The recently discovered material ${\mathrm{PtSn}}_4$ is known to exhibit extremely large magnetoresistance (XMR) that also manifests Dirac arc nodes on the surface. ${\mathrm{PdSn}}_4$ is isostructural to ${\mathrm{PtSn}}_4$ with the same electron count. We report on the physical properties of high-quality single crystals of ${\mathrm{PdSn}}_4$ including specific heat, temperature- and magnetic-field-dependent resistivity and magnetization, and electronic band-structure properties obtained from angle-resolved photoemission spectroscopy (ARPES). We observe that ${\mathrm{PdSn}}_4$ has physical properties that are qualitatively similar to those of ${\mathrm{PtSn}}_4$, but find also pronounced differences. Importantly, the Dirac arc node surface state of ${\mathrm{PtSn}}_4$ is gapped out for ${\mathrm{PdSn}}_4$. By comparing these similar compounds, we address the origin of the extremely large magnetoresistance in ${\mathrm{PdSn}}_4$ and ${\mathrm{PtSn}}_4$; based on detailed analysis of the magnetoresistivity $\rho (H,T)$, we conclude that neither the carrier compensation nor the Dirac arc node surface state are the primary reason for the extremely large magnetoresistance. On the other hand, we find that, surprisingly, Kohler's rule scaling of the magnetoresistance, which describes a self-similarity of the field-induced orbital electronic motion across different length scales and is derived for a simple electronic response of metals to an applied magnetic field is obeyed over the full range of temperatures and field strengths that we explore.

Polarization vortices that typically form in ferroelectric nanostructures are fundamental polar topological structures characterized by a curling polarization around a stable core. The control of vortex chirality by conventional fields including homogeneous electric field is a key to the utilization of vortices in technological applications. However, an effective control of the vortex chirality by such an electric field remains elusive since the toroidal moment of ferroelectric vortex is conjugated to a curled electric field rather than the homogeneous electric field. Here we demonstrate the control of vortex chirality by homogeneous electric field in free-standing nanodots with rationally designed nanostructures. The nanodots are designed by including a notch or an antinotch in the rectangular structure of nanodots. The results show that the chirality of polarization vortex is deterministically switched by a homogeneous electric field through the control of depolarization distribution by designed structures. The evolution path under homogeneous electric field in antinotched nanodot takes place in the opposite direction in comparison with that in notched nanodot. We further demonstrate that the designed nanostructures break the symmetry of electrostatic field in the ferroelectric systems, where the depolarization field concentrates at the notch but scatters at the antinotch. Such a symmetry breaking of electrostatic field results in the opposite evolution paths in the notched and antinotched nanodots under homogeneous electric field and provides the fundamental reason that allows such control. The present study suggests a new route on the practical control of the vortex domain pattern in ferroelectric nanostructures by homogeneous electric field.

We propose a new class of tight-binding models where a flat band exists either gapped from or crossing right through a dispersive band on two-band (i.e., two sites/unit cell) tetragonal and honeycomb lattices. By imposing a condition on the hopping parameters for generic models with up to third-neighbor hoppings, we first obtain models having a rigorously flat band isolated from a dispersive band with a gap, which accommodate both rank reducing and non-rank reducing of the Hamiltonian. The models include Tasaki's flat-band models, but the present model generally has a nonzero flat-band energy whose gap from the dispersive band is controllable as well. We then modify the models by appropriately changing the second- or third-neighbor hoppings, leading to a new class of two-dimensional lattices where a (slightly warped) flat band pierces a dispersive one. As with the known flat-band models, the connectivity condition is satisfied in the present models, so that we have unusual, unorthogonalizable Wannier orbitals. We have also shown that the present flat-band model can be extended to three (or higher) dimensions. Implications on possible high-$T_C$ superconductivity are discussed when a repulsive electron-electron interaction is introduced, where the flat band is envisaged to be utilized as intermediate states in pair scattering processes.

We propose a tensor network method for investigating strongly disordered systems that is based on an adaptation of entanglement renormalization [G. Vidal, Phys. Rev. Lett. 99, 220405 (2007)]. This method makes use of the strong disorder renormalization group to determine the order in which lattice sites are coarse-grained, which sets the overall structure of the corresponding tensor network ansatz, before optimization using variational energy minimization. Benchmark results from the disordered $XXZ$ model demonstrates that this approach accurately captures ground-state entanglement in disordered systems, even at long distances. This approach leads to a new class of efficiently contractible tensor network ansatz for one-dimensional systems, which may be understood as a generalization of the multiscale entanglement renormalization ansatz for disordered systems.

