Starting from a minimal model of a time-reversal-invariant topological superconductor in two dimensions, the author shows that an in-plane magnetic field could drive the superconductor into a second-order topological phase, which features Majorana zero modes, bound at two of the four corners of the system. These Majorana corner states occur where edges with distinct topology intersect. Hence, the corner states are topologically protected. With variation of magnetic-field orientation, the nontrivial corner states would move along the boundary and reside on different corners.

[Phys. Rev. B 97, 205134] Published Fri May 25, 2018

]]>While topological insulators have a gapped bulk band structure, they have gapless surface states. Recently, it was shown that the presence of additional crystalline symmetries can lead to topological phases that combine a gapped bulk spectrum with gapless states on edges or corners of the crystal. Such topological phases have been called “higher-order topological phases”. This article presents a complete classification of second-order topological insulators and superconductors with mirror, twofold-rotation, or inversion symmetry. The authors show that crystals with a mirror symmetry and a nontrivial bulk band structure either have gapless surfaces or gapless edges. On the other hand, there are crystals with twofold rotation or inversion symmetry with a nontrivial bulk topology but without surface or edge states.

[Phys. Rev. B 97, 205135] Published Fri May 25, 2018

]]>Here, higher-order topological insulators and superconductors protected by inversion symmetry are investigated. These phases are characterized by gapped bulk and surface with gapless modes confined to hinges or corners of the sample. Such surface states can be understood as topological defects that are globally stabilized by inversion. They can be built using a layer construction that embeds a standard topological insulator/superconductor into a higher dimension by symmetrically adding to it copies of itself. Using this procedure, a complete classification of such states in any dimension is obtained and several examples for possible physical realizations are provided.

[Phys. Rev. B 97, 205136] Published Fri May 25, 2018

]]>There are two well-established ways of characterizing transport properties. The first is via the spread of correlations in an isolated system. The second is through the system-size dependence of the current, when a potential bias is applied by connecting the two ends of the system to reservoirs. These are two different experiments, but usually the result of one experiment can be inferred from that of the other. This work presents a surprising counterexample. The authors show that, at the critical point of the Aubry-André-Harper model, transport in the presence of reservoirs is drastically different from transport in the isolated system. The scaling of current with system size shows clear subdiffusive behavior, while the spread of correlations does not show any subdiffusive behavior. It actually hints at superdiffusion. This implies that the understanding of the relation between transport in open and in isolated systems needs to be reexamined.

[Phys. Rev. B 97, 174206] Published Thu May 24, 2018

]]>The authors present a scheme to construct classical $n$-body force fields using Gaussian Process (GP) Regression, appropriately mapped over explicit n-body functions (M-FFs). The procedure is possible, and will yield accurate forces, whenever prior knowledge allows to restrict the interactions to a finite order $n$, so that the “universal approximator” resolving power of standard GPs or Neural Networks is not needed. Under these conditions, the proposed construction preserves flexibility of training, systematically improvable accuracy, and a clear framework for validation of the underlying machine learning technique. Moreover, the M-FFs are as fast as classical parametrized potentials, since they avoid lengthy summations over database entries or weight parameters.

[Phys. Rev. B 97, 184307] Published Thu May 24, 2018

]]>A proposed scheme for generating torus-shaped light pulses called flying doughnuts utilizes a metamaterial “sprinkled” with tiny resonators in a concentric ring pattern.

[Phys. Rev. B 97, 201409(R)] Published Wed May 23, 2018

]]>Artificial neural networks and machine learning tools have become ubiquitous far beyond the field of computer science. They already smartly assist us in our daily lives in various ways. Recently, these concepts have been adopted and applied in the realm of quantum physics, for example as a computational Ansatz for the state of a quantum many-body system. In this work, the authors harness the enormous flexibility of artificial neural networks to study exotic phases of quantum matter, known as chiral topological phases, that are particularly hard to investigate microscopically with more conventional computational methods.

[Phys. Rev. B 97, 195136] Published Fri May 18, 2018

]]>Excitons strongly influence the optical response of low-dimensional materials. Here, the authors propose a practical framework for calculating the excitonic linear and nonlinear optical response of multiband semiconductors based on the perturbative density matrix within the mean-field approximation. Using this framework, the equivalence of four different approaches is demonstrated analytically and numerically, if the correct velocity gauge interaction is used. The four approaches stem from two choices of gauge and two ways of computing the response functions. The present formalism can readily be extended to generate gauge invariant responses for higher-order nonlinear processes.

[Phys. Rev. B 97, 205432] Published Fri May 18, 2018

]]>The interplay of magnetism and superconductivity gives rise to the emergence of various interesting phenomena in condensed matter physics, such as the Yu-Shiba-Rusinov in-gap modes created by local magnetic moments in superconducting hosts. Using normal-metal and superconducting tips the authors have formed contacts to an individual magnetic molecule adsorbed on a conventional superconductor. Spectroscopy of the differential conductance with tunable junction transparency has unveiled the crossover from quasiparticle tunneling, which probes the Bardeen-Cooper-Schrieffer energy gap as well as the Yu-Shiba-Rusinov levels, to the contact range where multiple Andreev reflection takes over.

[Phys. Rev. B 97, 195429] Published Thu May 17, 2018

]]>The authors show here that geometric properties of Laughlin fractional quantum Hall states are accurately described by the Chern-Simons matrix model, a regularization of the noncommutative Chern-Simons theory description of these states. The noncommutative Chern-Simons theory itself is an example of an exotic noncommutative field theory: a quantum field theory on a space where the coordinates do not commute. The authors revisit these noncommutative theories and show that they capture the Hall viscosity of the Laughlin states, as well as other geometric properties of these states that are of current interest.

[Phys. Rev. B 97, 205122] Published Thu May 17, 2018

]]>Coherently excited phonons are a unique tool to control the properties of materials in their electronic ground state on the picosecond timescale. Here, the authors complete the map of four excitation mechanisms for Raman-active phonons, which are either directly mediated by photons or work with the help of nonlinearly coupled phonons. Each process can be achieved by using a sum- or difference-frequency mixing of the scattering particles. The general model here captures all four mechanisms, which is demonstrated for different types of material using density functional theory. This study may serve as a guide for the selective excitation of phonons and Raman-active modes in general.

[Phys. Rev. B 97, 174302] Published Wed May 16, 2018

]]>Mesoscopic superconducting islands with four or more Majorana zero modes are promising candidates for topological qubits. In recent theoretical proposals, interlinked ensembles of such Majorana boxes are used to establish Majorana code networks that, in principle, can run large-scale quantum computations and quantum error correction. Here, the authors discuss transport through coupled Majorana boxes, developing theoretical approaches in which general networks can be drastically reduced in their complexity. This is followed by a discussion of several examples of recent interest, including an analysis of the so-called loop quit device, which is currently under experimental investigation.

[Phys. Rev. B 97, 184506] Published Wed May 16, 2018

]]>The authors analyze the interplay between a $d$-wave uniform superconducting and a pair-density-wave order (PDW) parameter in the neighborhood of a vortex, and compute the resulting local density of states in the vortex halo. The intertwining of the two superconducting orders leads to a charge density modulation with the same periodicity as the PDW, which is twice the period of the charge density wave that arises as a second harmonic of the PDW itself. They discuss key features of the charge density modulation that can be directly compared with recent results from scanning tunneling microscopy and speculate on the role PDW order may play in the global phase diagram of the hole-doped cuprates.

[Phys. Rev. B 97, 174510] Published Mon May 14, 2018

]]>The authors analyze a recent STM experiment, reporting a period-8 charge density wave (CDW) in the vortex halo in striking contrast to the period-4 CDWs that are commonly observed in the cuprates. Inside the superconducting dome, the observation of CDWs necessarily implies the existence of a pair density wave (PDW) at the same momentum, as suggested by various theoretical proposals. The authors discuss distinct experimental features, which distiguish the case where this newly reported CDW is driven by a PDW and the case where the CDW is the primary order, and suggest follow-up experiments in order to distinguish unambiguously these two scenarios. The PDW-driven scenario is then put into a broader context in relation to the pseudo-gap phenomenology, H${}_{c}$${}_{2}$, and different kinds of CDWs.

