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

]]>Spin-orbit coupling (SOC) links spin space and real space and leads to solids with intriguing spin topologies and transport properties, such as anisotropic magnetoresistance (AMR). The orientation of a real-space symmetry axis with respect to the spin direction determines the size of SOC-induced changes of the electronic structure. To show this effect at the single-molecule level, the authors arrange Pb dimers on a ferromagnetic Fe layer and observe that the AMR resulting from their molecular orbitals depends strongly on the dimer orientation.

[Phys. Rev. B 97, 041114(R)] Published Mon Jan 22, 2018

]]>The authors here combine polarized neutron scattering experimental work with computational minimization of proposed Hamiltonians in an effort to understand the low-temperature magnetic behavior in small applied magnetic fields for LaCo${O}_{3}$. They provide an exceptionally comprehensive study of the phase diagrams generated by various Hamiltonians germane to a situation in which magnetic ordering occurs at twin boundaries due to antiferromagnetic chains crossing the boundary at an angle different than 90°.

[Phys. Rev. B 97, 024418] Published Fri Jan 19, 2018

]]>Low-dimensional intrinsically ferromagnetic semiconductors are rare and crucial to develop the next-generation spintronics devices. CrI${}_{3}$ is a promising candidate since its ferromagnetism can be maintained upon exfoliating of bulk crystals down to single layer. However, the nature of magnetism in bulk CrI${}_{3}$ is still unclear. Here, the authors experimentally study the critical properties of bulk CrI${}_{3}$ around the paramagnetic to ferromagnetic phase transition, and idenify clearly three-dimensional long-range magnetic coupling with the exchange distance decaying as $J(r)$ ≈ ${r}^{-4.69}$ in bulk CrI${}_{3}$.

[Phys. Rev. B 97, 014420] Published Thu Jan 18, 2018

]]>The authors propose here the first experimentally viable scheme of a topological insulator for sound waves at the nanoscale. This significantly advances the field of nanoscale phononics, adding helical waveguides to its toolbox. The proposed device could hardly be simpler as it relies purely on a geometrical design, based on the “snowflake” phononic crystal that has already been demonstrated experimentally.

[Phys. Rev. B 97, 020102(R)] Published Thu Jan 18, 2018

]]>The effective dimensionality of a semiconductor strongly influences its excitonic properties. Due to their unit-cell thickness, monolayer materials are the almost perfect realization of a two-dimensional system. However, the unit-cell dimensions can easily exceed the nominal value of the exciton Bohr radius. Employing a fully microscopic theory based on the equation-of-motion approach, it is shown that the effective finite-layer thickness in transition metal dichalcogenides (TMDCs) is responsible for the observed nonhydrogenic exciton series, not only in monolayers, but also in few-layer and bulk systems. The theoretical results identify the resonances in bulk TMDCs as due to quasi-two-dimensional intra- and interlayer excitons.

[Phys. Rev. B 97, 035425] Published Thu Jan 18, 2018

]]>Due to their small size, atomically thin materials are expected to exhibit electronic properties that are highly sensitive to their environment. However, atomically thin semiconductors on a variety of substrates, as well as in their bulk layered phase, exhibit an absorption onset at roughly the same energy. The authors provide a semianalytic theory of the quasiparticle band gap in atomically thin semiconductors and show that environment-induced changes are almost exactly canceled by changes in the exciton binding energy, leaving the absorption onset nearly constant. The presented theory sets limits on the prospect of environmental engineering in atomically thin materials.

[Phys. Rev. B 97, 041409(R)] Published Wed Jan 17, 2018

]]>Magnetic skyrmions discovered in chiral magnets show unique physical properties due to their nontrivial topology. While their basic properties have been well understood for pure systems, in real materials there is always disorder from, for example, crystal defects or impurities. This glassy nature substantially modifies the low-energy behavior, and hence the investigation of the skyrmion glass is crucial to understanding real materials. Here, the authors theoretically study disordered chiral magnets in terms of replica field theory, and provide a firm basis to explore the glassy state of magnetic skyrmions.

