${}_{2}$ and are very sensitive to strain. While flat undistorted portions of the sample show well defined Fermi arcs – a clear signature of Weyl fermions – strain due to bulking of the surface in other locations suppresses this state and destroys the Weyl state. Such high sensitivity to strain provides a simple way to tune properties of Weyl Fermions and explore the physics of these unusual objects.

[Phys. Rev. B 94, 121113(R)] Published Wed Sep 14, 2016

]]>Slicing the 3D Brillouin zone into 2D as a dimensional reduction makes the topological structure of the 3D Weyl point clear as a topological critical point. The Chern number of the sliced 2D system (section Chern number) changes discontinuously at the gap-closing momentum that guarantees topological stability of the Weyl point. This section Chern number is just a mathematical tool and cannot be directly measured experimentally. However, propagating edge modes of the 3D system with boundaries are necessarily momentum selective, associated with the section Chern number. This is a direct consequence of the bulk-edge correspondence. Using a localized basis with/without boundaries, the authors have demonstrated this here in a 3D helical photonic crystal without inversion symmetry

[Phys. Rev. B 94, 125125] Published Wed Sep 14, 2016

]]>Ferroelectrics that allow a coupling between the polarization and another order parameter are of great interest because they could make the electric field control of nonpolar order parameters possible. In recent years, “hybrid improper’” ferroelectrics - materials where the polarization couples to two different structural distortions - have emerged as a possible way to realize this goal. Theoretical predictions of hybrid improper ferroelectricity in layered perovskite materials were followed by its first experimental realization in Ca${}_{3}$Ti${}_{2}$O${}_{7}$ in 2015. However, the precise pathway by which the polarization reverses in this material during ferroelectric switching remains an open question. The authors address this question and lay the groundwork for understanding the unexpectedly complex domain structure of Ca${}_{3}$Ti${}_{2}$O${}_{7}$, consisting of a network of multiple types of domain walls and topological defects.

[Phys. Rev. B 94, 104105] Published Thu Sep 08, 2016

]]>Entanglement entropy is a powerful tool for characterizing spin liquids and other systems with long-range entanglement. For gapped spin liquids, this problem has been well-studied, finding a universal subleading constant term in the entanglement entropy, known as the topological entanglement entropy. In this paper, the idea of topological entanglement entropy is generalized to the gapless $U$(1) spin liquid. First, it is shown that the entanglement entropy separates into two pieces, one due to emergent charges of the theory (assumed to be gapped) and one due to the emergent gapless gauge mode. Both give universal subheading contributions to the entanglement entropy, which are logarithmic in the size of the region considered. The photon contribution is due to the local physics of the gapless mode. The particle contribution, on the other hand, is a topological term, generalizing the notion of topological entanglement entropy. A construction is presented which can isolate this topological piece by eliminating the photon contribution and other local terms, such as the area law. This topological term in the entanglement entropy can serve as a powerful diagnostic tool for $U$(1) spin liquids.

[Phys. Rev. B 94, 125112] Published Thu Sep 08, 2016

]]>In a strong magnetic field, two-dimensional electron systems are dominated by Coulomb interactions between the electrons. While the most familiar manifestation of these interactions is the fractional quantum Hall effect, this is actually a relatively subtle variation in the strong electronic correlations that exist throughout the high-field regime. This is clearly revealed in tunneling experiments in which an uncorrelated electron is rapidly thrust into a strongly correlated electron gas. A robust Coulomb pseudogap is observed, with tunneling strongly suppressed until the energy of the incoming electron is large enough. The Coulomb gap is insensitive to the precise magnetic field and is typically much larger than the energy gap associated with the fractional quantum Hall effect. In the present paper, the authors demonstrate that the size of the Coulomb gap observed in tunneling between two parallel two-dimensional electron systems depends sensitively on their spin configuration, with larger gaps associated with a larger net spin polarization. In effect, it is easier for electrons ‘to get out of the way’ of an incoming tunneling electron when the spin polarization of the electron gas is less than complete.

[Phys. Rev. B 94, 125409] Published Thu Sep 08, 2016

]]>There is much interest from the physics community in materials with a gapped nonmagnetic ground state, in which quantum critical points can be reached by application of a magnetic field. Here, the authors report magnetization data as a function of temperature, field, and hydrostatic pressure for the gapped quantum magnet CsFeCl${}_{3}$, a quite intriguing system in which the large easy-plane single-ion anisotropy competes with the exchange interactions. They establish the phase boundary, and identify a quantum phase transition at a critical value of the pressure. It is likely that experiments by other techniques will follow this discovery of both magnetic-field and pressure-controlled transitions in this interesting compound.

[Phys. Rev. B 94, 104409] Published Wed Sep 07, 2016

]]>Spintronics holds the promise of novel functionality and higher device performance, but a massive challenge is to find semiconducting materials with good electronic properties that are also ferromagnetic. Dilute magnetic semiconductors (DMS) such as Mn-doped GaAs have great potential but are notoriously difficult to make, only stabilizing in thin films in most cases. Recently, a promising new class of DMS materials based on the chemistry and structure of iron-based superconductors has been discovered, including the compound (Ba,K)(Zn,Mn)${}_{2}$As${}_{2}$. These materials can be synthesized in bulk form, providing an unprecedented opportunity to study the mechanism of ferromagnetism in semiconductors. In this work, the authors present detailed temperature-dependent characterization of the atomic and magnetic structure of (Ba,K)(Zn,Mn)${}_{2}$As${}_{2}$ using x-ray and neutron pair distribution function techniques, establishing the existence of an unexpected structural distortion on a short length scale and surprisingly robust short-range ferromagnetic correlations that persist even at room temperature. These results fill in important gaps in the experimental understanding of DMS materials and provide valuable guidance to their theoretical description.

[Phys. Rev. B 94, 094102] Published Tue Sep 06, 2016

]]>Scaling, universality, and renormalization are three pillars of modern critical phenomena. According to the hypothesis of universality, continuous phase transitions fall into classes mainly determined by spatial dimensionality and symmetry of the order parameter. The latter is usually reflected by the degeneracy of the ground state of the Hamiltonian. However, for certain systems at criticality, a higher symmetry emerges in the order parameter, and the associated critical behavior may become very rich. Such emergent symmetry has been found in spin ice systems, deconfined quantum critical points, high-${T}_{c}$ superconductors, and so forth, and are often accompanied by very interesting critical phenomena. New results presented here are based on Monte Carlo simulations and finite-size scaling of a series of $q$-state Potts models on the simple cubic lattice with ferromagnetic interactions in one lattice direction and antiferromagnetic interactions in the other two directions. The staggered magnetization appears to display an emergent continuous O($n$) symmetry with $n$=$q$-1, as illustrated by two-dimensional intersections of the distribution functions. Also the estimated critical exponents are consistent with the O($n$=$q$-1) universality classes.