Accurate thermodynamic descriptions are a key ingredient to kinetic theories that describe the mesoscale evolution of a solid undergoing ordering or decomposition reactions. We introduce a general approach to identify order parameters for order-disorder reactions and to calculate first-principles free-energy surfaces as a function of these order parameters. The symmetry of the disordered phase is used to formulate order parameters as linear combinations of sublattice compositions of a reference supercell. The order parameters can distinguish the disordered phase from the symmetrically equivalent variants of a particular ordered phase. A thermodynamic formalism is then developed to rigorously define a coarse-grained free energy as a function of order parameters. Bias potentials are added to the potential energy to enable sampling of the unstable regions within the order-parameter domain. Monte Carlo sampling in the biased ensemble is combined with free-energy integration to calculate high-temperature free energies. We illustrate the approach by analyzing the free energies of order-disorder transitions on a two-dimensional triangular lattice and in the technologically important Ni-Al alloy.

We demonstrate a high-purity source of indistinguishable single photons using a quantum dot embedded in a nanophotonic waveguide. The source features a near-unity internal coupling efficiency and the collected photons are efficiently coupled off chip by implementing a taper that adiabatically couples the photons to an optical fiber. By quasiresonant excitation of the quantum dot, we measure a single-photon purity larger than $99.4\%$ and a photon indistinguishability of up to $94\pm 1\%$ by using $p$-shell excitation combined with spectral filtering to reduce photon jitter. A temperature-dependent study allows pinpointing the residual decoherence processes, notably the effect of phonon broadening. Strict resonant excitation is implemented as well as another means of suppressing photon jitter, and the additional complexity of suppressing the excitation laser source is addressed. The paper opens a clear pathway towards the long-standing goal of a fully deterministic source of indistinguishable photons, which is integrated on a planar photonic chip.

The electronic structure of semimetallic transition-metal dichalcogenides, such as ${\mathrm{WTe}}_2$ and orthorhombic $\gamma -{\mathrm{MoTe}}_2$, are claimed to contain pairs of Weyl points or linearly touching electron and hole pockets associated with a nontrivial Chern number. For this reason, these compounds were recently claimed to conform to a new class, deemed type-II, of Weyl semimetallic systems. A series of angle-resolved photoemission experiments (ARPES) claim a broad agreement with these predictions detecting, for example, Fermi arcs at the surface of these crystals. We synthesized single crystals of semimetallic ${\mathrm{MoTe}}_2$ through a Te flux method to validate these predictions through measurements of its bulk Fermi surface (FS) via quantum oscillatory phenomena. We find that the superconducting transition temperature of $\gamma -{\mathrm{MoTe}}_2$ depends on disorder as quantified by the ratio between the room- and low-temperature resistivities, suggesting the possibility of an unconventional superconducting pairing symmetry. Similarly to ${\mathrm{WTe}}_2$, the magnetoresistivity of $\gamma -{\mathrm{MoTe}}_2$ does not saturate at high magnetic fields and can easily surpass ${10}^6$%. Remarkably, the analysis of the de Haas–van Alphen (dHvA) signal superimposed onto the magnetic torque indicates that the geometry of its FS is markedly distinct from the calculated one. The dHvA signal also reveals that the FS is affected by the Zeeman effect precluding the extraction of the Berry phase. A direct comparison between the previous ARPES studies and density-functional-theory (DFT) calculations reveals a disagreement in the position of the valence bands relative to the Fermi level ${\varepsilon }_F$. Here, we show that a shift of the DFT valence bands relative to ${\varepsilon }_F$, in order to match the ARPES observations, and of the DFT electron bands to explain some of the observed dHvA frequencies, leads to a good agreement between the calculations and the angular dependence of the FS cross-sectional areas observed experimentally. However, this relative displacement between electron and hole bands eliminates their crossings and, therefore, the Weyl type-II points predicted for $\gamma -{\mathrm{MoTe}}_2$.