[Phys. Rev. B 97, 024413] Published Tue Jan 16, 2018

]]>In the electronic band structure of materials, two-band touching points with linear dispersion always appear in pairs in momentum space. When the pair annihilates, the system undergoes a quantum phase transition from a Weyl semimetal (WSM) to a band insulator (BI), which is described by a new critical theory. This paper reveals that the critical theory hosts a novel disorder-driven quantum multicritical point (QMCP), which is encompassed by three quantum phases: WSM, BI, and diffusive metal (DM). The authors clarify scaling theories around the QMCP as well as all the phase boundaries among these three phases. The scaling theory is general and easily extended to other systems.

[Phys. Rev. B 97, 045129] Published Tue Jan 16, 2018

]]>The notion of symmetry protected distinctions among quantum phases has been well appreciated in understanding many-body systems. As a prominent example, recent studies on topological crystalline insulators (TCIs) indicate that these insulators are nontrivial if and only if the relevant crystalline (and some other) symmetries are preserved. By bulk-boundary correspondence, the authors study the properties of the bulk magnetic monopoles of some TCIs that are conceptually interesting and/or experimentally feasible, and argue that this bulk characterization is a more direct and unified physical description of a TCI. In doing so, the authors prove the stability of these TCIs with respect to interactions.

[Phys. Rev. B 97, 045130] Published Tue Jan 16, 2018

]]>The detection via capacitance-voltage measurements of nonequilibrium electron states in semiconductor quantum dots are reported here for the first time. A nonequilibrium setting is generated by external illumination of the quantum dots. By carefully tuning the dot properties, it is possible to render the nonequilibrium states long-lived, allowing for their subsequent detection.

[Phys. Rev. B 97, 045416] Published Tue Jan 16, 2018

]]>Lattice symmetry is important for classifying ordered phases in solids, but can it have consequences for the energetics of phase formation? In the heavy-fermion antiferromagnet CeAuSb${}_{2}$, it does. The unstressed lattice is tetragonal, and an orthorhombic distortion can be created through in-plane uniaxial pressure. For small distortions, the Néel transition is hardly affected, since the tetragonal lattice symmetry provides an energetic stabilization of the magnetic order. When the applied orthorhombicity exceeds a threshold, however, the transition splits into two transitions, and the order changes qualitatively.

[Phys. Rev. B 97, 024411] Published Thu Jan 11, 2018

]]>Stanene, a single tin atomic layer akin to graphene, is a quantum spin Hall insulator. Its spin-polarized edge states within the gap would be well suited for spintronic applications, but this attractive property has not been realized because the substrate for supporting stanene in prior experiments leads to a metallic contact that fills the band gap and shorts out the quantum spin Hall channels. By judiciously selecting InSb(111) as the substrate, the resulting system shows a large gap of 0.44 eV well suited for room-temperature device operations. Stanene on InSb(111) is thus a strong contender for next-generation spintronic technology.

[Phys. Rev. B 97, 035122] Published Thu Jan 11, 2018

]]>Spin density waves (SDW) provide a canonical example of quantum states with spontaneously broken symmetries. The dynamics of SDWs in Mott insulators is described by the well-known Holstein-Primakoff Hamiltonian, which is equivalent to the semiclassical Landau-Lifshitz equation. Based on a real-space formulation of the time-dependent Hartree-Fock method, the authors demonstrate how to extend the semiclassical dynamics description to the case of itinerant magnets or Mott systems with a moderate charge gap. They further show that, within the adiabatic approximation, the SDW dynamics is governed by a generalized quantum Landau-Lifshitz equation.

[Phys. Rev. B 97, 035120] Published Wed Jan 10, 2018

]]>Two-dimensional materials research has recently advanced to free-standing, atomically thin metallic nanostructures. To facilitate further progress, the authors construct a periodic table for the properties of 2D elemental metals using density functional theory. The theoretical values for cohesive energies, bond lengths, and elastic moduli of 45 metals correlate linearly with the corresponding properties of the familiar bulk metals. The presentation of these correlations and their causes provides for profound insights into material research that would help design new 2D nanosize heterostructures with optical, catalytic, and nanoelectronic applications.