[Phys. Rev. B 94, 104402] Published Thu Sep 01, 2016

]]>It has been corroborated in the past that Pauli blockade mechanisms of weakly spin-coupled charge carrier pairs undergoing transitions into doubly occupied singlet states are among the most dominant spin-selection rules influencing room-temperature magnetoresistance and luminescence of organic semiconductors. It has remained unclear though whether these pairs consist of equal or opposite charges, and thus, whether the spin-dependent transitions are recombination or charge transport processes. Here, the authors report on simultaneous transient measurements of electric current and optical emission changes in organic light emitting diodes due to pulsed magnetic resonance induced charge carrier spin manipulation. When a room-temperature steady-state current is changed magnetic resonantly, the electroluminescence changes show identical dynamic and magnetic resonant signatures, indicating that both observables are controlled by the same spin-dependent electronic process. However, a quantitative analysis of the optical emission change reveals that its magnitude significantly exceeds what is expected from the magnitude of the electrical current change, implying that the optical emission change occurs not only because of the device current change but also due to a change of the radiative recombination rate, thereby showing that the observed spin-dependent process is due to recombination, not transport.

[Phys. Rev. B 94, 075209] Published Wed Aug 31, 2016

]]>The discovery of new spin-to-charge current conversion effects such as the spin Hall effect (SHE) is expanding the potential of spintronic applications. In contrast to the anomalous Hall effect (AHE), a systematic experimental study of the different mechanisms contributing to the SHE is lacking. Finding routes to maximize the SHE is not possible as long as it remains unclear which is the dominant mechanism in a material. The authors systematically study Pt, the prototypical SHE material, using the spin absorption method and find a single intrinsic spin Hall conductivity in a wide range of Pt conductivities, in good agreement with theory. By tuning the conductivity, they observe for the first time the crossover between the moderately dirty and the superclean scaling regimes of the SHE, equivalent to that obtained for the AHE. These results explain the dispersion of values in the literature and show a clear path to enhance this important effect.

[Phys. Rev. B 94, 060412(R)] Published Tue Aug 30, 2016

]]>Driven-dissipative quantum many-body systems, i.e., many-particle systems where quantum coherent dynamics and dissipative effects occur on the same footing, can constitute a promising platform to initiate a systematic classification of quantum dynamical criticality. In this work, the authors show that both nonequilibrium and quantum features can be simultaneously present and persist in the low-frequency properties of a critical one-dimensional driven open Bose gas, and no effective equilibrium description is applicable for this novel type of nonequilibrium quantum criticality.

[Phys. Rev. B 94, 085150] Published Tue Aug 30, 2016

]]>Nanowire/superconductor hybrid devices are important systems that are currently used in many experimental programs. In these systems, deliberate or accidental quantum dots in the nanowire create Yu-Shiba-Rusinov subgap states when they couple to the superconductor. In this work, an array of bottom gates under an InAs nanowire is used to tune the coupling (${\mathrm{\Gamma}}_{S}$) of individual quantum states to a superconductor as the system is probed with a weakly coupled normal lead. The paper includes measurements of a coupling-induced quantum phase transition as well as the Zeeman splitting of a subgap state for different values of ${\mathrm{\Gamma}}_{S}$. The authors analyze their data using an extension of the Anderson model and the numerical renormalization group procedure and give quantitative values for key parameters of the system.

[Phys. Rev. B 94, 064520] Published Mon Aug 29, 2016

]]>Using optical absorption and scanning tunneling spectroscopy techniques, the authors investigate band-gap properties of single layers of transition metal dichalcogenide (TMDC) alloyed crystals (Mo${}_{x}$W${}_{1-x}$S${}_{2}$). They observe a significant modulation of the excitonic transitions, quasiparticle band gaps, and exciton binding energies. These findings may hold promise for fundamental studies of intralayer phenomena and for potential applications of TMDC alloys in electronic and optoelectronic devices.

[Phys. Rev. B 94, 075440] Published Mon Aug 29, 2016

]]>Individual dopants in semiconductors are attracting considerable attention due to the prospect of using them in a wide range of applications. In particular, phosphorus donors in silicon have attracted significant interest owing to their weak interaction with the host crystal. However, harnessing their attributes toward the construction of scalable circuitry will require low resistive interconnects at a comparable scale as the dopant atoms. In this Rapid Communication, the authors investigate the transition from quantum coherent to the semiclassical diffusive transport in a 4.6-nm quasi-one-dimensional Si:P metal wire. Analyzing the temperature dependence of universal conductance fluctuations (UCF) the authors show that electron transport evolves from a quantum coherent to a semiclassical regime at temperatures as low as ~4 K and confirm that concepts of UCF and weak localization remain valid in metallic conductors at the atomic-scale.

[Phys. Rev. B 94, 081412(R)] Published Mon Aug 29, 2016

]]>In the usual theory of electrical insulators, a small conductivity is permitted at finite temperature by the process of phonon-assisted quantum tunneling, or “hopping”, of electrons across a disordered energy landscape. The long-standing assumption has been that during this process each electron hops between essentially random locations in space. Here the authors show that the paths followed by electrons through a compensated semiconductor are in fact far from random. Instead, these electron pathways spontaneously form a fractal shape with effective dimension 2. The existence of these fractal pathways allows an unexpectedly high electrical current to move through the material at low temperature. The authors’ results can be used to explain recent surprising observations about electron transport through compensated topological insulators and nanocrystal arrays.

[Phys. Rev. B 94, 085146] Published Mon Aug 29, 2016

]]>What is the long-time behavior of a quantum system after it is brought out of equilibrium? In a generic case, it has been proposed that an effective thermalization occurs, and the local properties of the system can then be described by a thermal Gibbs ensemble. However, in integrable systems, with infinitely many conservation laws, the answer is far more complicated. It was shown in several special cases that the long-time steady state can be exactly described by the so called complete generalized Gibbs ensemble. The latter can be obtained by maximizing the entropy with constraints imposed by the conservation of energy, but also by additional carefully chosen subset of the integrals of motion. In this work, the authors analyze more general initial states, focusing on integrable XXZ Heisenberg spin chains. They find that the complete generalized Gibbs ensemble indeed correctly describes the system at long times after the quench.

[Phys. Rev. B 94, 054313] Published Tue Aug 23, 2016

]]>Quantum Monte Carlo simulations provide a powerful tool to compute static properties of quantum many-particles models and realistic systems. A vast array of ground-state energies and properties defined by equal-time correlation functions can be computed with high accuracy. On the other hand, the extensions needed to study time-displaced correlation functions, which provide access to excited states, have been challenging, especially for fermionic strongly correlated systems. The authors present a methodology that allows a high-accuracy calculation of the dynamical Green functions of many-fermions systems in the framework of the auxiliary-field quantum Monte Carlo method. They extract the the charge gap of the repulsive two-dimensional Hubbard model at half-filling as a function of the interaction strength and propose strategies for applying their methodology to more realistic systems.

[Phys. Rev. B 94, 085140] Published Tue Aug 23, 2016

]]>The Dzyaloshinski-Moriya interaction (DMI) originates from the relativistic spin-orbit coupling, and in low-symmetry crystal structures without inversion centers it gives rise to antisymmetric magnetic exchange. DMI plays an important role in the formation of inhomogeneous spin structures such as long-wavelength spirals, vortex states, and skyrmion textures. Even in high-symmetry lattices, where the antisymmetric DM term would normally vanish, DMI is present in the vicinity of any lattice defect and can influence the magnetic microstructure of polycrystalline materials with a large defect density. In this work the authors have studied the impact of the DMI on the magnetic small-angle neutron scattering (SANS) cross section, which is able to probe long-wavelength magnetization fluctuations in the bulk and on the nanometer length scale. The analysis of the authors provides a promising way to characterize the DMI in defect-rich materials.