[Phys. Rev. B 97, 174511] Published Mon May 14, 2018

]]>Frustrated spin systems prevent the formation of conventional magnetic order and can form an entangled spin state. A frustrated square lattice with only nearest-neighbor interactions is called a fully frustrated square lattice (FFSL) and expected to exhibit strong quantum fluctuations. Here, the authors present an experimental realization of an $S$=½ FFSL composed of a verdazyl-based salt. $A\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ calculations indicate the formation of an FFSL with competing ferro- and antiferromagnetic nearest-neighbor interactions. The magnetic behavior demonstrates that the system forms a quantum valence-bond solid state. This material provides a way to realize a variety of $S$=½ FFSLs through the molecular design method, proposed here.

[Phys. Rev. B 97, 201109(R)] Published Mon May 14, 2018

]]>SmB${}_{6}$ is a mixed-valence, strongly correlated Kondo insulator. Moreover, its band structure makes it a promising contender in the quest for one of the first systems with topologically nontrivial and correlated surface states. Despite the “topological protection”, surface layer reconstructions can fundamentally change the material’s properties, so that the topological state is expelled into the undisrupted bulk. Following previous indications, the authors combine resonant reflectivity, photoemission, and theory to uncover chemical and valence reconstruction and its dynamics, resulting in boron termination and Sm${}^{3+}$ subsurface dominance. This work reconciles earlier studies using more surface-sensitive techniques, such as photoemission and scanning tunneling spectroscopy.

[Phys. Rev. B 97, 205416] Published Mon May 14, 2018

]]>Optical nonlinearities usually occur at large intensities, but discrete transitions allow for giant nonlinearities operating at the single-photon level. Here, the authors demonstrate a two-mode giant nonlinearity with a single semiconductor quantum dot, embedded in a broadband photonic wire antenna. They exploit two detuned optical transitions associated with the exciton-biexciton quantum-dot level scheme. The reflection of one laser beam is controlled by the other beam with a threshold power as low as ten photons per exciton lifetime (1.6 nW). Such a two-color nonlinearity opens up appealing perspectives for the realization of integrated ultralow-power logical gates and optical quantum gates.

[Phys. Rev. B 97, 201106(R)] Published Fri May 11, 2018

]]>The authors deal with the fundamental problem of a rigorous classification of quantum topological phases. Here, they extend to the realm of interacting fermions in two dimensions techniques, developed for bosons, whereby they introduce extrinsic defects carrying symmetry fluxes into the topological phase. The result is a complete classification of symmetry-protected topological phases of interacting fermions in two dimensions. In addition, the classification of interacting fermion systems with time-reversal symmetry is also developed. The advances, reported here, are immediately relevant for the selection of reliable candidates for topologically protected storage and communication.

[Phys. Rev. B 97, 205109] Published Wed May 09, 2018

]]>Understanding the phases of a model usually requires knowledge of their characteristic features, which are nonlocal in topologically ordered systems. Here, the authors reframe the phase classification problem in disordered topological superconductors as a data-driven task, motivated by the recent surge of interest in the application of machine-learning techniques including deep learning. It is demonstrated that an artificial neural network learns to extract the essence of the clean system and successfully distinguishes phases even under disorder by statistical recovery of translational symmetry.

[Phys. Rev. B 97, 205110] Published Wed May 09, 2018

]]>Silver is a popular choice of substrate for the growth of nanostructured 1D and 2D silicon – possibly silicenelike – layers. First-principles simulations and measurements of reflectance anisotropy and surface differential reflectivity spectroscopy are applied to the study of single monolayer Si/Ag(111) and Si/Ag(110) interfaces. The authors demonstrate the absence of structural and electronic properties of freestanding, graphenelike silicene, finding evidence for strong Si-Ag bonding in the Si/Ag(111) case and the formation of pentagonal nanoribbons on Si/Ag(110). Many-body effects in the optical response of silicene are found to cancel out.

[Phys. Rev. B 97, 195407] Published Mon May 07, 2018

]]>The authors observe unreported features of magnetic anisotropy in the dilute ferromagnetic semiconductor (Ga,Mn)As. Magnetization and ferromagnetic resonance studies provide experimental evidence that, in a specific range of temperatures and hole concentrations, the $<110>$ in-plane directions become the easy axes of the cubic contribution to magnetic anisotropy in (Ga,Mn)As(001). This result has significant supportive value for the theoretical description of ferromagnetism in (III,Mn)V dilute ferromagnetic semiconductors by the $p$-$d$ Zener model. However, to provide a comprehensive quantitative account of these findings, incorporation of scattering broadening of the hole density of states, single-ion magnetic anisotropy of Mn, and disorder-driven nonuniformities of the carrier density have to be accomplished.

[Phys. Rev. B 97, 184403] Published Thu May 03, 2018

]]>Detailed knowledge of the gap function in iron-based superconductors is a requisite to identify the mechanism of superconductivity in these materials. Here, the authors present a systematic study of the superconducting gap distribution on the three-dimensional Fermi surfaces of FeSe using synchrotron-based angle-resolved photoemission spectroscopy. The results imply a significant anisotropy of the superconducting gap on all parts of the FeSe Fermi surface not only in-plane, but also as a function of ${k}_{z}$. The observed in-plane anisotropy can be qualitatively understood in terms of orbital-selective Cooper pairing and nematicity-induced mixing of $s$- and $d$-pairing channels.

[Phys. Rev. B 97, 180501(R)] Published Wed May 02, 2018

]]>A multiorbital quantum impurity with effective spin S>1/2 connected to several reservoirs of conduction electrons can lead to a Kondo effect, exhibiting multistage screening. The ground state of a prototypical nanoscale device consisting of an S=1 quantum impurity connected to two conducting reservoirs is a spin singlet, formed via a two-stage Kondo screening mechanism. Understanding the equilibrium and nonequilibrium properties of the strong-coupling regime resulting from this two-stage screening has been an open problem for many years. Here, the authors offer a detailed description in terms of an effective local two-color Fermi liquid theory.

[Phys. Rev. B 97, 195403] Published Wed May 02, 2018

]]>Majorana-based qubits are candidates for qubits with long coherence times. Just like conventional qubits, these qubits require active error correction in order to run arbitrarily long quantum computations. Unlike conventional quits though, Majorana-based qubits can use not only qubit-based codes, but also Majorana fermion codes for error correction. Several proposals for Majorana-based quantum computing with and without error correction exist, each featuring individual strategies to implement a universal set of gates. This paper unites all these approaches in a general framework, in which a certain set of operations—Clifford gates—are implemented with zero time overhead.

[Phys. Rev. B 97, 205404] Published Wed May 02, 2018

]]>Periodic driving of an interacting spin system may lead to unusual out-of-equilibrium states of matter, such as the discrete time crystal (DTC). Two recent experiments have reported a key signature of the DTC: for long enough interaction times, the system’s response becomes rigidly locked to one half of the driving frequency. Using nuclear magnetic resonance, the authors observe this DTC signature in the most unexpected system to date: a three-dimensional, spatially ordered crystal. This challenges the paradigm that the DTC arises from many-body localization. Furthermore, hidden coherence in the system is revealed by the “DTC echo” experiment, and the effect of interactions during drive pulses is demonstrated.

[Phys. Rev. B 97, 184301] Published Tue May 01, 2018

]]>Cd${}_{3}$As${}_{2}$ with its linearly dispersing bulk state is generally considered an archetype of the three-dimensional Dirac semimetal phase. The present investigation of its electronic properties calls for a revision of this crude assumption. Indeed, Cd${}_{3}$As${}_{2}$ exhibits both a surface state and two bulk bands, dispersing across the Fermi level. Hence, all these states must contribute to the unique material electrical transport properties, but with very different effective masses, band velocities, and different binding energies with respect to the chemical potential. This results in possible ambipolar charge carrier transport at the surface.