[Phys. Rev. B 97, 035411] Published Wed Jan 10, 2018

]]>Implementing real-space renormalization using tensor networks has proven to be a useful approach for studying lattice systems. The key step in such methods is removing local correlations from a network. Here, the authors propose a way to do this that is fast, simple to implement, applicable to any network, and preserves geometry. This opens a new avenue for extending such methods to three-dimensional networks, as well as providing a competitive alternative to existing methods in two dimensions.

[Phys. Rev. B 97, 045111] Published Wed Jan 10, 2018

]]>The quantum Hall effect has bagged several Nobel prizes and is a cornerstone of modern metrology, but after four decades it continues to strain our understanding of quantum physics. This comprehensive review of scaling experiments provides substantial support for the conjecture that the morass of Hall data conceals a new type of symmetry, known as modular symmetry [C. A. Lütken and G. G. Ross, Phys. Rev. B 45, 11837 (1992)]. These infinite discrete symmetries are central to many recent developments in mathematics and string theory, but have not previously been seen in nature.

[Phys. Rev. B 97, 045113] Published Wed Jan 10, 2018

]]>Many structural phase transitions are of first order, and many superconducting transitions are of second order. Some iron-based materials have a first-order (magneto)structural phase transition and then cross over to a superconducting phase at low temperatures. Using nanoscale imaging, the authors show that this results in separated superconducting and normal regions, each showing a different crystalline structure. In systems having different crystalline phases, it can be energetically favorable to separate a superconductor into patches.

[Phys. Rev. B 97, 014505] Published Tue Jan 09, 2018

]]>The quantum anomalous Hall effect can, in principle, be driven entirely by interparticle interactions into a new phase of matter called the topological Mott insulator. Calculations to date have shown that this effect can arise in simple models only when the interaction strength between particles is not spatially decaying. Such interactions are typically unphysical. Here, the authors study particles in a decorated honeycomb lattice to find that spatially decaying interactions do in fact lead to a topological Mott insulator with a quantum anomalous Hall effect. The results set the stage for realizing interaction-driven topological effects in quantum matter.

[Phys. Rev. B 97, 035114] Published Tue Jan 09, 2018

]]>The anomalous nature of spin transport in the XXZ quantum spin chain has been a topic of theoretical interest for some time. Here, the integrability of the underlying dynamics leads to a ballistic component of the spin current, characterized by a spin Drude weight, which measures the degree of divergence of the zero-frequency spin conductivity. However, this quantity had previously proven to be beyond the reach of standard Bethe ansatz techniques. Here, the authors show that a recently developed hydrodynamic formalism for quantum integrable models may be used to compute the spin Drude weight. They also propose a numerical scheme to obtain hydrodynamic predictions for finite-time energy transport. This suggests that the hydrodynamic approach captures completely the ballistic component that dominates transport at long times and distances in the gapless regime of the XXZ model.

[Phys. Rev. B 97, 045407] Published Mon Jan 08, 2018

]]>It was recently proposed that multiferroic GaV${}_{4}$S${}_{8}$ hosts a new Néel-type of magnetic skyrmion lattice in its bulk, though direct microscopic experimental evidence of this type of skyrmion state has been absent up to now. By using polarized small-angle neutron scattering, the authors show unambiguously that the spins twist cycloidally in all modulated phases of GaV${}_{4}$S${}_{8}$, consistent with that expected for Néel-type skyrmions. In a further contrast to any other skyrmion host material, all of the spiral and skyrmion phases in GaV${}_{4}$S${}_{8}$ are shown to be stable only at finite temperature due to a delicate interplay between thermal agitation and a strong magnetic anisotropy.