[Phys. Rev. B 94, 054424] Published Mon Aug 22, 2016

]]>Members of the Mo${}_{x}$W${}_{1-x}$Te${}_{2}$ series are predicted to be Weyl semimetals, hosting type-II Weyl fermions, which have yet to be experimentally realized and which are unusual because they strongly violate Lorentz invariance. Crucially, the Weyl points in this system are predicted to sit above the Fermi level. Here, the authors show that for a type-II Weyl cone, although not for a type-I Weyl cone, if the Weyl point is above the Fermi level, then it’s necessary to see the band structure above the Fermi level to observe a topological Fermi arc. The authors also discover that pump-probe angle-resolved photoemission beautifully displays the unoccupied band structure in Mo${}_{x}$W${}_{1-x}$Te${}_{2}$. Their work sets the stage for demonstrating that this system is the first type-II Weyl semimetal, as well as the first tunable Weyl semimetal.

[Phys. Rev. B 94, 085127] Published Mon Aug 15, 2016

]]>Mid-infrared plasmonics has the potential to revolutionize molecular sensing technology, if integrated into optoelectronic chips. Recently,several groups working on plasmonics have substituted metals with heavily doped semiconductors for the sake of integration, also opening up the possibility of tuning the device response via the doping level. In this work, the authors analyze the relevant case of heavily doped Ge films by combining transport measurements with infrared spectroscopy. They demonstrate a broad tunability of the screened plasma frequency up to the mid-infrared range. The main loss channels are identified through comparison of the experimental scattering rates with quantum calculations and pump-probe measurements. Heavily doped Ge is highlighted as a viable route for the integration of mid-infrared plasmonics into silicon optoelectronic platforms.

[Phys. Rev. B 94, 085202] Published Mon Aug 15, 2016

]]>Here, the authors demonstrate a new first-principles computational method for performing predictive calculations of the optical absorption spectra of solids at finite temperature, including both zero-point fluctuations and phonon-assisted optical transitions. The comparison between calculations and measurements for the case of silicon, diamond, and gallium arsenide shows that this new method is not only efficient, but also very accurate, and is capable of yielding absorption coefficients in agreement with experiment over several orders of magnitude.

[Phys. Rev. B 94, 075125] Published Fri Aug 12, 2016

]]>Spin-resolved and circular dichroism angle-resolved photoemission spectroscopies (ARPES) are powerful techniques to detect the spin information of electrons in materials. In this work, the authors distinguish the following three types of spin in the ARPES experiment: the real spin, the pseudospin, and the spin polarization of photoemitted electrons. The authors present an interesting theoretical study on these three spin textures for topological surface states of Bi${}_{2}$Se${}_{3}$ and SmB${}_{6}$. Based on a model-type surface state Hamiltonian, they discuss the connections and differences of these spin textures as a function of the symmetry of the investigated system and the polarization of the incident detection light. This work provides a detailed and quantitative analytical study of the spin polarization and circular dichroism spectrum of the photoelectrons detected in an ARPES experiment.

[Phys. Rev. B 94, 085123] Published Fri Aug 12, 2016

]]>One of the key mechanisms for transverse spin-current generation in metals is the electron skew scattering off defects and impurities, where the spin-orbit interaction leads to an effective deflection of the electrons away from the direction of an applied electric field. Recently, there has been an intensive interest in the skew-scattering part of the Hall effects in an important spintronics material, $L{1}_{0}$-ordered FePt alloy, where conflicting data with regard to the skew-scattering magnitude have been reported in previous studies. To understand the reason for this controversy, the authors considered various sources of scattering in FePt and determined the skew scattering Hall angles from microscopic density functional theory calculations and Boltzmann transport theory. Based on their results, the authors are able to explain the extreme spread of available data for FePt with drastic dependence of the skew-scattering properties on the details of disorder.

[Phys. Rev. B 94, 060406(R)] Published Thu Aug 11, 2016

]]>The Dzyaloshinskii-Moriya Interaction (DMI) has recently attracted considerable interest owing to its fundamental role in the stabilization of chiral spin textures in ultrathin ferromagnets, which are interesting candidates for novel spintronic technologies. Here, the authors present an experimental study of the DMI strength that is induced in nanometer-thick CoFeB ferromagnetic thin films in contact with different nonmagnetic metal underlayers. They use a novel technique for noninvasive high-sensitivity sensing of magnetic field: a scanning nanomagnetometer based on the magnetic response of a single nitrogen-vacancy defect in diamond. The magnetic domain walls are mapped, the spin structure and type of domain wall determined, and the DMI strength extracted. Importantly, the authors find local variation of the DMI constant, which clearly suggests that local field mapping techniques are extremely important to study this physics.

[Phys. Rev. B 94, 064413] Published Thu Aug 11, 2016

]]>The fractional Josephson effect is of the most striking phenomena heralding topological superconductivity and Majorana bound states. In contrast to the conventional Josephson current that is 2$\pi $-periodic in the applied phase difference, the periodicity is 4$\pi $ in the junctions connecting topological superconductors. Unfortunately, this effect can be easily masked in experiments due to the quasiparticle poisoning processes resulting from inelastic transitions between the localized subgap states of the junction and delocalized states in the quasiparticle continuum. The authors propose a simple and feasible solution to this problem by embedding the topological Josephson junction in a SQUID setup. They show that signatures of the topological junction can be found in the switching probability of the supercurrent through the SQUID, even in the presence of quasiparticle poisoning processes.

[Phys. Rev. B 94, 085409] Published Thu Aug 11, 2016

]]>Magnetoresistance studies are one of the simplest yet powerful tools to investigate the electronic properties of solids. The authors here report on high-field angle-dependent magnetotransport measurements on epitaxial thin films of Bi${}_{2}$Se${}_{3}$, a three-dimensional topological insulator. At low temperature, they observe quantum oscillations that demonstrate the simultaneous presence of bulk and surface carriers. Their key observation is a strong anisotropy in the magnetoresistance in Bi${}_{2}$Se${}_{3}$ that depends on the orientation of the applied current (electric field) with respect to the applied magnetic field. When the magnetic field is applied parallel to the electric field, they find a strong negative longitudinal magnetoresistance (NLMR) that, quite strikingly, persists even up to room temperature. With this finding, the authors demonstrate that the observation of a NLMR is not unique to topological semimetals, such as Weyl semimetals, and therefore cannot by itself be taken as a diagnostic tool and as conclusive evidence for the existence of Weyl fermions. These results could pave the way towards a general understanding of the emergence of the axial anomaly that is suggested to be a universal phenomenon for generic three-dimensional metals in the presence of parallel electric and magnetic fields.

[Phys. Rev. B 94, 081302(R)] Published Wed Aug 10, 2016

]]>Magnetism due to vacancies in graphene has been a controversial topic, with conflicting results in the literature. This work aims to shine some light on the discussion by calculating the charging energy $U$, a key measure that encodes the local electron-electron Coulomb interaction in the vacancy-induced state. By combining analytical and numerical tools, the authors obtain values of $U$ reaching about 1 eV for typical micron-sized samples, which is significantly higher than the previous estimates. These results provide a strong theoretical support to the emergence of vacancy-mediated magnetism in bulk graphene with a dilute concentration of vacancies and are in agreement with recent experiments in the literature.