[Phys. Rev. B 97, 165439] Published Mon Apr 30, 2018

]]>A remarkable feature of the excitation spectrum of chromite spinels, where antiferromagnetically coupled Cr${}^{3+}$ ions form a highly frustrated pyrochlore lattice, is the existence of so-called resonance modes. These resonances resemble the zero-energy modes that have been theoretically predicted for the pyrochlore Heisenberg antiferromagnet and other highly frustrated systems. Here, the authors investigate the complicated structural and magnetic states in MgCr${}_{2}$O${}_{4}$ spinel, interpret the magnetic excitations as acoustic and optical spin wave branches, and show that the neutron scattering cross sections of transitions within a unit of two corner-sharing tetrahedra match the observed intensity distribution of the resonances.

[Phys. Rev. B 97, 134430] Published Fri Apr 27, 2018

]]>In this paper, the authors introduce the concept of recoverable information, which is well-defined for any stabilizer code. The concept illuminates universal properties of a phase analogous to that captured by entanglement entropy, since each bit of recoverable information is related to an independent ${\mathbb{Z}}_{2}$ Gauss-law type relation for excitations as satisfied by the entanglement surface. Since the interpretation of entanglement structure is poorly understood in three-dimensional fracton models, the authors calculate recoverable information for type-I and type-II stabilizer fracton models via three different methods, demonstrating the topological properties such models possess.

[Phys. Rev. B 97, 134426] Published Thu Apr 26, 2018

]]>The Bohr-Sommerfeld quantization rule lies at the heart of the semiclassical theory of a Bloch electron in a magnetic field. This rule is predictive of Landau levels and quantum oscillations for conventional metals as well as for a host of topological metals, which have emerged in the recent synergy between band theory, crystalline symmetries, and topology. Here, the authors systematically formulate quantization rules that apply to energy-degenerate bands and to orbits that intersect at saddle points and type-II Dirac points where quantum tunneling is unavoidable. The formulation extends previous work by incorporating the orbital magnetic moment and the geometric Berry phase. The latter is an indispensable property of topological metals.

[Phys. Rev. B 97, 144422] Published Thu Apr 26, 2018

]]>Silicon qubits can show a strong interplay between spin and valley physics. The spin is much more robust to decoherence than the valley degree of freedom, but is more difficult to address electrically. Here, the authors show via atomistic simulations that a spin qubit can be continuously transformed into a valley qubit for manipulation, then brought back to the spin regime to benefit from the longer lifetimes. This transformation relies on intrinsic spin-orbit coupling and suitable engineering of the electric field in the device. This opens up new possibilities for the design of robust and electrically addressable silicon qubits.

[Phys. Rev. B 97, 155433] Published Thu Apr 26, 2018

]]>The ultrafast manipulation of the magnetization using femtosecond laser pulses is of utmost importance for both fundamental physics and technological applications. Here, the authors investigated the laser-induced ultrafast magnetization dynamics in a rare-earth iron garnet over a broad temperature range including the magnetization compensation point T${}_{\mathrm{M}}$. They find that the heat energy resulting from exciting the phonon-assisted $d$-$d$ transitions induces large-amplitude magnetization dynamics at temperatures slightly below T${}_{\mathrm{M}}$. They also demonstrate that the speed and the amplitude of the magnetization dynamics can be controlled by tuning either the laser energy density or the amplitude of the external magnetic field. The obtained results are explained by a magnetization reversal process across T${}_{\mathrm{M}}$.

[Phys. Rev. B 97, 134419] Published Thu Apr 19, 2018

]]>A material with broken space-inversion and time-reversal symmetries can exhibit symmetry-dependent unique phenomena such as the magnetoelectric effect. Ferroic order of magnetic quadrupoles fulfills this symmetry condition. The authors report here the discovery of ferroic magnetic quadrupole order and its magnetoelectric activity in the novel compound Pb(TiO)Cu${}_{4}$(PO${}_{4}$)${}_{4}$, which is in contrast with isostructural Ba(TiO)Cu${}_{4}$(PO${}_{4}$)${}_{4}$ and Sr(TiO)Cu${}_{4}$(PO${}_{4}$)${}_{4}$ in exhibiting antiferroic quadrupole order. Their first-principles calculations reveal that ${s}^{2}$ electrons in Pb${}^{2+}$ ions alter specific magnetic interactions and consequently switch the quadrupole order from antiferroic to ferroic. This result provides not only an opportunity to explore magnetoelectric phenomena arising from magnetic quadrupoles, but also a useful way of fine-tuning magnetic interactions with ${s}^{2}$ electrons.

[Phys. Rev. B 97, 134418] Published Wed Apr 18, 2018

]]>Carbon point defects in graphene induce local magnetic moments in the unbound σ orbital as well as in a ferromagnetically coupled bound state in the $\pi $ conduction band. The local curvature of the graphene sheet determines the hybridization strength to the originally orthogonal conduction band. Here, the authors investigate a two-orbital model, comprising the two localized orbitals as a function of the gate-controlled chemical potential. They identify three different low-temperature regimes. The corresponding distinct spectral functions agree well with recent scanning tunneling data, obtained on different carbon point defects.

[Phys. Rev. B 97, 155419] Published Wed Apr 18, 2018

]]>Surface-enhanced Raman scattering (SERS) describes the huge enhancement of the Raman intensity by plasmonic near-fields. The authors investigate SERS caused by a localized surface plasmon by coupling a plasmonic gold nanodimer with the nonresonant Raman scatterer graphene. They perform comprehensive Raman scattering experiments on the coupled system to understand polarization and wavelength dependence of plasmonic enhancement. Remarkably, the near-field resonance from Raman scattering differs by almost 0.2 eV from the far-field resonance measured by dark-field spectroscopy, which is well beyond the expected difference. Graphene is an excellent material to fundamentally study the effects and interactions that give rise to SERS.

[Phys. Rev. B 97, 155417] Published Tue Apr 17, 2018

]]>Recent experiments have revealed a variety of symmetry-breaking phenomena inherent to the pseudogap state of underdoped cuprates. The authors consider emergent phases in a model where the interaction between fermions and antiferromagnetic fluctuations affects two extended regions of the Fermi surface instead of isolated ‘hot spots.’ Surprisingly, this leads to a rich phase diagram including various charge and staggered bond current orders. The uncovered phases share some features with the experimental signatures of the pseudogap state. The results open up perspectives for new types of particle-hole order in itinerant systems with strong spin fluctuations.

[Phys. Rev. B 97, 165125] Published Mon Apr 16, 2018

]]>The authors discuss a lower bound of nonlocal entanglement entropy (a generalization of topological entanglement entropy). The lower bound applies to Abelian conventional topological order as well as to type-I and type-II fracton models. It provides the physical intuition that each type of topological excitation, created by a deformable string, membrane, or fractal operator, gives a contribution to the nonlocal entanglement entropy. For fracton models, the lower bound is geometry dependent; it can take extensive values.

[Phys. Rev. B 97, 144106] Published Thu Apr 12, 2018

]]>The nature of the coupling within and between lattice and charge degrees of freedom is central to condensed matter and materials physics. However, these interactions have so far resisted a comprehensive time-resolved experimental investigation. The authors demonstrate that ultrafast electron diffuse scattering provides direct, momentum-resolved measurements of the electron-phonon and phonon-phonon coupling strengths in graphite. Through the technique’s ability to follow individual phonon population dynamics with femtosecond time resolution, this method, complimented by angle-resolved photoemission spectroscopy, can provide a complete picture of the dynamics within and between electron and phonon subsystems, and help unravel the physics of complex phases where the intertwined nature of the electron-lattice systems determine material properties.

[Phys. Rev. B 97, 165416] Published Thu Apr 12, 2018

]]>Knowing the electron filling alone is sometimes sufficient to eliminate the possibility of featureless insulators, since the material with a certain filling must exhibit either spontaneous symmetry breaking with concomitant gapless excitations or fractionalization. The last possibility is theoretically and experimentally the most exciting scenario; it is believed to occur in quantum spin liquids. The absence of magnetic ordering is usually taken as a prerequisite for such exotic phases, but the filling constraint explored in this paper will be a guiding principle in the search for a phase with coexisting topological and magnetic order.

