We show that the topological central charge of a topological phase can be directly accessed from the ground-state wave functions for a system on a surface as a Berry curvature produced by adiabatic variation of the metric on the surface, at least up to addition of another topological invariant that arises in some cases. For trial wave functions that are given by conformal blocks (chiral correlation functions) in a conformal field theory (CFT), we carry out this calculation analytically, using the hypothesis of generalized screening. The topological central charge is found to be that of the underlying CFT used in the construction, as expected. The calculation makes use of the gravitational anomaly in the chiral CFT. It is also shown that the Hall conductivity can be obtained in an analogous way from the U(1) gauge anomaly.

We study the braiding statistics of particlelike and looplike excitations in two- (2D) and three-dimensional (3D) gauge theories with finite, Abelian gauge group. The gauge theories that we consider are obtained by gauging the symmetry of gapped, short-range entangled, lattice boson models. We define a set of quantities, called topological invariants, that summarize some of the most important parts of the braiding statistics data for these systems. Conveniently, these invariants are always Abelian phases, even if the gauge theory supports excitations with non-Abelian statistics. We compute these invariants for gauge theories obtained from the exactly soluble group cohomology models of Chen, Gu, Liu, and Wen, and we derive two results. First, we find that the invariants take different values for every group cohomology model with finite, Abelian symmetry group. Second, we find that these models exhaust all possible values for the invariants in the 2D case, and we give some evidence for this in the 3D case. The first result implies that every one of these models belongs to a distinct symmetry-protected topological (SPT) phase, while the second result suggests that these models may realize all SPT phases. These results support the group cohomology classification conjecture for SPT phases in the case where the symmetry group is finite, Abelian, and unitary.

We consider near-field heat transfer with nonzero chemical potential for photons, as can occur between two semiconductor bodies, held at different temperatures with at least one of the bodies under external bias. We show that the dependence of radiative heat flux on chemical potential enables electronic control of both the direction and magnitude of near-field heat transfer between the two bodies. Moreover such a configuration can operate as a solid-state cooling device whose efficiency can approach the Carnot limit in the ideal case. Significant cooling can also be achieved in the presence of inherent nonidealities including Auger recombination and parasitic phonon-polariton heat transfer.

We report the structure and magnetic properties of a new iridate compound, $\mathrm{S}{\mathrm{r}}_x\mathrm{L}{\mathrm{a}}_{11-x}\mathrm{I}{\mathrm{r}}_4{\mathrm{O}}_{24}$, where the $d$-electron count of Ir and therefore its number of unpaired electrons can be tuned continuously from $5d^5\mathrm{I}{\mathrm{r}}^{4+}$ to $5d^4\mathrm{I}{\mathrm{r}}^{5+}$, i.e., from $\mathrm{SrL}{\mathrm{a}}_{10}\mathrm{I}{\mathrm{r}}_4{\mathrm{O}}_{24}$ to $\mathrm{S}{\mathrm{r}}_5\mathrm{L}{\mathrm{a}}_6\mathrm{I}{\mathrm{r}}_4{\mathrm{O}}_{24}$. The $\mathrm{Ir}{\mathrm{O}}_6$ octahedra in $\mathrm{S}{\mathrm{r}}_x\mathrm{L}{\mathrm{a}}_{11-x}\mathrm{I}{\mathrm{r}}_4{\mathrm{O}}_{24}$ are isolated from each other and from other transition elements, minimizing band effects, and the doping is on the framework sites, not the Ir sites, minimizing the effects of disorder. Measurements of the temperature-dependent magnetic susceptibility are employed to determine the evolution of the Ir magnetic moment on progressing from $5d^5\mathrm{I}{\mathrm{r}}^{4+}$ to $5d^4\mathrm{I}{\mathrm{r}}^{5+}$, and are clearly best described by a transition from a $J=1/2$ to a $J=0\mathrm{Ir}$ magnetic state; that is, the evolution of the magnetic susceptibility shows the dominance of spin-orbit coupling in determining the magnetic properties of a material with highly isolated $\mathrm{Ir}{\mathrm{O}}_6$ octahedra.

Theoretical and experimental studies agree that ${\text{Bi}}_{1-x}{\text{Sb}}_x(0.07\le x\le 0.21)$ is a three-dimensional topological insulator. However, there is still a debate on the corresponding ${\text{Bi}}_{1-x}{\text{Sb}}_x\left(111\right)$ surface band structure. While three spin polarized bands have been claimed experimentally, theoretically, only two surface bands appear, with the third band being attributed to surface imperfections. Here, we address this controversy using angle-resolved photoemission spectroscopy (ARPES) on ${\text{Bi}}_{1-x}{\text{Sb}}_x$ films. To minimize surface imperfections, we have optimized the sample growth recipe. We have measured the evolution of the surface band structure of ${\text{Bi}}_{1-x}{\text{Sb}}_x$ with $x$ increasing gradually from $x=0$ to $x=0.6$. Our ARPES data show better agreement with the theoretical calculations, where the system is topologically nontrivial with two surface bands.