[Phys. Rev. B 94, 075114] Published Mon Aug 08, 2016

]]>The transport properties of zirconium pentatelluride ZrTe${}_{5}$, where the sign of carriers changes with temperature, have so far eluded a clear understanding. In this work, the authors investigate by angle-resolved photoemission the temperature dependence of the low-energy electronic bands, and find that the transport behavior can be explained by the presence of a van Hove singularity in the density of states. By surface doping with alkali atoms, the authors also access the unoccupied states in the conduction band and define this material as being a weak topological insulator.

[Phys. Rev. B 94, 081101(R)] Published Thu Aug 04, 2016

]]>Unique spin configurations in thin films, including skyrmions and chiral domain walls, present fascinating physical questions while also promising future, low-energy computing systems. The emergent interfacial Dzyaloshinskii-Moriya interaction (DMI), which stabilizes these features, greatly changes the energetic and symmetry-based arguments by which magnetic domains have previously been described. The authors here analyze the behavior of magnetic bubbles in thin films influenced by DMI using classical interface thermodynamics via the Wulff construction, originally conceived to describe crystal facets more than 100 years ago. The striking resemblance of experimentally observed domain shapes to those predicted theoretically suggests that a significant energetic driving force for domain growth exists due to an anisotropic wall energy term from the interfacial DMI. This description explains the observed reversal in growth symmetry found here in Co/Ni multilayer films while presenting a new paradigm for describing the broader behavior of chiral domain walls.

[Phys. Rev. B 94, 060401(R)] Published Mon Aug 01, 2016

]]>Superconductivity at high-pressures has attracted considerable interest after the report of a record critical temperature (${T}_{c}$) of 203 K in sulfur hydride (H${}_{3}$S) at 200 GPa. In this work, the authors predict several novel lithium-sulfur compounds at high pressure using evolutionary crystal structure prediction techniques and investigate their superconducting properties with density functional linear response calculations. The calculations reveal an intrinsic correlation between superconductivity and real-space electron localization. We find that high-${T}_{c}$ superconductivity in the Li-S system occurs at pressures much higher than in H${}_{3}$S, i.e., only when the electrode-like interstitial charge localization typical of alkali-metal compounds is suppressed. This is illustrated in the image, which shows a comparison of the electron-phonon spectral function ${\alpha}^{2}\phantom{\rule{0}{0ex}}F(\omega )$ of Li${}_{3}$S in the low- pressure (panel $a$, ${T}_{c}=0$ K) and high-pressure phases (panel $c$, ${T}_{c}=80$ K), calculated at 500 GPa. The right panels of the image show that the increase in ${T}_{c}$ is accompanied by a shift of the electronic charge from $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l$ regions to Li-S bonds.

[Phys. Rev. B 94, 060502(R)] Published Mon Aug 01, 2016

]]>Since 2008, iron-based superconductors have been the focus of intense research. Unfortunately, the higher transition temperatures and most promising high-magnetic field properties have been associated with compounds that have to be chemically disordered, making it hard to fully understand their intrinsic properties. A fully ordered compound with near optimal properties would be ideal and CaKFe${}_{4}$As${}_{4}$ appears to be that compound. In this work, the authors present detailed anisotropic, structural, thermodynamic, and transport measurements that, in addition to revealing high upper critical fields, also provide the basis for further understanding of this fully ordered, high-${T}_{c}$, iron-based superconductor. It is shown that CaKFe${}_{4}$As${}_{4}$ has exceptionally large upper critical fields (in excess of 650 kOe and possibly as high as 900 kOe) as well as high ${T}_{c}$ values (35 K), which can be obtained and studied without any disorder-related effects obscuring or complicating the results.

[Phys. Rev. B 94, 064501] Published Mon Aug 01, 2016

]]>Here, the authors investigate the possibility that particle-hole symmetry in the lowest Landau level at half filling arises as an emergent low-energy symmetry. Specifically, they start with the traditional approach to the half-filled Landau level: an effective field theory known as the composite Fermi liquid pioneered by Halperin, Lee, and Read. In this theory, composite fermions arise when flux is attached to electrons in a half-filled Landau level. However, an equivalent starting point is one where one starts with a filled Landau level, depletes electrons (or equivalently, adds holes) to the point of half filling. One then attaches flux to the holes, and the resulting composite hole description is another viable description of the half-filled Landau level. The authors construct a particle-hole symmetric theory by studying a system consisting of alternating quasi-one-dimensional strips of composite Fermi liquid (CFL) and composite hole liquid (CHL), both of which break particle-hole symmetry. Gauge invariance requires the existence of gapless edge modes at the interface between the CFL and CHL. Out of these modes, they construct a neutral Dirac fermion coupled to an emergent U(1) gauge field. Thus, at distances long compared to the strip width, they demonstrate that the system is described by a Dirac fermion coupled to an emergent gauge field, with an anti-unitary particle-hole symmetry, precisely the form conjectured recently by Son to be the low-energy description of a particle-hole symmetric half-filled Landau level.

[Phys. Rev. B 94, 075101] Published Mon Aug 01, 2016

]]>The authors perform near-field nanoimaging of various waveguide modes inside thin flakes of WSe${}_{2}$ by using state-of-the-art near-field scanning optical microscopy. This compound is a prototypical transition metal dichalcogenide with strongly bound excitons. The nanoimaging data provides direct evidence of strong coupling between waveguide photon modes and excitons. Such coupling generates hybrid polaritonic modes that are imaged for the first time as propagative modes in real space. This work uncovers in real space the physics of nanoscale light-matter interactions in van der Waals semiconductors and thus provides guidelines for future applications of this class of materials in nanophotonics and optoelectronics.

[Phys. Rev. B 94, 081402(R)] Published Mon Aug 01, 2016

]]>The authors here explore the properties of Weyl semimetals and axion insulators coupled to geometry. The novel interplay between axion strings and lattice dislocation defects are studied through both effective field theory and microscopic arguments. Various new phenomena are proposed for a Weyl semimetal with dislocations. This work should be of great interest both in the context of topological semimetal/insulators and for the problem of topological defects in strongly correlated systems.

[Phys. Rev. B 94, 085102] Published Mon Aug 01, 2016

]]>Strain engineering in graphene is of high current interest because of the possibility to induce new physical phenomena by means of mechanical strain. The authors of this work study the effect of strain on the electron-phonon interaction that plays an important role in the vibrational, thermal, and transport properties of graphene. It has been commonly assumed that Kohn anomalies and electron-phonon coupling in strained graphene occur only for the longitudinal and transverse optical phonons, as in pristine graphene. In contrast, this study shows that uniaxial strain in graphene induces a Kohn anomaly and enhancement of the electron-phonon coupling in the longitudinal acoustic phonon branch. This feature is a major difference between pristine and uniaxially strained graphene, due to the presence of a new intervalley phonon-scattering channel for electronic states close to the Dirac point.

[Phys. Rev. B 94, 085401] Published Mon Aug 01, 2016

]]>The phenomenon of many-body localization generalizes Anderson localization to interacting systems. Understanding this conceptually novel phenomenon requires a study of many-body eigenstates at finite energy densities. This is a very challenging task since the most efficient numerical methods such as, e.g., the density matrix renormalization group method, can only access the ground state and low lying excitations. In this work, the authors introduce a unitary tensor network based variational method that approximately finds all many-body eigenstates of fully localized Hamiltonians and scales polynomially with system size. The usefulness of their approach is demonstrated by considering the Heisenberg chain in a strongly disordered magnetic field.