[Phys. Rev. B 97, 165117] Published Wed Apr 11, 2018

]]>Nodal line systems are a new class of three-dimensional topological semimetals of great current interest. They are characterized by energy-band touching that forms a closed loop in momentum space and exhibits a topological $\pi $ Berry phase in loops linking the nodal line. The authors study their associated magnetic quantum oscillation signatures for Fermi surfaces of various geometries. Landau levels of extremal orbits and their associated Berry phase are computed. They can serve as basic topological signatures in magnetic transport measurement of nodal line systems.

[Phys. Rev. B 97, 165118] Published Wed Apr 11, 2018

]]>Up to now, transport spectroscopy of superconducting islands with strong spin-orbit coupling has revealed a robust zero-bias peak in various setups upon applying a magnetic field. While this observation is consistent with the formation of topologically nontrivial Majorana bound states, a careful distinction from topologically trivial Andreev bound states is a vital step. Here, the authors show that two-terminal Andreev bound states can be distinguished from two-terminal Majorana bound states by an interference experiment. Such experiments can be implemented by using current fabrication capabilities and may also be useful to probe quasiparticle poisoning rates for nonisolated islands.

[Phys. Rev. B 97, 161401(R)] Published Tue Apr 10, 2018

]]>Spin-orbit coupling (SOC) effects on charge carriers in organic light emitting diodes (OLEDs) impact spin lifetimes, spin-transport properties, and magnetic-field effects. Previous attempts to measure SOC in organic semiconductors have been indirect. Here, the authors utilize quantum chemistry to predict the effect of SOC on magnetic resonance line shapes as determined by the microscopic $g$-tensor. To test the predictions, they measure electrically detected magnetic resonance on OLEDs at high fields of up to 12T. The results indicate that the structural disorder of the polymer has only weak influence on SOC and that spin-related phenomena in OLEDs are fundamentally monomolecular in nature.

[Phys. Rev. B 97, 161201(R)] Published Mon Apr 09, 2018

]]>In analogy to the case of crystal-lattice vibrations in polar compounds, the coupling to low-energy plasmons in highly doped semiconductors may influence profoundly the electron band structure, leading, e.g., to the relaxation of hot carriers, increase of electronic linewidths, and formation of satellites in the angle-resolved photoemission spectrum. A first-principles theory to account for these phenomena is developed here. Its application to the paradigmatic case of the photoemission spectrum of anatase TiO${}_{2}$ reveals a complex interplay of plasmon and phonon satellites.

[Phys. Rev. B 97, 165113] Published Mon Apr 09, 2018

]]>The electronic configuration of Eu${}^{+2}$ is 4${f}^{7}$ with spin $S$=7/2 and angular momentum $L$=0. Thus, the paramagnetic effective moment is expected and usually found to be ${\mu}_{\text{eff}}$ $\approx $ 7.94 ${\mu}_{\text{B}}$/Eu. Instead, crystals of the helical antiferromagnet EuCo${}_{2}$As${}_{2}$ exhibit ${\mu}_{\text{eff}}$ $\approx $ 8.5 ${\mu}_{\text{B}}$/Eu. $A\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ calculations show that this enhancement arises from ferromagnetic polarization of the conduction electron spins around each Eu spin. Agreement of the anisotropic magnetic susceptibility of the Eu helix in EuCo${}_{2}$As${}_{2}$ with unified molecular field theory is also found.

[Phys. Rev. B 97, 144403] Published Fri Apr 06, 2018

]]>Density functional calculations with the exact exchange (EXX) could potentially lead to an improved description of the electronic structure of surfaces and slabs. As soon as EXX is utilized, plane-wave pseudopotential (PWPP) calculations for slabs suffer from limitations of the plane-wave representation of the Kohn-Sham states in the vacuum between the replicas of the slab. Far outside the slab, however, the PWPP states can be extended by the analytically known asymptotic states of a single slab in a rather robust fashion. The resulting real-space states allow us to perform EXX calculations with the Krieger-Li-Iafrate approximation for very wide vacua.

[Phys. Rev. B 97, 155112] Published Fri Apr 06, 2018

]]>There has recently been renewed interest in improper ferroelectricity due to the discovery of a “hybrid” mechanism by which two nonpolar lattice modes couple to a third polar mode. Here, the authors demonstrate that the polar ground state of perovskite HgMn${}_{3}$Mn${}_{4}$O${}_{12}$ emerges due to electronic instabilities that drive charge and orbital ordering on the A- and B-sites. These order parameters couple to polar displacements (P) in a manner conceptually similar to hybrid improper ferroelectricity. The results open up a possible route for electronic control of orbital ordering - an appealing prospect given the technologically important properties dependent on such states.

[Phys. Rev. B 97, 144102] Published Thu Apr 05, 2018

]]>Electron and nuclear spins at impurities embedded in solids have been identified as promising elementary bricks for possible quantum computing applications. Interstitial Ga${}^{2+}$ defects in GaAsN have been emerging as an alternative system possessing the potential to combine the characteristics of deep and well-isolated paramagnetic centers with those of an electrically and optically addressable semiconductor.

[Phys. Rev. B 97, 155201] Published Thu Apr 05, 2018

]]>As is well known, graphene is not flat and develops ripples even when freestanding, due to thermal and quantum out-of-plane fluctuations. Atomistic path integral quantum Monte Carlo simulations show how the well-known large and locally smooth thermal ripples gradually cross over upon cooling to globally flatter, yet locally rougher quantum ripples, giving graphene a surprising quantum corrugation, as shown in the image. As a result, even at cryogenic temperatures, graphene is locally tilted by as much as several degrees over the average plane. This spectacular manifestation of quantum zero-point motion in a macroscopic membrane will probably be measurable by electron diffraction at temperatures below 50 K.

[Phys. Rev. B 97, 140301(R)] Published Wed Apr 04, 2018

]]>Inhomogeneous broadening in ensembles of semiconductor quantum dots (QDs) has hindered coherent operations due to the detuning effects, caused by the large fluctuation in the QD transition energy due to size dispersion. This difficulty is especially evident when using femtosecond laser pulses for excitation. Here, the authors successfully measure detuning-dependent coherent evolution for a QD ensemble by employing prepulse two-dimensional coherent spectroscopy. The dephasing mechanism is found to be significantly different from that observed using picosecond lasers. This work presents a novel method to explore collective phenomena, such as superfluorescence, as well as motivating future studies of coherent interactions in the femtosecond regime.

[Phys. Rev. B 97, 161301(R)] Published Wed Apr 04, 2018

]]>The authors present a generic lattice construction for a generalized X-cube model of fracton topological order and propose a coarser definition of phase of matter for fracton orders. Similar to other fracton models, this model hosts emergent subdimensional excitations, which are topologically robust excitations that are confined to only move along certain lines or surfaces. The model is constructed from stacks of intersecting surfaces, and the subdimensional excitations are confined to move only along these (possibly curved) surfaces or their intersections. Surprisingly, for certain curved intersecting surfaces, a robust ground-state degeneracy on a manifold with trivial topology can result.

[Phys. Rev. B 97, 165106] Published Wed Apr 04, 2018

]]>In a Fermi liquid, the stability of the Fermi surface to deformation is encoded in the angular moments of the interaction between quasiparticles via the Pomeranchuk criterion ${F}_{l}$ $\ge $ $-1$. Interestingly, charge and spin conservation prevent interactions from destabilizing the $l$=1 channel even when ${F}_{l}$ = -1, indicating that high-energy processes play a nontrivial role in the low-energy Fermi liquid dynamics. The authors identify, analytically and numerically, that conservation laws imply an equivalence between processes near and away from the Fermi surface, preventing $l$=1 instabilities. Then, they construct $l$=1 order parameters without this equivalence, identifying possible novel instabilities in strongly correlated electron systems.