We present a numerical and theoretical study that supports and explains recent experimental results on anomalous magnetization fluctuations of a uniaxial ferromagnetic film in its low-temperature phase, which is forced by an oscillating field above the critical period of the associated dynamic phase transition (DPT) [P. Riego, P. Vavassori, and A. Berger, Phys. Rev. Lett. 118, 117202 (2017)]. For this purpose, we perform kinetic Monte Carlo simulations of a two-dimensional Ising model with nearest-neighbor ferromagnetic interactions in the presence of a sinusoidally oscillating field, to which is added a constant bias field. We study a large range of system sizes and supercritical periods and analyze the data using a droplet-theoretical description of magnetization switching. We find that the period-averaged magnetization, which plays the role of the order parameter for the DPT, presents large fluctuations that give rise to well-defined peaks in its scaled variance and its susceptibility with respect to the bias field. The peaks are symmetric with respect to zero bias and located at values of the bias field that increase toward the field amplitude as an inverse logarithm of the field oscillation period. Our results indicate that this effect is independent of the system size for large systems, ruling out critical behavior associated with a phase transition. Rather, it is a stochastic-resonance phenomenon that has no counterpart in the corresponding thermodynamic phase transition, providing a reminder that the equivalence of the DPT to an equilibrium phase transition is limited to the critical region near the critical period and zero bias.

Dynamical phases with novel topological properties are known to arise in driven systems of free fermions. In this paper, we obtain a ‘periodic table’ to describe the phases of such time-dependent systems, generalizing the periodic table for static topological insulators. Using K theory, we systematically classify Floquet topological insulators from the ten Altland-Zirnbauer symmetry classes across all dimensions. We find that the static classification scheme described by a group $\mathcal{G}$ becomes ${\mathcal{G}}^{\times n}$ in the time-dependent case, where $n$ is the number of physically important gaps in the quasienergy spectrum (including any gaps at quasienergy $\pi$). The factors of $\mathcal{G}$ may be interpreted as arising from the bipartite decomposition of the unitary time-evolution operator. Topologically protected edge modes may arise at the boundary between two Floquet systems, and we provide a mapping between the number of such edge modes and the topological invariant of the bulk.

We studied the local Ru $4d$ electronic structure of $\alpha -{\mathrm{RuCl}}_3$ by means of polarization-dependent x-ray absorption spectroscopy at the $\mathrm{Ru} L_{2,3}$ edges. We observed a vanishingly small linear dichroism indicating that electronically the Ru $4d$ local symmetry is highly cubic. Using full multiplet cluster calculations we were able to reproduce the spectra excellently and to extract that the trigonal splitting of the $t_{2g}$ orbitals is $-12\pm 10$ meV, i.e., negligible as compared to the Ru $4d$ spin-orbit coupling constant. Consistent with our magnetic circular dichroism measurements, we found that the ratio of the orbital and spin moments is 2.0, the value expected for a $J_{\text{eff}}=\frac{1}{2}$ ground state. We have thus shown that as far as the Ru $4d$ local properties are concerned, $\alpha -{\mathrm{RuCl}}_3$ is an ideal candidate for the realization of Kitaev physics.

We present a unified perspective on symmetry protected topological (SPT) phases in one dimension and address the open question of what characterizes their phase transitions. In the first part of this work, we use symmetry as a guide to map various well-known fermionic and spin SPTs to a Kitaev chain with coupling of range $\alpha \in \mathbb{Z}$. This unified picture uncovers new properties of old models, such as how the cluster state is the fixed point limit of the Affleck-Kennedy-Lieb-Tasaki state in disguise, and elucidates the connection between fermionic and bosonic phases, with the Hubbard chain interpolating between four Kitaev chains and a spin chain in the Haldane phase. In the second part, we study the topological phase transitions between these models in the presence of interactions. This leads us to conjecture that the critical point between any SPT with $d$-dimensional edge modes and the trivial phase has a central charge $c\ge {log}_{2}d$. We analytically verify this for many known transitions. This agrees with the intuitive notion that the phase transition is described by a delocalized edge mode, and that the central charge of a conformal field theory is a measure of the gapless degrees of freedom.

We derive the multiband $\mathbf{k}·\mathbf{p}$ Hamiltonian for an electron in monolayer $X\mathrm{Se}$ ($X=\mathrm{In}$, Ga) near the $\mathrm{\Gamma }$ point in the presence of spin-orbit coupling and strain. With four conduction bands and ten valence bands included, the 14-band $\mathbf{k}·\mathbf{p}$ Hamiltonian is capable of describing the nonparabolic energy band dispersion, strong interband mixing, and strain-induced effects, in good agreement with first-principles calculations. This $\mathbf{k}·\mathbf{p}$ theory provides a simple and convenient way to understand and manipulate the optical and transport properties of monolayer $X\mathrm{Se}$ and their nanostructures by strain and external fields. Interestingly, we find that uniaxial strain can even induce an indirect-to-direct band-gap transition in these two-dimensional materials.