[Phys. Rev. B 94, 041116(R)] Published Thu Jul 28, 2016

]]>Two-dimensional potentials are excellent test beds for the physics of interacting excitonic gases, but engineering highly homogeneous potentials can be challenging. In this work, the authors investigate excitons bound to a single 10-μm scale stacking fault, finding excitonic luminescence with unprecedented homogeneity. The authors further show that stacking-fault-bound excitons are mobile and have a giant permanent dipole moment by means of the magneto-Stark effect. These results indicate that stacking faults may be a promising platform to probe the many-body physics of interacting dipoles in an atomically smooth potential.

[Phys. Rev. B 94, 041201(R)] Published Mon Jul 25, 2016

]]>The creation of electron-hole pairs by photoexcitation in quasi-two-dimensional semiconductor heterostructures has long provided one motivation for the study of a positive impurity immersed in an electron gas. Improved understanding of such systems is vital to realize the full potential of solar cells and optoelectronic transistors and memory devices. Collective, many-body phenomena dominate the behavior of the hole-in-electron-gas system at high carrier density, while excitonic species such as the exciton (a bound state of an electron and the hole) and the trion (a bound state formed from two electrons and the hole) emerge at low density. Confinement enhances the binding energy in quantum wells compared to bulk samples but, despite much experimental and theoretical work, the properties of excitons and trions are still uncertain. The authors perform diffusion Monte Carlo calculations of a single positive impurity immersed in a two-dimensional, paramagnetic electron gas at zero temperature using the Coulomb ($1/r$) potential. A gradual crossover from the collective, many-body behavior to the trion state is studied over a range of carrier densities and electron-hole mass ratios consistent with recent experiments in GaAs and CdTe quantum wells. Relaxation energies, electron-hole pair correlation functions, and electron-hole center-of-mass momentum densities are reported for these systems and may be used, for example, in density functional theory studies of optoelectronic devices.

[Phys. Rev. B 94, 041410(R)] Published Fri Jul 22, 2016

]]>In a wide range of experiments where electrical currents are used to inject angular momentum, or spin currents, from a metallic ferromagnet into a nonmagnetic metal, the interface plays a critical role. Whether in the giant magnetoresistance effect or in nonlocal spin valves, a loss of spin polarization of an electrical current crossing the interface is seen to reduce the spin current and lead to smaller overall response in sensors. The authors here compare electrical spin injection to the more recently discovered thermal spin injection in a nonlocal spin valve. They show that, despite a strong reduction of electrical spin injection that they tie to the loss of interfacial spin polarization, thermal spin injection remains a large effect. This highlights that thermal spin injection (also called the spin-dependent Seebeck effect) can proceed by new physical mechanisms not possible in the electrically driven case, potentially involving incoherent spin pumping and collective behavior of magnetization in the oxidized interface layer. Better understanding of these interface effects could lead to new ways to increase sensitivity of next-generation magnetic sensors and efficiency of sources of spin current in metallic systems.

[Phys. Rev. B 94, 024426] Published Thu Jul 21, 2016

]]>Atomic force microscopy is one of the techniques that allows us to visualize objects that we cannot see directly. The question of what we actually see when we look at an atomic force microscopy image is, however, a subtle one. This paper discusses a scheme that permits the calculation of atomic force microscopy images with relatively little effort. Using electronic density functional theory, the force on the cantilever is calculated as the gradient of the effective potential that governs the electronic structure in density functional theory. In this way, nonclassical contributions to the force can efficiently be taken into account and are translated into an image. Comparison to experimental images shows that the nonclassical contribution to the force can make a visible difference.

[Phys. Rev. B 94, 035426] Published Mon Jul 18, 2016

]]>The Kitaev model is a toy model for $S$=$\frac{1}{2}$ moments on a honeycomb lattice, which interact via strongly bond-dependent Ising-like interactions. In real material candidates like Na${}_{2}$IrO${}_{3}$, small Heisenberg interactions are also present in addition to dominant Kitaev interactions. The doped Kitaev-Heisenberg model has been predicted to show unconventional superconductivity and spin or charge density wave, and bond-order instabilities. In this paper, the authors have succeeded in surface-doping Na${}_{2}$IrO${}_{3}$ crystals by argon plasma etching and in changing the conductivity by several orders of magnitude. The doped samples show several unusual transport behaviors, including first order spin or charge density wave-like transitions.

[Phys. Rev. B 94, 041109(R)] Published Mon Jul 18, 2016

]]>URu${}_{2}$Si${}_{2}$ undergoes a second-order phase transition at 17.5 K that has defied attempts to identify its order parameter despite a vast literature extending over the last 30 years. A wide variety of theories have been posed to explain this so-called “hidden order”, some with exotic ground states. An important dividing line between these theories is the character of the 5$f$ orbital: on one side sit theories that require a localized ${f}^{2}$ configuration ($f$-manifold crystalline electric field effects, hastatic order, etc.), while on the other sit those that start from an itinerant, partially occupied ${f}^{3}$ orbital (band structure + hybridization). Unfortunately, the experimental measures remain muddy on this issue, with indications of both localized ${f}^{2}$ and itinerant ${f}^{3}$ behavior depending on the experiment. Here, the authors report U ${L}_{I\phantom{\rule{0}{0ex}}I\phantom{\rule{0}{0ex}}I}$-edge absorption/${L}_{\alpha 1}$ emission resonant x-ray emission spectroscopy measurements comparing data from URu${}_{2}$Si${}_{2}$ to UCd${}_{11}$, UF${}_{4}$, and UO${}_{2}$ data to unravel the various roles of $f$-orbital occupation, delocalization, and ligand-field splitting of the $d$ manifold. The data indicate a dominant delocalized ${f}^{3}$ configuration that is likely partially occupied, with no measurable change in occupancy to temperatures as low as 10 K. While these measurements do not rule out a minority localized ${f}^{2}$ configuration, any theory relying on such a configuration must account for its relatively small contribution to the Fermi surface.

[Phys. Rev. B 94, 045121] Published Fri Jul 15, 2016

]]>The ability to explain and predict the behavior in magnetic field is essential for any possible application of multiferroic materials. Motivated by the complex phase diagrams of MnWO${}_{4}$ and other spiral multiferroics, the authors formulate and investigate the anisotropic next-nearest-neighbor Heisenberg (ANNNH) model, which is a generalization of the celebrated ANNNI model to three component quantum spins. Using real-space mean-field simulations allowed for an unbiased construction of the magnetic phase diagram with multiple commensurate and incommensurate states. In particular, the biaxial anisotropy can explain the puzzling nonferroic reentrant phase of MnWO${}_{4}$ at high magnetic fields. The employed numerical technique is capable of predicting the temperature and field dependence of the ordering wave vector produced by the competition between helicity and anisotropy. An important issue raised in the present study is the effect of quantum fluctuations on the devil’s staircase known to exist for the ANNNI model, but which is replaced by a “floating” incommensurate phase for the ANNNH model.