[Phys. Rev. B 97, 165101] Published Mon Apr 02, 2018

]]>Recent measurement by Banerjee et al. (2018) of the thermal conductance of a $\nu $ = 5/2 fractional quantum Hall edge has given a surprising result that contradicts prior theory. The current paper proposes a possible solution to this connundrum. The expected structure of the edge of a 5/2-droplet includes charge modes and an exotic neutral “Majorana” edge mode. The current paper shows that if the Majorana edge mode remains out of thermal equilibrium with the charge modes, the experiment can be fully explained.

[Phys. Rev. B 97, 121406(R)] Published Thu Mar 29, 2018

]]>As previously explored with sophisticated computational methods, projecting out incomplete components of Bloch wave functions yields ‘states’ that appear to be ‘spin polarized’. Here, the consequences and physical relevance of this so-called hidden spin polarization obtained by wave-function truncation is revealed by inspection of analytically solvable models. Importantly, the limitations and shortcomings of this electronic structure decomposition are illuminated using arguments focused on operator commutativity and broken gauge freedom in degenerate wave-function projection. Moreover, it is shown how surface-sensitive measurements using experimental techniques, such as ARPES, are inevitably convoluted by intrinsic spin splitting of the band structure due to broken inversion symmetry at the surface.

[Phys. Rev. B 97, 125434] Published Thu Mar 29, 2018

]]>Conversion between free-space and guided-wave propagation modes is an essential function in numerous microwave, optical, and acoustical devices. The performance of currently available wave couplers is far from perfect due to the presence of parasitic scattering and reflections, which have been considered unavoidable up till now. The authors propose a technique for converting a homogeneous plane wave into a surface wave with linear power growth using metasurfaces. With a dramatic enhancement of conversion efficiency, the proposed metasurface wave couplers will find numerous applications ranging from perfect leaky-wave antennas in the microwave range to ideal photovoltaic cells in the visible range.

[Phys. Rev. B 97, 115447] Published Wed Mar 28, 2018

]]>Narrow band gap III-V based nanowires are subject to intense research, notably thanks to the exceptional mobility of the carriers. Moreover, due to the strong spin-orbit coupling combined with the ability to host superconductivity, they provide a platform for the realization of Majorana fermions. Here, the authors performed very high field magnetotransport measurements (50T) on InAs nanowires in both longitudinal and perpendicular configuration, giving access, in the quasiballistic regime, to the energy spectrum. The experimental results are supported by band-structure calculations in the tight-binding approximation. Finally, under very strong magnetic field, the authors observed the formation of Landau states and cyclotron orbits in both configurations.

[Phys. Rev. B 97, 125308] Published Tue Mar 27, 2018

]]>The recently discovered fermion-induced quantum critical points of Dirac materials present a new opportunity to revisit and extend the standard paradigms of quantum criticality. The appearance of a second diverging length scale at a continuous quantum phase transition is one such extension. The authors present a functional renormalization group study of fermion-induced quantum critical points in the broken-symmetry phase, in which the presence of gapless fermions and discrete symmetry breaking provide the natural setup for this phenomenon to occur.

[Phys. Rev. B 97, 125137] Published Thu Mar 22, 2018

]]>A novel phase transition with a stepwise evolution of odd-number charge modulation is discovered in $\beta $-vanadium bronzes under pressures of about 1 GPa. All the charge modulation vectors of the many kinds of charge-ordered phases can be represented as a primitive lattice translation vector along the $b$ axis multiplied by several odd numbers. This discovery demonstrates interplay between the charge degree of freedom and the crystallographic symmetry. This phenomenon is rationally explained by self-charge transfer (carrier redistribution) between two kinds of subsystems in the structure and sequential symmetry reduction as outlined in Landau theory of phase transitions.

[Phys. Rev. B 97, 125138] Published Thu Mar 22, 2018

]]>The freezing temperature of solid hydrogen may be significantly lowered below its bulk value by encapsulating the sample in a restricted geometry. The authors report on an electron-spin resonance (ESR) study of H atoms, stabilized in highly porous solid mixtures of neon and molecular hydrogen at temperatures below 1 K. The ESR analysis shows that H atoms stabilize into two different substitutional sites of the Ne lattice and into H${}_{2}$ clusters, formed in solid neon. The hydrogen atoms in H${}_{2}$ clusters rapidly recombine upon raising the temperature from 0.1 to 0.3–0.6 K, which is attributed to a solid-liquid transition in the H${}_{2}$ clusters formed in solid Ne.

[Phys. Rev. B 97, 104108] Published Wed Mar 21, 2018

]]>In plasmonic dimers, tunneling of electrons through the gap between the nanoparticles is often seen as a major disadvantage due to the quenching of the near fields associated to it. Nevertheless, tunneling is a highly nonlinear process and thus it might be advantageous for certain nonlinear applications. Using time-dependent density functional theory, the authors investigate how the second- and third-harmonic generation is influenced by electron tunneling in plasmonic dimers. They show that the nonlinear currents associated to the tunneling process in ultranarrow gaps can strongly enhance the high-harmonic generation.

[Phys. Rev. B 97, 115430] Published Mon Mar 19, 2018

]]>Friction between sliding objects is a phenomenon so ubiquitous that it is easily forgotten how few of its microscopic details are properly understood. To gain new insight into the dynamical mechanism of sliding dissipation, the authors investigate a minimal model of a rigid slider traveling over a harmonic substrate. With the analytical tools of linear response theory, the authors obtain an expression for the dynamical friction that depends on known static physical quantities. This result sheds new light on the physical intertwining between dissipation and the phononic degrees of freedom of the substrate.

[Phys. Rev. B 97, 104104] Published Thu Mar 15, 2018

]]>Periodic photonic structures such as metamaterials, plasmonic lattices, and excitons strongly coupled to photonic crystals are governed by non-Hermitian wave operators with explicitly energy-dependent potentials. The resulting eigenstates no longer form a basis of the vector space of physical states, nor are they normalizable via the common scalar product. This poses particular problems for the calculation of Green functions and densities of states. Here, the authors solve this challenge via the adjoint operator and modes for arbitrary dispersive and lossy periodic systems. This work can be readily adapted to nonoptical complex band-structure problems with energy-dependent potentials, e.g., within solid-state theory.

[Phys. Rev. B 97, 104203] Published Thu Mar 15, 2018

]]>Three-dimensional topological Dirac semimetals are a new state of matter. The authors report on experiments that directly access the electronic structure of two-dimensional states in the prototype topological Dirac semimetal Cd${}_{3}$As${}_{2}$. By modulating the carrier density of very thin, epitaxial Cd${}_{3}$As${}_{2}$ films, using the electric field effect, the authors tune the electronic states through a two-dimensional Dirac point, as evidenced by a minimum in the density of states. Moreover, the linear energy dispersion and the presence of a zero-energy Landau level establish the topological nature of the two-dimensional states in this three-dimensional Dirac semimetal.

[Phys. Rev. B 97, 115132] Published Thu Mar 15, 2018

]]>Magnetic atomic chains grown on top of conventional superconductors may form an interesting and promising platform for the study of Majorana zero modes. Despite the strong evidence already provided with a state-of-art scanning tunneling microscope (STM) technique, clear features beyond the energetic and spatial ones are required to exclude the only remaining alternative interpretation of the zero modes as trivial Shiba states accidentally occurring at zero energy. The authors here propose a robust spin signature for this purpose. This signature is rooted in two sum rules that dictate the distribution of spin densities in a superconducting state with respect to a normal state, and has been recently detected with spin-polarized STM technique that implicitly takes advantage of the sum rules.

[Phys. Rev. B 97, 125119] Published Thu Mar 15, 2018

]]>The authors investigate the development of magnetic phases in (Ga,Mn)As – the canonical dilute ferromagnetic semiconductor – in a thoroughly prepared set of high-quality layers grown by molecular beam epitaxy. The Mn content in the (Ga,Mn)As solid solution extends from the extremely diluted (0.0007% Mn) to the medium Mn content of 1.6%. The authors trace simultaneously the evolution of magnetic and electronic properties of (Ga,Mn)As by comprehensive magnetic (SQUID magnetometry) optical (Raman scattering and photoreflectance) and ARPES measurements. The onset of a low-temperature ferromagnetic phase in (Ga,Mn)As is revealed for 1.6% Mn, the composition just slightly above the one displaying purely superparamagnetic behavior (0.9% Mn). No signature of a Mn-related impurity band has been identified in superparamagnetic and ferromagnetic (Ga,Mn)As.