For quasi-one-dimensional quantum spin systems theory predicts the occurrence of a confinement of spinon excitation due to interchain couplings. Here we investigate the system ${\mathrm{SrCo}}_2{\mathrm{V}}_2{\mathrm{O}}_8$, a realization of the weakly coupled Ising-like XXZ antiferromagnetic chains, by terahertz spectroscopy with and without applied magnetic field. At low temperatures a series of excitations is observed, which split in a Zeeman-like fashion in an applied magnetic field. These magnetic excitations are identified as the theoretically predicted spinon-pair excitations. Using a one-dimensional Schrödinger equation with a linear confinement potential imposed by weak interchain couplings, the hierarchy of the confined spinons can be fully described.

We investigate the interaction between electromagnetic pulses and two-dimensional current sheets whose conductivity is controlled as a function of time by the generation of photocarriers, and we discuss its applicability to tunable frequency comb generation. To this aim, we develop an analytical model that permits the calculation of the scattered waves off a thin sheet with time-dependent, dispersive sheet conductivity. We evaluate the transmitted spectrum as a function of the dispersive behavior and the modulation frequency of the number of photocarriers. We conclude that such active materials, e.g., time-dependent graphene sheets, open up the possibility to manipulate the frequency of incident pulses and, hence, could lead to highly tunable, miniaturized frequency comb generation.

The intersystem crossing (ISC) is an important process in many solid-state atomlike impurities. For example, it allows the electronic spin state of the nitrogen-vacancy (NV) center in diamond to be initialized and read out using optical fields at ambient temperatures. This capability has enabled a wide array of applications in metrology and quantum information science. Here, we develop a microscopic model of the state-selective ISC from the optical excited state manifold of the NV center. By correlating the electron-phonon interactions that mediate the ISC with those that induce population dynamics within the NV center's excited state manifold and those that produce the phonon sidebands of its optical transitions, we quantitatively demonstrate that our model is consistent with recent ISC measurements. Furthermore, our model constrains the unknown energy spacings between the center's spin-singlet and spin-triplet levels. Finally, we discuss prospects to engineer the ISC in order to improve the spin initialization and readout fidelities of NV centers.

We present a comprehensive study of the evolution of the nematic electronic structure of FeSe using high-resolution angle-resolved photoemission spectroscopy (ARPES), quantum oscillations in the normal state, and elastoresistance measurements. Our high-resolution ARPES allows us to track the Fermi surface deformation from fourfold to twofold symmetry across the structural transition at $\sim 87\mathrm{K}$, which is stabilized as a result of the dramatic splitting of bands associated with $d_{xz}$ and $d_{yz}$ character in the presence of strong electronic interactions. The low-temperature Fermi surface is that of a compensated metal consisting of one hole and two electron bands and is fully determined by combining the knowledge from ARPES and quantum oscillations. A manifestation of the nematic state is the significant increase in the nematic susceptibility approaching the structural transition that we detect from our elastoresistance measurements on FeSe. The dramatic changes in electronic structure cannot be explained by the small lattice distortion and, in the absence of magnetic fluctuations above the structural transition, point clearly towards an electronically driven transition in FeSe, stabilized by orbital-charge ordering.

In this paper, we systematically classify and describe bosonic symmetry protected topological (SPT) phases in all physical spatial dimensions using semiclassical nonlinear sigma model (NLSM) field theories. All the SPT phases on a $d\text{-dimensional}$ lattice discussed in this paper can be described by the same NLSM, which is an O($d+2$) NLSM in $(d+1)\text{-dimensional}$ space-time, with a topological $\Theta$ term. The field in the NLSM is a semiclassical Landau order parameter with a unit length constraint. The classification of SPT phases discussed in this paper based on their NLSMs is Completely Identical to the more mathematical classification based on group cohomology given in X. Chen, Z.-C. Gu, Z.-X. Liu, and X.-G. Wen, Phys. Rev. B 87, 155114 (2013) and Science 338, 1604 (2012). Besides the classification, the formalism used in this paper also allows us to explicitly discuss the physics at the boundary of the SPT phases, and it reveals the relation between SPT phases with different symmetries. For example, it gives many of these SPT states a natural “decorated defect” construction.