[Phys. Rev. B 94, 020406(R)] Published Thu Jul 14, 2016

]]>An infinite projected entangled-pair state (iPEPS) is a powerful variational tensor network ansatz for two-dimensional ground states in the thermodynamic limit, and can be seen as a natural generalization of a matrix-product state to two dimensions. One of the main challenges in iPEPS simulations is the optimization of the tensors, i.e., finding the optimal variational parameters, in order to have the best representation of the ground state of a given Hamiltonian. The author presents a variational optimization scheme, in which the energy is minimized in an iterative way by sweeping over all the tensors in the ansatz, in a similar spirit as done in the density-matrix renormalization group method. Benchmark results for challenging problems are presented that show that the variational scheme yields considerably more accurate results than the previously best imaginary-time evolution algorithm, with a similar computational cost and with a faster convergence towards the ground state.

[Phys. Rev. B 94, 035133] Published Thu Jul 14, 2016

]]>Many earlier high-resolution x-ray studies of criticality seemingly invalidated the single-length scaling hypothesis that is predicted by the three-dimensional Ising model, a cornerstone in the theory of critical phenomena. Speculations about the reasons for these discrepancies have concentrated on the role of impurities and on surface-induced strain in the crystalline samples. The general validity of the Ising model for real systems has also been questioned. Here, the authors present x-ray scattering data on the temperature-induced order-disorder transition in $\beta $-brass, nature’s archetypical realization of the Ising model. These investigations unambiguously show that the Ising model does precisely describe the critical behavior observed and, in particular, that the short-range order parameter indeed exhibits single-length scaling. The surface sensitivity has been varied in the experiments without any substantial differences observed.

[Phys. Rev. B 94, 014111] Published Wed Jul 13, 2016

]]>This paper focuses on developing a new algorithm (En-DMRG) for obtaining highly excited states of disordered and strongly interacting spin systems. This line of research is stimulated by the growing interest in understanding the many-body localized (MBL) state, which emerges in disordered interacting systems at high energy densities through a disorder-driven dynamic phase transition. By studying the one-dimensional Heisenberg spin chain (with up to 72 spins) in a random field, the authors demonstrate the accuracy of the method in obtaining energy eigenstates and the corresponding statistical results of the quantum system in the MBL phase. The method presented in this work has the potential to be applicable in a variety of quantum statistical physics problems, possibly including the MBL phase in higher dimensions.

[Phys. Rev. B 94, 045111] Published Wed Jul 13, 2016

]]>Ultrathin films of the heavy elements Pd and Pt are widely used in spintronics as converters between charge and spin currents and vice versa. One open question has been whether the strongly paramagnetic nature of these elements influences the interconversion process. The authors show, using a combination of ferromagnetic resonance and x-ray magnetic circular dichroism measurements, that a related spin current absorption process (spin pumping damping) responds to the presence of ferromagnetic order in the polarizable Pd and Pt layers. They observe a change in the thickness dependence of spin current absorption when the Pd and Pt layers are in direct contact with a ferromagnet, compared with when they are in indirect contact. This change correlates with the presence of induced magnetic moments in Pd and Pt as revealed by element-specific x-ray magnetic circular dichroism magnetometry. The results show that the decoherence process for spin current in such layers is not independent of geometry, but rather is influenced by the presence of induced ferromagnetic order.

[Phys. Rev. B 94, 014414] Published Tue Jul 12, 2016

]]>Arrays of subwavelength slit cavities have been of interest to the acoustic metamaterial community for some years now, giving rise to such phenomena as “enhanced acoustic transmission”, where sound of certain wavelengths can transmit through the slit array structure as if it were not there. This behaviour is widely accepted as a coupling effect between diffraction evanescent waves arising from structure factor, and Fabry-Perot like resonances supported along the length of each slit cavity. Here, the authors explore the effect of altering the spacing, or width, of some of the slits that form the array, thereby forming a “compound” grating, where each periodic unit cell is comprised of multiple slit cavities. The transmission of such structures is experimentally recorded, where it is found that as one increases the number of slit cavities per unit cell, broad and deep minima can appear in the peaks of the primary enhanced acoustic transmission resonance that may have a strong angular dependence. These features owe their existence to the “phase resonance”, where new degrees of freedom available to the near field allow adjacent slit cavities to be out-of-phase with one another and thus destructively interfere with the re-radiated pressure wave. Such behaviour has been studied extensively in the electromagnetic case, where a surface wave band folding picture could be used to predict the dispersive nature of these phase resonances. A similar explanation of the underlying surface wave physics is extended here to the acoustic case.

[Phys. Rev. B 94, 024304] Published Tue Jul 12, 2016

]]>The authors have studied SmB${}_{6}$ and Ce${}_{3}$Bi${}_{4}$Pt${}_{3}$. Both of these Kondo insulators show a saturation of the increasing resistivity at low temperature that suggests an additional conduction channel and is consistent with the theoretical prediction that these systems should host robust surface states due to their nontrivial topology. While previous work has shown that the resistance saturation in SmB${}_{6}$ is due to conducting surface states, this work demonstrates that the origin of the resistance saturation in Ce${}_{3}$Bi${}_{4}$Pt${}_{3}$ is through the bulk of the material and not the surface. The contrasting behavior of the two materials when the surface is disordered with ion-irradiation supports this conclusion, and also demonstrates the sensitivity of the resistance of the surface state in SmB${}_{6}$ to small levels of disorder. Finally, the low-temperature specific heat of SmB${}_{6}$ shows a residual linear term, and the origin of this term has been highly debated recently. Through measurements of single crystals and powders, it is shown that this term dominantly arises from the bulk and not the surface state.

[Phys. Rev. B 94, 035127] Published Tue Jul 12, 2016

]]>The properties of an electron beam can be manipulated by electromagnetic fields in vacuum via the ponderomotive force. Such an interaction is also at the core of the Kapitza-Dirac effect, which describes the diffraction of electrons by an optical standing wave. Here, the authors predict a new type of interaction between electrons and the electromagnetic field, opening up new possibilities for the manipulation of electron beams. If surface plasmon polaritons are tailored to interfere forming a periodic field pattern, it becomes possible to diffract electrons from such near field. With the proper manipulation of the plasmonic fields, orbital angular momentum can be imparted to the electrons, and even the phase of their wave functions can be manipulated. An additional degree of freedom is provided by the possibility to tailor the spatial properties of the light and the materials supporting the surface plasmons. This arbitrary control can be extended to different substrates such as graphene or layered systems and may open up a viable route to create tunable phase plates for electron microscopes.

[Phys. Rev. B 94, 041404(R)] Published Mon Jul 11, 2016

]]>“Strange metal phases”, displaying strong deviations from Fermi liquid theory, are discussed in the context of cuprates, itinerant magnets, or heavy-fermion metals. Several Yb-based materials, e.g., $\beta $-YbAlB${}_{4}$ or the Au-Al-Yb quasicrystal, display $T$/$B$ scaling of magnetic and thermodynamic properties. This has been taken as evidence for zero-field quantum criticality without requirement to fine-tune composition and pressure. However, it appears unlikely that materials are accidentally located at such a special point in multidimensional phase space. In this paper, the authors demonstrate by a thermodynamic study on the new Kondo lattice compound YbCo${}_{2}$Ge${}_{4}$ that $T$/$B$ scaling and a divergence of the magnetic Grüneisen parameter can arise without a quasiparticle mass divergence and in the absence of zero-field quantum criticality. They discuss alternative scenarios for such strange metal behavior.