[Phys. Rev. B 97, 020401(R)] Published Fri Jan 05, 2018

]]>A spin fractionalizes into matter and gauge fermions in the Kitaev spin liquid on the honeycomb lattice, which is topologically ordered for certain parameters. To what extent does only one constituent of a fractionalized topological system signal the existence of topological order? To answer this, the authors focus on the reduced density matrix of the gauge sector on a cylinder after tracing out the matter degrees of freedom. The ensuing gauge entanglement Hamiltonian contains infinitely long-range correlations along the symmetry axis of the cylinder. Rather small correctly chosen gauge partitions can still account for the topological entanglement entropy in spite of long-range correlations in the gauge entanglement Hamiltonian.

[Phys. Rev. B 97, 035109] Published Fri Jan 05, 2018

]]>The phase degree of freedom is a core property of the superconducting state, intimately linked to its coherent yet macroscopic nature. The authors directly observe evidence of a complete suppression of the superconducting energy gap in a nanosized aluminum wire upon subjecting the latter to an appropriate phase gradient by means of a micro-interferometer. They exploit the sharp magnetic response obtained with the design to realize ultrasensitive magnetic flux detectors.

[Phys. Rev. B 96, 214517] Published Fri Dec 29, 2017

]]>In the one-monolayer FeSe film on SrTiO${}_{3}$, which is a new member of high-temperature superconductor family, there exists two competing proposals on the nature of the ground state in the parent undoped film: strongly correlated insulator (as in copper-oxide superconductors) versus moderately correlated metal (as in iron-based superconductors). Here, the authors provide experimental evidence for the metallic nature by performing $i\phantom{\rule{0}{0ex}}n$ $s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}u$ angle-resolved photoemission spectroscopy on an undoped one-monolayer FeSe film prepared by careful post-growth annealing. The authors demonstrate the materialization of an ideal two-dimensional Dirac electron system, where only the Dirac-cone band crosses the Fermi level, unlike in the multilayer counterpart.

[Phys. Rev. B 96, 220509(R)] Published Fri Dec 29, 2017

]]>The discovery of superconductivity in the $j$=$\frac{3}{2}$ spin-orbit coupled half-Heusler semimetals has given the field of low-density superconductivity a new twist. Exotic pairings are likely, yet the favored superconducting instabilities remain to be determined. The authors provide a method to analyze superconductivity in systems with symmetry-protected band touchings. They apply it to $j$=$\frac{3}{2}$ systems to find that polar phonons play an important role in the superconducting mechanism of the half-Heusler compounds. They also explicitly show a number of new features associated with such band structures, such as the strong dependence on hole or electron doping in the determination of the leading odd-parity superconducting instability.

[Phys. Rev. B 96, 214514] Published Thu Dec 28, 2017

]]>Conversion of conventional spin-singlet Cooper pairs into spin-triplet pairs inside a Josephson junction enables supercurrent to propagate over long distances through ferromagnetic materials. Several groups have shown how to turn the spin-triplet supercurrent on or off by varying the relative orientations of the magnetizations in multiple ferromagnetic layers inside the junction. This paper introduces a new magnetic configuration involving a synthetic antiferromagnet (SAF) with perpendicular magnetic anisotropy (PMA) that can be patterned into nanomagnets with very low stray fields. The authors suggest that this configuration will enable the controlled manipulation of the ground-state phase shift across the spin-triplet Josephson junctions to be either 0 or $\pi $.

[Phys. Rev. B 96, 224515] Published Thu Dec 28, 2017

]]>The authors investigate theoretically and experimentally epitaxial nanostructures offering a full range of confinement regimes for excitons due to controlled cross-sectional sizes and strong variation in the elongation of the nanosize objects - from strong through intermediate to weak confinement. It is demonstrated that the symmetry breaking leads to significantly slower recombination in one of the bright states of the excitonic fine structure. The respective components are detected in time-resolved decays of unpolarized photoluminescence,which allows a full characterization of the exciton states, including their intrinsic polarization properties.