[Phys. Rev. B 97, 115201] Published Tue Mar 13, 2018

]]>Quantum gas microscopes are a promising tool to study interacting quantum many-body systems and bridge the gap between theoretical models and real materials. One of the most powerful experimental methods in solids is angle-resolved photoemission spectroscopy (ARPES), which measures the single-particle spectral function. The authors propose a measurement scheme to experimentally access the momentum- and energy-resolved spectral function in a quantum gas microscope. As an example for possible applications, the spectrum of a single hole excitation in one-dimensional $t$-$J$ models is calculated and analyzed. A sharp asymmetry in the distribution of spectral weight, reminiscent of the Fermi arcs observed in the pseudogap phase of cuprates, appears in the case of an isotropic Heisenberg spin chain.

[Phys. Rev. B 97, 125117] Published Tue Mar 13, 2018

]]>An insulating state in graphite, which appears in a strong magnetic field around 30 T, may have its origin in an exotic density-wave state - a valley-density wave state. Experimental verification, however, has been a challenging problem. By reducing the thickness of graphite, the authors demonstrate that this insulator state becomes unstable. This thickness dependence implies evolution of the ordered state along the out-of-plane direction. The phase transition lines of each thickness can be qualitatively reproduced by the density-wave model. This thinning approach, free from introducing defects or carriers, will help understand the entire phase diagram of graphite.

[Phys. Rev. B 97, 115122] Published Mon Mar 12, 2018

]]>Recent experiments [Science 357, 294 (2017)] have observed a half-quantized electrical conductance plateau, a proposed signature of chiral Majorana fermions. Such Majoranas have been argued to occur generically when a quantum anomalous Hall insulator is tuned to its plateau transition and proximitized by a superconductor. The authors show here that such half-quantized conductance can also occur in this system even in the absence of chiral Majorana fermions, once experimental disorder and temperature are considered. This work therefore motivates the search for more compelling evidence of chiral Majorana fermions in this system, such as quantum coherent interferometry or half-quantized thermal conductance.

[Phys. Rev. B 97, 100501(R)] Published Fri Mar 09, 2018

]]>Two-dimensional chiral topological superconductors can be realized in clean quantum anomalous Hall insulators under $s$-wave superconducting proximity, as is predicted by band theory. Under disorder, random domain walls hosting chiral Majorana fermion arise in space, and conventional band theory becomes invalid. Employing percolation theory and the renormalization group method, the authors show that the disordered system still has a stable chiral topological superconductor phase, and exhibits a half-quantized conductance plateau as a feature. The critical exponents and temperature dependence of the conductance plateau and transitions are obtained, which can be tested by experiment.

[Phys. Rev. B 97, 125408] Published Fri Mar 09, 2018

]]>The control of localized light modes is extremely important both for fundamental studies and for practical applications. Most past studies focused on gaining control over the mode frequency, while the mode spatial distribution, which is equally important in governing radiative interactions, has usually been harder to manipulate. Here, the authors have investigated theoretically and experimentally a photonic crystal device where a sub-100 nm displacement, controlled by an applied voltage, can strongly reshape the field profiles. As a first application, the authors demonstrated that this effect can be used to strongly modify the optical $Q$ factors. A loss modulation by more than a factor five is measured experimentally, a value that could be increased further with the help of optimized fabrication.

[Phys. Rev. B 97, 115304] Published Thu Mar 08, 2018

]]>Understanding electromagnetic heat transfer in many-body systems is critical to exploring new physics effects that can arise in reciprocal and nonreciprocal many-body systems. Here, the authors develop explicit and compact formulas for electromagnetic heat transfer in systems consisting of a large number of bodies without the constraint of reciprocity. They go on to demonstrate the emergence of a persistent heat current at thermal equilibrium and a directional heat flow at nonequilibrium in systems composed of multiple magneto-optical particles. This opens up possibilities for controlling near-field electromagnetic heat transfer using complex reciprocal and nonreciprocal systems.

[Phys. Rev. B 97, 094302] Published Wed Mar 07, 2018

]]>Helium bubbles can form in a wide variety of materials and are of significance in a great variety of circumstances, ranging from electronics through nuclear physics or geology to outer-space exploration. However, their properties remain elusive, especially at the nanometer scale. Here, the authors study by means of transmission electron microscopy the structural modification and, simultaneously, the helium emission from individual nanosized bubbles in silicon during $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0.333em}{0ex}}s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}u$ annealing. It is shown that, surprisingly, helium emission takes place at temperatures where bubble migration has barely begun. Analytical modeling of helium emission suggests that helium is in its solid state in the most highly pressurized bubbles.

[Phys. Rev. B 97, 104102] Published Wed Mar 07, 2018

]]>Disorder and interactions can lead to the breakdown of statistical mechanics in quantum systems by transitioning to a many-body localized (MBL) phase. The thermal-to-MBL transition occurs by either increasing the amount of disorder present in a system or by decreasing its energy until it crosses the so-called mobility edge. The authors construct a set of approximate integrals of motion that are quasi-universal across the energy spectrum and, thus, across the mobility edge, too. Interestingly, MBL eigenstates under the mobility edge are aware of the existence of a thermal phase at a higher energy. The phase diagram is probed close to the transition and in the MBL phase. One finds a universal distribution of the strength of the couplings of the integrals of motion in the strong disorder limit.

[Phys. Rev. B 97, 104406] Published Wed Mar 07, 2018

]]>During the last two decades, single-photon emission from two-photon Raman processes has been extensively studied both experimentally and theoretically in atomic as well as in quantum-dot multilevel systems. However, despite theoretical predictions on-demand emission, which is crucial for quantum networking, has not been achieved in experiments yet. Based on the cluster-expansion approach, the authors have performed an in-depth microscopic analysis of the Raman transition in fundamental three-level configurations. They find that excitation-induced destructive quantum interference and Stark shifts of the Raman resonance have to be taken into account in order to close in on on-demand single-photon emission.

[Phys. Rev. B 97, 125303] Published Tue Mar 06, 2018

]]>Materials that are simultaneously transparent and electrically conductive play a key role in optoelectronic devices. Barium stannate has recently come to the forefront as a promising new transparent conductor, but its conductivity has been difficult to control. The authors have performed computational studies to determine the mechanisms that limit doping. They find that atomic-scale defects, such as vacancies in the crystal lattice, are responsible, but their formation can be controlled by tailoring the growth conditions. The authors also investigated related materials, such as strontium stannate and calcium stannate, which are transparent even in the ultraviolet. Calcium stannate is found to always be insulating, but guidelines are provided for maximizing the conductivity of the strontium compound.

[Phys. Rev. B 97, 054112] Published Mon Feb 26, 2018

]]>Many properties of the copper oxide high-temperature superconductors are still poorly understood, despite more than thirty years of research. This is true in particular for the metallic “pseudogap” state, out of which the well-known superconducting phase develops upon increasing the concentration of hole-like charge carriers. Here, the authors analyze the single-electron spectral function of a recently introduced quantum dimer model, which features a very interesting metallic ground state with topological order: a so-called fractionalized Fermi liquid. Earlier works have argued that this model describes several key properties of the pseudogap metal, such as Fermi-arc like features in photoemission experiments. Using a combination of numerical and analytical methods, the authors show that this model indeed features a sizable antinodal pseudogap with an angular dependence deviating from a simple $d$-wave form, in accordance with various experimental results on underdoped cuprates.

[Phys. Rev. B 97, 075144] Published Fri Feb 23, 2018

]]>One of the key properties of graphene for spintronics is its capability to transport spins over long distances unaltered. However, the need to manipulate the spins for information processing and storage is also of fundamental importance. The two-dimensional heterostructure composed of tungsten diselenide and graphene allows for manipulating the spins while preserving graphene’s exceptional electron properties. Using weak antilocalization measurements, the authors find an anisotropic spin lifetime with out-of-plane spins living much longer than in-plane spins. This originates from a special spin-orbit coupling that leads to a Zeeman field of opposite sign in the two valleys $K$ and ${K}^{\prime}$.