[Phys. Rev. B 94, 041106(R)] Published Fri Jul 08, 2016

]]>Engineering the gauge field of a physical system is a crucial step in the effort to control the behavior of waves. In the past few years, scientists have demonstrated the ability to design and implement artificial gauge fields in a variety of physical systems, which have played a major role in demonstrating various fascinating phenomena in both photonic and cold-atoms settings. In this Rapid Communication, the authors explore, theoretically and experimentally, a method to create artificial gauge fields. They implement this method for demonstrating the Rashba effect – the photonic equivalent of the spin-orbit coupling known from electronic systems.

[Phys. Rev. B 94, 020301(R)] Published Thu Jul 07, 2016

]]>The control of octahedral rotations in perovskite heterostructures is an emerging strategy for inducing new functionality as evidenced by recent predictions of improper ferroelectricity, polar metals, and multiferroics. Many of these predictions are predicated on the presence of a specific rotation pattern (${a}^{-}\phantom{\rule{0}{0ex}}{a}^{-}\phantom{\rule{0}{0ex}}{c}^{+}$) in superlattices that exhibit the orthorhombic ($P\phantom{\rule{0}{0ex}}b\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}m$) perovskite structural variant. The authors use synchrotron diffraction to measure the octahedral rotation patterns in strained ferrite, manganite, and gallate perovskite films finding that compressive strain strongly favors ${a}^{+}\phantom{\rule{0}{0ex}}{a}^{-}\phantom{\rule{0}{0ex}}{c}^{-}$ rotation patterns and tensile strain weakly favors ${a}^{-}\phantom{\rule{0}{0ex}}{a}^{-}\phantom{\rule{0}{0ex}}{c}^{+}$ structures. In contrast, films grown on orthorhombic substrates exhibit the same rotation pattern orientation as the substrate, even for epitaxial conditions where strain would favor the opposite structural orientation. The results indicate that substrate imprinting is a more robust method than strain for controlling the rotation pattern in $P\phantom{\rule{0}{0ex}}b\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}m$-type perovskite films, a finding that should enable more efficient experimental pursuits of rotation-driven ferroic states in oxide heterostructures.

[Phys. Rev. B 94, 024105] Published Thu Jul 07, 2016

]]>Ferromagnetism and superconductivity are two generally incompatible states of matter. Their coexistence has been observed only under very unusual circumstances and so far only in metals. In contrast, most semiconductors are not naturally magnetic or superconducting, but introducing magnetism or superconductivity into semiconductors is seen as an important step towards radical improvement of our electronics capabilities and therefore a hotly pursued goal. Here, the authors report the discovery of superconductivity coexisting with ferromagnetism in the semiconducting material samarium nitride (SmN). The large intrinsic exchange splitting of the conduction band in SmN requires the superconducting order to be of unconventional (likely $p$-wave) type. Superconductivity is observed to be even further enhanced in superlattices where layers of SmN alternate with layers made of the strongly ferromagnetic but non-superconducting material gadolinium nitride. These features render SmN an interesting laboratory for understanding more about the fundamentals of ferromagnetism and superconductivity in semiconductors and exploring opportunities for integrating superconducting spintronics into the design of semiconductor-based electronic devices.

[Phys. Rev. B 94, 024106] Published Thu Jul 07, 2016

]]>A ${\mathbb{Z}}_{2}$ fractionalized Fermi liquid (FL*) is a novel state of strongly correlated quantum matter. Although it is metallic, it violates Luttinger’s theorem on the volume enclosed by the Fermi surface obeyed by conventional metals; this is possible due to the presence of emergent gauge excitations. The authors study here the superconducting states that can arise out of instabilities of such a topological metal. In particular, they focus on a FL* topological metal that has favorable energetics on the square lattice with nematic order, relevant to the cuprate superconductors. They find that a Higgs transition out of this FL* results in confinement of the anyonic degrees of freedom, and the resulting state is a superconductor with broken translation symmetry or time-reversal invariance. In the process, they also establish a complete mapping between bosonic and fermionic descriptions of time-reversal invariant gapped insulating ${\mathbb{Z}}_{2}$ spin liquid states on the rectangular lattice, which, on doping with charge carriers, can give rise to FL* phases. They also note a possible connection to the recent observation of pair density waves in the superconducting state of the underdoped cuprates.

[Phys. Rev. B 94, 024502] Published Tue Jul 05, 2016

]]>Magnetic vortex is one of the simplest nontrivial topological structure that can exist in magnetic nanodots. Due to nonzero topological charge, vortices exhibit complex and interesting dynamic under external excitation, which includes excitation of translational (gyrotropic) and spin-wave modes. Here, the authors study the vortex dynamic in thick magnetic nanodots, when the vortex becomes essentially three dimensional. The third dimension is shown to lead to the appearance of additional spin-wave modes with characteristic curled structure (“curled” modes), which becomes the lowest ones in frequency among spin-wave modes for a sufficiently thick nanodot. Besides an uncommon profile, the curled modes also show uncommon hybridization with gyrotropic modes: modes propagating in opposite directions (clockwise and counterclockwise) that can be hybridized with different order gyrotropic modes.

[Phys. Rev. B 93, 214437] Published Tue Jun 28, 2016

]]>Magnetic tunnel junctions are under active consideration owing to their applications including the potential next generations of spin transfer torque memories. In view of the high symmetry of the magnetic properties and of the high frequency of the system eigenexcitations, out-of-plane magnetized systems are thought to enable a faster and simpler spin-torque-induced switching process than the formerly used in-plane magnetized systems. Unfortunately, the lack of high frequency and low current switching experiments have precluded, so far, the assessment of the speed potential of perpendicular anisotropy systems at low junction dimensions. By time resolving the switching and evidencing its stochastic aspects at the nanosecond-scale, the authors here demonstrate that a complex dynamics happens and persists down to small sized elements with always a very strong asymmetry between the two switching directions. The reversal is explained by the complex interplay between the spatial profile of the stray field emanating from the fixed system of the tunnel junction and the constraint that the switching is preferably initiated from the device edge.

[Phys. Rev. B 93, 224432] Published Tue Jun 28, 2016

]]>Much interest has emerged in the realization of materials with quantum spin liquid ground states, which would provide fertile ground for realizing exotic excitations. Three “Kitaev” spin liquid candidate materials have been proposed – Na${}_{2}$IrO${}_{3}$, $\alpha $-RuCl${}_{3}$, and $\alpha $-Li${}_{2}$IrO${}_{3}$ – and there has been many diverging proposals for the most relevant interactions. Here, the authors re-evaluate carefully the magnetic interactions in these materials, using nonperturbative exact diagonalization methods. The results clarify misconceptions in the literature and help estimate the potential for realizing the spin liquid state in real materials.