[Phys. Rev. B 96, 245425] Published Wed Dec 27, 2017

]]>The author provides a full theoretical description of current transport in finite-size superconductors with variable attractive pairing interaction. The solution shows an intricate interplay between strong quantum fluctuations induced by the finite system size and the interaction strength, which tunes a BEC-BCS crossover. The results explain a recent experiment on mesoscopic superconducting semiconductor nanowires and point the way to a systematic understanding of mesoscopic fluctuations in superconductors. In particular, the study identifies the first experimental detection of a fundamental quantity in mesoscopic superconductivity, the parity parameter.

[Phys. Rev. B 96, 220508(R)] Published Tue Dec 26, 2017

]]>Until recently, many authors remained skeptical about the presence of charge order in superconducting cuprates. As early as 1999, Hunt $e\phantom{\rule{0}{0ex}}t$ $a\phantom{\rule{0}{0ex}}l$. [Phys. Rev. Lett. 82, 4300] demonstrated that superconducting La${}_{1.885}$Sr${}_{0.115}$CuO${}_{4}$ (T${}_{c}$=30 K) shares nearly identical, peculiar NMR anomalies observed at the charge order transition of Nd codoped La${}_{1.48}$Nd${}_{0.4}$Sr${}_{0.12}$CuO${}_{4}$ (suppressed T${}_{c}$=6 K). In particular, ${}^{63}$Cu NMR signals gradually disappear in the charge ordered state. Where did the wiped out ${}^{63}$Cu NMR signals go? The authors here take full advantage of advancements in the instrumental technologies of NMR over the last two decades, and finally solve this mystery.

[Phys. Rev. B 96, 224508] Published Tue Dec 26, 2017

]]>The magnetic shape-memory effect – magnetic-field-induced plastic deformations that return to the initial shape upon heating – is intensively studied in intermetallic Heusler materials. Here, the authors see this effect for the metallic oxide SrRuO${}_{3}$, which exhibits a structural phase transition at high temperature and ferromagnetic ordering at six times a lower temperature. Neutron diffraction and macroscopic analyses on SrRuO${}_{3}$ single crystals show that moderate magnetic fields induce strong structural deformations resulting from the rearrangement of structural domains. The shape-memory effect is observed by modest heating to the paramagnetic phase, where the initial arrangement of structural domains already recovers, although the temperature remains well below the structural phase transition.

[Phys. Rev. B 96, 220406(R)] Published Thu Dec 21, 2017

]]>Electrical injection of spin current via the spin Hall effect has a huge technological implication for spin transfer torque devices and in magnon spintronics. This effect generates a spin current perpendicular to a charge current but has so far relied on expensive heavy metals. This paper uncovers a new mechanism present in ferromagnets, the anomalous spin Hall effect, where the magnetization of the ferromagnet is used to tune the spin injection efficiency up to a level greater than that found in heavy metals. The authors demonstrate this anomalous mechanism via an experimental advance where nonlocal magnon transport in a ferrimagnetic insulator (YIG) was achieved using a ferromagnetic metal (permalloy) for efficient and tunable electrical injection and detection of magnons.

[Phys. Rev. B 96, 220408(R)] Published Thu Dec 21, 2017

]]>The authors present here a general quantum kinetic equation for the Bloch-state density matrix of a crystal that accounts for disorder, and for electric and magnetic fields. The theory naturally incorporates momentum-space Berry phase effects that can have an especially important influence on transport in topological materials. As an illustration, they have considered the steady state of a Weyl semimetal that is driven by parallel electric and magnetic fields, showing explicitly how the chiral magnetotransport anomaly emerges when intervalley scattering is very weak.

[Phys. Rev. B 96, 235134] Published Thu Dec 21, 2017

]]>Generation of spin-photon entanglement lies at the heart of quantum repeater protocols. Here, the authors experimentally demonstrate quantum entanglement between a photon and a spin degree of freedom of a quantum dot molecule, by measuring a state overlap with a fully entangled state of about 70%. In contrast to earlier demonstrations of spin-photon entanglement, the system realized here, using single-triplet qubits in a quantum dot molecule, combines potentially long spin-coherence times with a very efficient photonic interface. The authors also develop a novel heterodyne technique that may have wide-ranging applications for characterizing photonic color qubits.