[Phys. Rev. B 97, 075434] Published Thu Feb 22, 2018

]]>Using large-scale quantum Monte Carlo simulations, the authors unbiasedly demonstrate the existence of a topological Mott insulator – an interaction-driven topological insulator emerging from a strongly correlated Dirac semimetal. The model designed here consists of Dirac fermions and fluctuating Ising fields mediating interactions between fermions. By tuning the fluctuation strength of the Ising fields, the authors show the existence of a topological quantum phase transition between the topological Mott insulator phase and the Dirac-semimetal phase. Furthermore, the quantum critical point was found to be in the (2+1)D $N$=8 Chiral Ising universality class.

[Phys. Rev. B 97, 081110(R)] Published Thu Feb 22, 2018

]]>Superconductivity in two-dimensional materials remains largely unexplored and attracts much attention. Although graphene is not a superconductor, adsorption of alkali metals on its surface may enhance electron-phonon coupling (EPC) sufficiently to induce superconductivity. Here, using photoemission spectroscopy, the authors study the electronic structure and EPC in a Li-doped graphene monolayer weakly bonded to a cobalt substrate. It is shown that EPC is high enough to create superconductivity in graphene, however the nonmagnetic graphene layer is clamped between two oppositely magnetized atomic layers. This condition makes the studied system a promising platform for understanding the influence of magnetic environment on superconductivity at low dimensions.

[Phys. Rev. B 97, 085132] Published Wed Feb 21, 2018

]]>The unique properties of zero-magnetization ferromagnets (ZMF) are well suited for device applications to process the spin of charged particles. The present paper is the first single-crystal polarized neutron diffraction study of the orbital and spin magnetic moments as a function of temperature through the compensation point of the ZMF Sm${}_{1-x}$Gd${}_{x}$Al${}_{2}$. This study is difficult because of enormous absorption of both Sm and Gd with thermal neutrons and was therefore not undertaken before. By using hot polarized neutrons and an applied magnetic field of 8 Tesla, the authors successfully determined spin and orbital moments of Sm${}^{3+}$ ions separately with high accuracy. This study enables a better understanding of the basic microscopic mechanism of ZMF materials.

[Phys. Rev. B 97, 064417] Published Tue Feb 20, 2018

]]>Interest in quantum memory devices based on topological superconductors and their non-Abelian zero modes is motivated by the observation that at zero temperature the information stored in these devices is exponentially protected. In noninteracting systems, due to the fact that the system can be described using normal modes, much of this protection also exists at higher temperatures. The authors examine here how this picture gradually breaks down at higher temperatures when interactions are present, and explore the conjecture that the topological protection may be recovered by disorder-induced localization. They see that the disorder triggers multiple mechanisms which, depending on the parameters of the system, can both enhance and degrade the stability of the topological zero modes.

[Phys. Rev. B 97, 085425] Published Tue Feb 20, 2018

]]>The Ising spin glass in two dimensions is a model system with extraordinarily rich behavior. Although it does not show a spin-glass phase at finite temperatures, it exhibits many of the salient features of fully fledged spin glass systems. The authors use recently developed mappings to graph-theoretic problems together with highly efficient implementations of combinatorial optimization algorithms to determine exact ground states for systems with up to $10,000\times 10,000$ spins. These and further results from new algorithms for fully periodic lattices and for systems with degeneracies allow for the determination of critical exponents with unprecedented accuracy.

[Phys. Rev. B 97, 064410] Published Fri Feb 16, 2018

]]>Semiconductor quantum dots (QDs) are typically entirely uncoupled, and interfacial QDs are no exception. However, exciting higher-energy quantum-well states can turn on QD interactions. Here, the authors measure individual QDs with a coherent spectroscopy that is extremely sensitive to interactions. They demonstrate an induced binding between two distinct QDs with a very weak excitation of delocalized quantum-well states. This interaction is presented as a possible method for turning on coupling between quantum states and for measuring fundamental processes in semiconductors with nanometer spatial resolution.

[Phys. Rev. B 97, 081301(R)] Published Thu Feb 15, 2018

]]>The spin induced ferroelectricity of the $R$Mn${}_{2}$O${}_{5}$ family is ascribed to an exchange-striction mechanism related to the frustrated Mn${}^{3+}$-Mn${}^{4+}$ antiferromagnetic interactions. From an accurate powder neutron diffraction experiment on an isotope-enriched sample of GdMn${}_{2}$O${}_{5}$, the authors detect in this compound a supplementary exchange-striction mechanism responsible for its exceptionally large electric polarization. This additional contribution comes from the release of the frustration between the huge and isotropic Gd${}^{3+}$ moments and the Mn${}^{3+}$/Mn${}^{4+}$ spins. The key ingredients to the atomic displacements are the isotropic Gd${}^{3+}$ electronic structure and the mediation of the Gd/Mn magnetic interactions by the oxygens. This results in a differential dependence of the Gd/Mn magnetic exchanges to the Mn-O distances and not the Gd-Mn ones, as predicted.

[Phys. Rev. B 97, 085128] Published Thu Feb 15, 2018

]]>Rare-earth spins in optical crystals like yttrium orthosilicate (YSO) are of much interest for providing coherent interfaces with both microwave and optical photons. Here, the authors study the spin dynamics of a new system, Yb in YSO, which has favorable optical properties and strong hyperfine coupling to the ${}^{171}$Yb (spin-1/2) and ${}^{173}$Yb (spin-5/2) isotopes. The results demonstrate long coherence times for both the electron and nuclear spins (~1 ms). Employing a range of pulsed electron spin resonance measurements as well as comparison with relevant theory, the study elucidates a number of spin relaxation and decoherence mechanisms at work.

[Phys. Rev. B 97, 064409] Published Wed Feb 14, 2018

]]>The critical properties of the spin-density-wave/semimetal phase transition in Dirac materials are still under debate despite all the efforts from continuum and lattice methods. Using state-of-the-art functional renormalization group methods, the author provides improved estimates of the leading critical exponents. They are in line with earlier results obtained with the same method and in disagreement with recent quantum Monte Carlo estimates. The study supports the conjecture the only adequate nonperturbative treatment of the different universality classes, related to this phase transition, is via the functional renormalization group.

[Phys. Rev. B 97, 075129] Published Wed Feb 14, 2018

]]>Understanding the properties of non-Hermitian topological systems is of critical importance for photonic and acoustic systems, where it is relatively easy to add material gain and absorption. Here, the authors provide a systematic study of the effects of adding gain and loss to Weyl semimetals, whereby Weyl points with arbitrary charge expand into topologically charged rings of degeneracies. This yields a new class of topological transitions, which occur by changing the strength of the gain and loss, rather than altering the symmetry of the system.

[Phys. Rev. B 97, 075128] Published Tue Feb 13, 2018

]]>Magnetic skyrmions are topological spin textures with potential applications in future spintronic devices, but they have a tendency to collapse that can be cured only by a subtle interplay of weak interactions. Here, the authors propose a novel mechanism to stabilize skyrmions in a magnetic monolayer by placing the system on a conducting substrate, such as a normal metal or graphene. This makes the spins interact via the long-range Ruderman-Kittel-Kasuya-Yosida exchange. With a metallic substrate, skyrmions can be stabilized by fine tuning the Fermi surface parameters, while with a graphene substrate the stabilization occurs naturally in several geometries.

[Phys. Rev. B 97, 054408] Published Tue Feb 06, 2018

]]>Angle-resolved secondary electron emission (ARSEE) spectra of atomic sheets have been considered to reflect the unoccupied energy bands of the targets. However, far less is known about the dynamics of electron excitation from the valence bands to the unoccupied bands upon electron impact, leading to emission into the vacuum. Here, the authors keep track of the electrons excited to the unoccupied bands and emitted to the vacuum in real time, position space, and k-space simultaneously by a time-dependent density-functional theory simulation.