The electronic and magnetic properties of a new diluted magnetic semiconductor (DMS) ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x\left({\mathrm{Zn}}_{1-y}{\mathrm{Mn}}_y\right){}_2{\mathrm{As}}_2$, which is isostructural to so-called 122-type Fe-based superconductors, are investigated by x-ray absorption spectroscopy (XAS) and resonance photoemission spectroscopy (RPES). Mn $L_{2,3}$-edge XAS indicates that the doped Mn atoms have a valence 2+ and strongly hybridize with the $4p$ orbitals of the tetrahedrally coordinating As ligands. The Mn $3d$ partial density of states obtained by RPES shows a peak around 4 eV and is relatively high between 0 and 2 eV below the Fermi level ($E_F$) with little contribution at $E_F$, similar to that of the archetypal DMS ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{As}$. This energy level creates a $d^5$ electron configuration with $S=5/2$ local magnetic moments at the Mn atoms. Hole carriers induced by K substitution for Ba atoms go into the top of the As $4p$ valence band and are weakly bound to the Mn local spins. The ferromagnetic correlation between the local spins mediated by the hole carriers induces ferromagnetism in ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x\left({\mathrm{Zn}}_{1-y}{\mathrm{Mn}}_y\right){}_2{\mathrm{As}}_2$.

By means of density functional theory calculations, we predict that several two-dimensional $AB$ binary monolayers, where $A$ and $B$ atoms belong to group IV or III-V, are ferroelectric. Dipoles arise from the buckled structure, where the $A$ and $B$ ions are located on the sites of a bipartite corrugated honeycomb lattice with trigonal symmetry. We discuss the emerging valley-dependent properties and the coupling of spin and valley physics, which arise from the loss of inversion symmetry, and explore the interplay between ferroelectricity and Rashba spin-splitting phenomena. We show that valley-related properties originate mainly from the binary nature of $AB$ monolayers, while the Rashba spin-texture developing around valleys is fully controllable and switchable by reversing the ferroelectric polarization.

We study the electronic properties of strongly spin-orbit coupled electrons on the elastic pyrochlore lattice. Akin to the Peierls transition in one-dimensional systems, the coupling of the lattice to the electronic degrees of freedom can stabilize a spontaneous deformation of the crystal. This deformation corresponds to a breathing mode, which breaks the inversion symmetry. We find that for intermediate values of the staggered strain, the inversion-symmetry broken phase realizes a topological Weyl semimetal. In the temperature-elasticity phase diagram, the Weyl semimetal shows a reentrant phase behavior: it can be reached from a symmetric phase realized both at higher and at lower temperatures. The symmetric phase is a Dirac semimetal, which is protected by the nonsymmorphic space group of the pyrochlore lattice. Beyond a critical value of the staggered strain, the symmetry-broken phase is a fully gapped trivial insulator. The surface states of the Weyl semimetal form open Fermi arcs and we observe that their connectivity depends on the termination of the crystal. In particular, for the $\left\{111\right\}$ films, the semiclassical closed electronic orbits of the surface states in a magnetic field cross the bulk either twice, four, six, or twelve times. We demonstrate how one can tune the number of bulk crossings through a Lifshitz-like transition of the Fermi arcs, which we call Weyl-Lifshitz transition, by applying a surface potential. Our results offer a route to a topological Weyl semimetal in nonmagnetic materials and might be relevant for pyrochlore oxides with heavy transition-metal ions such as alloys of iridates.

We report about the experimental observation and characterization of nonlinear optical properties of individual silicon nanowires of different dimensions. Our results show that the nonlinear light has different components, with one of them corresponding to the second-harmonic generation (SHG). The SHG strongly depends on the polarization of the optical excitation and nanowire diameter, and gives access to the local electromagnetic field intensity distribution. Furthermore, we show that the second harmonic, when observed, is enhanced compared to bulk silicon and is sensitive to optical resonances supported by the nanowires. This offers different perspectives on the definition of silicon-based nonlinear photonic devices.

Using density-functional theory, we calculate the electronic band structure of single-layer graphene on top of hexagonal ${\mathrm{In}}_2{\mathrm{Te}}_2$ monolayers. The geometric configuration with In and Te atoms at the centers of the carbon hexagons leads to a Kekulé texture with an ensuing band gap of 20 meV. The alternative structure, nearly degenerate in energy, with the In and Te atoms on top of carbon sites, is characterized instead by a gapless spectrum with the original Dirac cones of graphene reshaped, depending on the graphene-indium chalcogenide distance, either in the form of an undoubled pseudospin-one Dirac cone or in a quadratic band crossing point at the Fermi level. These electronic phases harbor charge fractionalization and topological Mott insulating states of matter.