[Phys. Rev. B 96, 241410(R)] Published Thu Dec 21, 2017

]]>Disordered quantum systems undergo Anderson localization-delocalization transitions, which exhibit very rich physics. A remarkable feature of these transitions is the multifractality of critical wave functions. The eigenfunction multifractality in a $d$-dimensional disordered system holds only at the transition point and is characterized by universal critical exponents. The authors explore the evolution of wave-function statistics on a finite Bethe lattice from the central site (“root”) to the boundary (“leaves”). They show that eigenfunction moments exhibit generally a multifractal scaling with the volume $N$. The multifractality spectrum ${\tau}_{q}$ depends on the disorder strength and on the parameter $s$ characterizing the position of the observation point, $s$ = $r/R$, where $r$ is the distance to the root and $R$ is the “radius” of the lattice.

[Phys. Rev. B 96, 214204] Published Wed Dec 20, 2017

]]>Superconducting circuits provide a very attractive platform to study the transition from chaotic ergodic to full localized behavior expected in closed quantum systems. The present work reports the emergence of a chaotic, yet nonergodic regime that preempts the transition into a fully localized state in Josephson junction chains. The lack of ergodicity is reflected in multifractal eigenstates in the many-body space. Furthermore, the multifractality in the phase space is accompanied by the fractal properties of the local energy spectrum that translates into anomalous power-law behavior of the physical correlators. The observation of a fractal structure in the local energy spectrum of Josephson junction arrays may provide a bridge between nonergodic dynamics of quantum systems and the hierarchical structure of the local minima in the theory of classical spin glasses.

[Phys. Rev. B 96, 214205] Published Wed Dec 20, 2017

]]>The honeycomb-lattice material $\alpha $-RuCl${}_{3}$ is a prime candidate for exhibiting Kitaev spin-liquid physics. Here, the authors employ high-field electron spin resonance spectroscopy to probe its spin dynamics across different regions of the phase diagram. Apart from two modes of antiferromagnetic resonance in the zigzag-ordered phase, a rich excitation spectrum was observed in the field-induced quantum disordered state. The authors compare their observations with results of recent numerical calculations, revealing a complex multiparticle nature of magnetic excitations in the field-induced phase.

[Phys. Rev. B 96, 241107(R)] Published Tue Dec 19, 2017

]]>Type-II Weyl semimetals (WSM) are topological materials characterized by special Weyl points (WP) in momentum space where the valence and the conduction band touch. A WSM phase is predicted in MoTe${}_{2}$ below 250 K, but the WP located in the unoccupied part of the electronic structure has eluded direct observation. By means of time- and angle-resolved photoemission, the authors reveal the fingerprint of the WP in the ultrafast electron dynamics. Electrons that are optically excited into the conduction band relax faster in the WSM phase than in the high-temperature phase or in the topologically trivial sister compound WTe${}_{2}$.

[Phys. Rev. B 96, 241408(R)] Published Mon Dec 18, 2017

]]>There are two main mechanisms of spin relaxation in conductors. The Elliott-Yafet (EY) mechanism describes spin-flip scattering, while the D’yakonov-Perel’ (DP) mechanism relies on spin precession and motional narrowing. While both need spin-orbit coupling, historically the two mechanisms have been treated differently. It is now demonstrated, using an intuitive generic model, that the two mechanisms can be cast in the same language based on motional narrowing in the presence of an effective, spin-orbit related magnetic field. This shows an intimate relationship between spin flip and spin precession and enables use of the same, readily programmable formalism for both regimes. The technique is demonstrated for MgB${}_{2}$, which shows an anomalous spin-relaxation behavior that cannot be explained by conventional models.

[Phys. Rev. B 96, 245123] Published Mon Dec 18, 2017

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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