[Phys. Rev. B 97, 075406] Published Tue Feb 06, 2018

]]>Subjecting a topologically nontrivial class of noninteracting electronic states to interactions can sometimes yield a “fractional” topologically ordered version of that class. For example, electrons in topological bands with integer-quantized Hall conductivity can generate fractional quantum Hall (FQH) states upon addition of repulsive interactions. However, of all existing material band structures, the topologically nontrivial ones are rare. Here, the author demonstrates theoretically in a minimal setting how strong interactions and symmetry breaking can induce FQH-type topological order even in topologically trivial bands. This finding shows that even topologically trivial band structures – the vast majority – can host topological order.

[Phys. Rev. B 97, 085108] Published Mon Feb 05, 2018

]]>The last few years have seen an explosion of interest in hydrodynamic effects in interacting electron systems in ultrapure materials, where the collective motion of charge carriers may resemble the flow of a viscous fluid. In a confined geometry, such as that provided by ultrahigh quality nanostructures, one expects the electronic fluid to exhibit Poiseuille-type flow profile. Here, the authors show that in two-component systems near charge neutrality the hydrodynamic viscous flow is strongly affected by the mutual friction and recombination between the two constituents leading to a strongly nonuniform current density and nonmonotonic magnetoresistance that happens to be negative (positive) in low (high) magnetic fields.

[Phys. Rev. B 97, 085109] Published Mon Feb 05, 2018

]]>The restricted Boltzmann machine is a fundamental building block of deep learning. The authors demonstrate its equivalence with tensor network states with explicit mappings, thus drawing a constructive connection between deep learning and quantum physics. On one side, deep learning approaches can be used to study novel states of matter. In return, investigations of tensor network states and their expressibility can be adapted to guide neural network architecture design.

[Phys. Rev. B 97, 085104] Published Fri Feb 02, 2018

]]>Sample miniaturization has been recognized to be helpful in obtaining deeply supercooled liquids or metastable crystalline solids. Here, the authors generalize the idea of the size effect with phenomenological simulations, and show that it certainly works in two distinct correlated electron systems. Because the validity of the size effects does not rely on microscopic details of each material, they are expected to be applicable to other correlated electron systems, potentially facilitating the creation of metastable electronic states.

[Phys. Rev. B 97, 085102] Published Thu Feb 01, 2018

]]>Clean experimental testing grounds for strongly interacting electrons confined to one dimension are hard to find. Thus, suspended carbon nanotubes (CNTs) are a unique model system. Previous work showed that electron transport in suspended metallic CNTs can be frozen by strong electron-electron interactions. However, a detailed understanding of this exotic insulating phase has remained elusive. Here, the authors characterize the energy gap associated with the insulating phase in nominally metallic CNTs, and compare the gap to precise measurements of CNT diameter and chirality.

[Phys. Rev. B 97, 035445] Published Wed Jan 31, 2018

]]>Machine learning is a fast developing area that finds applications in all disciplines of science. Here, the authors demonstrate that the machine learning (in particular deep learning) technique can be applied to understand the emergence of spatial geometry from learning the features of quantum many-body entanglement, an idea that was proposed in a recent study of the holography duality in quantum gravity. This work is the first to successfully demonstrate the idea of “geometry emerging from learning”.

[Phys. Rev. B 97, 045153] Published Wed Jan 31, 2018

]]>It has long been known that the magnetic order in a material can be transiently disrupted by an intense laser pulse, on surprisingly fast sub-picosecond timescales. However, two decades after its discovery, the microscopic physics underlying this behavior is still not completely understood. Here, the authors develop a new extreme ultraviolet (EUV) magneto-optical spectroscopy that allows them to extract the full $d\phantom{\rule{0}{0ex}}y\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c$ complex magneto-optical permittivity of a material. By exciting a Co film with a femtosecond laser pulse, and then scanning both the polarization and arrival time of a high-harmonic probe beam, they extract the instantaneous complex magneto-optical constant of Co across the ${M}_{2,3}$ absorption edge. Through a direct comparison with $a\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ theory, they show that the observed demagnetization is dominated by the excitation of magnons, with a possible small contribution from a reduction of the exchange splitting. This work further demonstrates the deepened insight on magnetism obtained through the recent application of tabletop-scale femtosecond EUV light sources.

[Phys. Rev. B 97, 024433] Published Tue Jan 30, 2018

]]>From superconductivity to magnetoelectricity, perovskite oxides exhibit a wealth of appealing physical properties, often controlled by subtle structural details. Especially critical are the ‘tilt’ distortion modes involving rotations of the oxygen octahedra that constitute the backbone of the perovskite lattice, which motivates today’s interest in better understanding and tuning such tilts. Here, the authors present a thorough first-principles investigation of the energy landscape relevant to this matter, revealing the main competitors among different tilt modes as well as their trends across the perovskite family.

[Phys. Rev. B 97, 024113] Published Fri Jan 26, 2018

]]>The authors extend the recent successful application of neural networks fordistinguishing phases of matter to the case of topologically driven phase transitions. They demonstrate that supervised networks correctly identify the Kosterlitz-Thouless transition in the two-dimensional classical XY model. However, their analysis suggests that a naive classification of phases is based on bulk features, rather than on detecting local topological defects. While they show that it is possible to fine-tune a network to recognize vortices, it generally remains a difficult task to detect topological defects without $a\phantom{\rule{0.333em}{0ex}}p\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}i$ knowledge.

[Phys. Rev. B 97, 045207] Published Thu Jan 25, 2018

]]>The authors show here that Kerr rotation is a sensitive probe of valley-dependent energy splitting induced by the optical Stark effect in two-dimensional semiconductors. Kerr rotation rejects the polarization-independent background and probes a complementary dielectric response from established absorption-based techniques, allowing detection of shifts as small as 4 μeV - the lowest reported value so far. The authors apply this improved valley Stark spectroscopy to a wider range of materials by observing Stark shifts of two energetically distinct exciton species in MoS${}_{2}$, representing the first valley- and energy-selective Stark effect in a single material.

[Phys. Rev. B 97, 045307] Published Thu Jan 25, 2018

]]>Electron localization is a crucial ingredient for the observation of the integer quantum Hall effect. However, using exact diagonalization to characterize the transition between consecutive Hall conductance plateaus is hard due to system size limitations. But what if one could remove most of the states in the Landau level, while still retaining the essential ingredients of the transition? The authors here propose a scheme to do this by binding a fraction of the electrons to point-like “impurity” potentials. The remaining states form a flat band, equivalent to a smaller Landau level, which undergoes a plateau transition of the same kind.

[Phys. Rev. B 97, 014205] Published Mon Jan 22, 2018

]]>The nitrogen-vacancy (NV) center in diamond is known to spin-polarize under optical illumination, but how is this polarization transferred to carbon spins in the bulk of the crystal? Here, the authors use field-cycled nuclear magnetic resonance to study the formation of light-driven ${}^{13}$C spin polarization in NV-rich diamond at room temperature. By considering the system’s response to various magnetic fields, it is shown that ${}^{13}$C spins polarize via a cross-relaxation process involving the NV center and the electron and nuclear spins of the substitutional nitrogen impurities. Furthermore, it is shown that high levels of ${}^{13}$C spin polarization can be attained for almost any magnetic field orientation.

[Phys. Rev. B 97, 024422] Published Mon Jan 22, 2018

]]>Fractional quantum Hall effect fluids remain the prototype systems with topological order and strong interactions. They are usually well described as integer quantum Hall states of composite fermions, made of electrons and magnetic flux quanta. The authors investigate a two-dimensional electron gas in a magnetic field at half filling. This system should be described by a Fermi liquid of composite fermions in zero field and is uniquely suited to study the nature of composite fermions as, for instance, to determine if they are Dirac fermions. The composite Fermi liquid is indeed found to give an excellent approximation of the low-energy physics. It also shows unexpected features in the Berry curvature of the composite fermions.

[Phys. Rev. B 97, 035149] Published Mon Jan 22, 2018

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