[Phys. Rev. B 93, 214431] Published Mon Jun 27, 2016

]]>The role of spin-orbit coupling (SOC) in 4$d$ and 5$d$ transition metal oxides is relatively poorly understood outside of the LS or JJ coupling limits. In this work, the importance of the intermediate regime is demonstrated through the identification of SOC as an essential feature of the collective properties in nominally orbitally quenched 5${d}^{3}$ systems. The authors probe the magnetic excitations in model system Sr${}_{2}$ScOsO${}_{6}$ via inelastic neutron scattering and model the results including the effects of SOC-induced anisotropy. Experimentally determining the strengths of nearest and next-nearest neighbor interactions leads to the important discovery that the magnetic ground state is controlled by anisotropy, which would not be possible in the LS coupling limit. This indicates that SOC plays a key role in relieving frustration and promoting heightened transition temperatures in 5${d}^{3}$ double perovskites.

[Phys. Rev. B 93, 220408(R)] Published Mon Jun 27, 2016

]]>Motivated by the particle-hole symmetry at the half-filled lowest Landau level, many articles have taken the point of view, following D. T. Son, that composite fermions are Dirac particles and their Fermi sea is akin to that occurring at the surface of a three dimensional topological insulator. This presents a paradox, because the microscopic theory of composite fermions, which treats composite fermions as non-relativistic particles, has been confirmed in great detail. The authors discuss what features of the Dirac-composite fermion view can be reconciled with the microscopic theory. They also show how particle-hole symmetry can emerge in the Chern-Simons formulation of composite fermions.

[Phys. Rev. B 93, 235152] Published Mon Jun 27, 2016

]]>Phases of matter are traditionally seen as families of static systems exhibiting the same long-distance and low-energy correlations. In this work, the authors propose and classify a new family of phases of matter. They are novel insofar as they are intrinsically driven and out of equilibrium — they can only be realized in systems with time-dependent Hamiltonians. The phases we consider arise in 1D periodically driven systems, in the presence of strong disorder and interactions, and are similar to but qualitatively distinct from the symmetry-protected topological phases now well known in the equilibrium setting. In a companion paper, the authors examine similar intrinsically driven families of states with long-range order, and an order parameter that oscillates at a frequency that is robustly an integer multiple of the underlying drive frequency.

[Phys. Rev. B 93, 245145] Published Mon Jun 27, 2016

]]>Duality is a powerful theoretical tool whereby the strongly interacting and intractable regime of a theory can be accessed in terms of weakly interacting “dual” variables. In condensed matter, this is familiar from the Kramers-Wannier duality of the Ising model, and boson-vortex duality. Here, a duality for another fundamental problem – Dirac fermions in two spatial dimensions – is obtained, which provides a new window to the physics of strongly interacting electrons. This leads to unexpected connections between topological insulators in three dimensions and the physics of the half-filled Landau level, including composite fermions and Pfaffian quantum Hall states.

[Phys. Rev. B 93, 245151] Published Mon Jun 27, 2016

]]>This study is motivated by a challenge to the third law of the thermodynamics: how does the degenerated ground state in a textbook example behave in a real compound? At very low temperatures, nature tries to preserve the basic law using small perturbations and a novel quantum state should appear. The authors here study the classic frustrated magnet Ba${}_{3}$Yb${}_{2}$Zn${}_{5}$O${}_{11}$, a spin tetrahedron having two-fold degeneracy realized in a breathing pyrochlore lattice. They measure by inelastic neutron scattering the low-energy excitations in order to identify the effective spin Hamiltonian and macroscopic properties at very low temperatures. They demonstrate how the degeneracy of the ground state is lifted and a unique quantum state is selected.

[Phys. Rev. B 93, 220407(R)] Published Fri Jun 24, 2016

]]>Unraveling the peculiar properties of topological materials often requires tuning of the bulk chemical potential, which has been addressed by compensation of acceptors and donors. Compensation necessarily creates Coulomb disorder which, however, impedes chemical potential tuning and gives rise to self-organized charge puddles. In the topological insulator BiSbTeSe${}_{2}$, optical data and Monte Carlo simulations presented here reveal the existence of puddles at low temperatures as well as their surprising “evaporation” on a temperature scale of about 40 K. The highly nonlinear screening effect of thermal excitations is also important for Dirac matter or Weyl semimetals. Understanding and controlling the puddles will be a key to realizing novel phenomena in 3D topological materials, as was the case with 2D materials such as graphene.

[Phys. Rev. B 93, 245149] Published Fri Jun 24, 2016

]]>Cubic chiral magnets crystallizing in space group $P$2${}_{1}$3 give rise to a spin structure of nontrivial topology, the so-called skyrmion lattice. The strongly doped semiconductor Fe${}_{1-x}$Co${}_{x}$Si is a fascinating member of this material class of compounds as it exhibits helimagnetism over a wide compositional range. The authors study the magnetic properties of this itinerant magnet for different values of $x$, combine their results with previous reports, and infer the magnetic phase diagrams. The latter are rather isotropic but depend on the field and temperature history. In particular, when cooled in an applied magnetic field, the skyrmion lattice may persist down to lowest temperatures. This history dependence, in combination with the variation of temperature, field, and length scale of the magnetic order by means of compositional tuning, makes Fe${}_{1-x}$Co${}_{x}$Si an excellent material for future experiments studying skyrmions.

[Phys. Rev. B 93, 235144] Published Tue Jun 21, 2016

]]>Quantum dots have long been considered systems that could deliver a significant advantage for practical quantum information applications. Their atomic dipolelike transitions and their compatibility to be integrated into a variety of photonic structures has highlighted this fact. Here, the authors measure an unexpectedly high single-photon level phase shift (~6${}^{\circ}$), induced upon reflection from a singly negatively charged quantum dot incorporated in a low-$Q$-factor cavity ($Q$~290). This measurement provides the first evidence for a system with unit fidelity and a high efficiency in its interactions. The result opens a new road to realistic spin-photon entanglement devices, in a simple and easy to fabricate photonic structure.

[Phys. Rev. B 93, 241409(R)] Published Tue Jun 21, 2016

]]>A special type of anisotropic spin exchange called the “Kitaev interaction” provides rare exactly-soluble models of gapless quantum spin liquids in 2d and 3d. The main goals of this work were to identify other lattice geometries that support the elusive Kitaev interactions and understand the magnetic properties of such materials. By performing inelastic neutron scattering on La${}_{2}$MgIrO${}_{6}$ and La${}_{2}$ZnIrO${}_{6}$, the authors find that these magnets, which possess long-range antiferromagnetic order, yield clear signatures of gapped and nearly-dispersionless magnon excitations. They also show that spin-wave models with a dominant Kitaev interaction can naturally account for the magnetic ordering pattern as well as the inelastic neutron scattering data.

[Phys. Rev. B 93, 214426] Published Mon Jun 20, 2016

]]>Coupled-cluster theory is a “gold standard” of molecular quantum chemistry, but has only recently been extended to condensed-phase systems. In this work, excited-state coupled-cluster techniques are brought to bear on the spectral function of the uniform electron gas. The diagrammatic content of coupled-cluster theory goes beyond that of the $G\phantom{\rule{0}{0ex}}W$ approximation, which has become a canonical approach in first-principles condensed-matter physics. Benchmark calculations demonstrate that the coupled-cluster results are more accurate than those of $G\phantom{\rule{0}{0ex}}W$-based approaches, insensitive to the mean-field reference, and systematically improvable towards the exact result. Calculations on large systems approaching the thermodynamic limit reveal an accurate quasiparticle bandwidth as well as plasmonic satellite structure in the core-hole spectral function.

[Phys. Rev. B 93, 235139] Published Mon Jun 20, 2016

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