Magnetoelastic coupling, i.e., the change of crystal lattice induced by a spin order, is not only scientifically interesting, but also technically important. In this work, we propose a general microscopic model from first-principles calculations to describe the magnetoelastic coupling and provide a way to construct the microscopic model from density functional theory calculations. Based on this model, we reveal that there exists a previously unexpected contribution to the electric polarization induced by the spin order in multiferroics due to the combined effects of magnetoelastic coupling and piezoelectric effect. Interestingly and surprisingly, we find that this lattice-deformation contribution to the polarization is even larger than that from the pure electronic and ion-displacement contributions in $\mathrm{BiFe}{\mathrm{O}}_3$. This model of magnetoelastic coupling can be generally applied to investigate the other magnetoelastic phenomena.

Using electrical transport experiments and shot noise thermometry, we investigate electron-phonon heat transfer rate in a suspended bilayer graphene. Contrary to monolayer graphene with heat flow via three-body supercollision scattering, we find that regular electron–optical-phonon scattering in bilayer graphene provides the dominant scattering process at electron energies $\gtrsim 0.15\mathrm{eV}$. We determine the strength of these intrinsic heat flow processes of bilayer graphene and find good agreement with theoretical estimates when both zone edge and zone center optical phonons are taken into account.

We investigate electron vortex beams elastically scattered on chiral crystals. After deriving a general expression for the scattering amplitude of a vortex electron, we study its diffraction on point scatterers arranged on a helix. We derive a relation between the handedness of the helix and the topological charge of the electron vortex on one hand and the symmetry of the higher-order Laue zones in the diffraction pattern on the other for kinematically and dynamically scattered electrons. We then extend this to atoms arranged on a helix as found in crystals which belong to chiral space groups and propose a method to determine the handedness of such crystals by looking at the symmetry of the diffraction pattern. In contrast to alternative methods, our technique does not require multiple scattering, which makes it possible to also investigate extremely thin samples in which multiple scattering is suppressed. In order to verify the model, elastic scattering simulations are performed, and an experimental demonstration on ${\mathrm{Mn}}_2{\mathrm{Sb}}_2{\mathrm{O}}_7$ is given in which we find the sample to belong to the right-handed variant of its enantiomorphic pair. This demonstrates the usefulness of electron vortex beams to reveal the chirality of crystals in a transmission electron microscope and provides the required theoretical basis for further developments in this field.

We report the x-ray photoexcitation effects on cycloidal magnetic order in a multiferroic perovskite-type manganite ${\mathrm{Gd}}_{0.5}{\mathrm{Tb}}_{0.5}{\mathrm{MnO}}_3$. The material exhibits a competition between the $P\left(\mathrm{electric}\mathrm{polarization}\right)\left|\right|a$ ferroelectric state with the commensurate (C) $ab$-plane cycloidal and the $P\left|\right|c$ ferroelectric state with the incommensurate (IC) $bc$-plane cycloidal spin structure. The phase transition between these two phases can be induced by an application of a magnetic field. We found that the x-ray irradiation can induce the bidirectional and persistent phase transition between IC and C phases. The nonresonant x-ray magnetic diffraction revealed that both the initial and the x-ray induced phases have the $bc$-plane cycloidal magnetic structure, whereas the magnetic wave number varies between IC and C. The x-ray induced $bc$-plane cycloidal C phase is thus the hidden multiferroic phase in the ${\mathrm{Gd}}_{1-x}{\mathrm{Tb}}_x{\mathrm{MnO}}_3$ system, which does not show up with the application of any static field.

It is demonstrated that persistent photoconductivity (PPC), well studied in lightly-doped semiconductors, is observable in GeSbTe compounds using infrared excitation at cryogenic temperatures. The low levels of energy flux necessary to induce an appreciable effect seems surprising given the high carrier concentration $n$ of these ternary alloys $\left(n>{10}^{20}{\mathrm{cm}}^{-3}\right)$. On the other hand, their high density of carriers makes GeSbTe films favorable candidates for exhibiting intrinsic electron-glass effects with long relaxation times. These are indeed observed in GeSbTe thin films that are Anderson-localized. In particular, a memory dip is observed in samples with sheet resistances larger than $\approx {10}^{\text{5}}\Omega$ at $T\approx 4$ K with similar characteristics as in other systems that exhibit intrinsic electron-glass effects. Persistent photoconductivity, however, is observable in GeSbTe films even for sheet resistances of the order of ${10}^{\text{3}}\Omega$, well below the range of disorder required for observing electron-glass effects. These two nonequilibrium phenomena, PPC and electron glass, are shown to be of different nature in terms of other aspects as well. In particular, their relaxation dynamics is qualitatively different; the excess conductance $\Delta G$ associated with PPC decays with time as a stretched exponential whereas a logarithmic relaxation law characterizes $\Delta G\left(t\right)$ of all electron glasses studied to date. Surprisingly, the magnitude of the memory dip is enhanced when the system is in the PPC state. This counter-intuitive result may be related to the compositional disorder in these materials extending over mesoscopic scales. Evidence in support of this scenario is presented and discussed.