Phonon-assisted tunneling plays a crucial role for electronic device performance and even more so with future size down-scaling. We show how one can include this effect in large-scale first-principles calculations using a single “special thermal displacement” (STD) of the atomic coordinates at almost the same cost as elastic transport calculations, by extending the recent method of Zacharias et al. [Phys. Rev. B 94, 075125 (2016)] to the important case of Landauer conductance. We apply the method to ultrascaled silicon devices and demonstrate the importance of phonon-assisted band-to-band and source-to-drain tunneling. In a diode the phonons lead to a rectification ratio suppression in good agreement with experiments, while in an ultrathin body transistor the phonons increase off currents by four orders of magnitude, and the subthreshold swing by a factor of 4, in agreement with perturbation theory.

IV-VI semiconductor SnSe has been known as the material with record high thermoelectric performance. The multiple close-to-degenerate (or “convergent”) valence bands in the electronic band structure has been one of the key factors contributing to the high power factor and thus figure of merit in the SnSe single crystal. To date, there have been primarily theoretical calculations of this particular electronic band structure. In this paper, however, using angle-resolved photoemission spectroscopy, we perform a systematic investigation of the electronic structure of SnSe. We directly observe three predicted hole bands with small energy differences between their band tops and relatively small in-plane effective masses, in good agreement with the ab initio calculations and critical for the enhancement of the Seebeck coefficient while keeping high electrical conductivity. Our results reveal the complete band structure of SnSe and help to provide a deeper understanding of the electronic origin of the excellent thermoelectric performances in SnSe.

The dislocation-mediated quantum melting of solids into quantum liquid crystals is extended from two to three spatial dimensions, using a generalization of boson-vortex or Abelian-Higgs duality. Dislocations are now Burgers-vector-valued strings that trace out worldsheets in space-time while the phonons of the solid dualize into two-form (Kalb-Ramond) gauge fields. We propose an effective dual Higgs potential that allows for restoring translational symmetry in either one, two, or three directions, leading to the quantum analogues of columnar, smectic, or nematic liquid crystals. In these phases, transverse phonons turn into gapped, propagating modes, while compressional stress remains massless. Rotational Goldstone modes emerge whenever translational symmetry is restored. We also consider the effective electromagnetic response of electrically charged quantum liquid crystals, and find among other things that as a hard principle only two out of the possible three rotational Goldstone modes are observable using propagating electromagnetic fields.

Unintentional but unavoidable fabrication imperfections in state-of-the-art photonic-crystal waveguides lead to the spontaneous formation of Anderson-localized modes thereby limiting slow-light propagation and its potential applications. On the other hand, disorder-induced cavities offer an approach to cavity-quantum electrodynamics and random lasing at the nanoscale. The key statistical parameter governing the disorder effects is the localization length, which together with the waveguide length determines the statistical transport of light through the waveguide. In a disordered photonic-crystal waveguide, the localization length is highly dispersive, and therefore, by controlling the underlying lattice parameters, it is possible to tune the localization of the mode. In the present work, we study the localization length in a disordered photonic-crystal waveguide using numerical simulations. We demonstrate two different localization regimes in the dispersion diagram where the localization length is linked to the density of states and the photon effective mass, respectively. The two different localization regimes are identified in experiments by recording the photoluminescence from quantum dots embedded in photonic-crystal waveguides.

Frustrated magnetic systems exhibit many fascinating phases. Prime among them are quantum spin liquids, where the magnetic moments do not order even at zero temperature. A subclass of quantum spin liquids called Kitaev spin liquids are particularly interesting, because they are exactly solvable, can be realized in certain materials, and show a large variety of gapless and gapped phases. Here we show that nonsymmorphic symmetries can enrich spin liquid phases, such that the low-energy spinon degrees of freedom form three-dimensional Dirac cones or nodal chains. In addition, we suggest a realization of such Kitaev spin liquids in metal-organic frameworks.

We report on the low-temperature magnetic properties of the ${\mathrm{DyScO}}_3$ perovskite, which were characterized by means of single crystal and powder neutron scattering, and by magnetization measurements. Below $T_{\mathrm{N}}=3.15$ K, ${\mathrm{Dy}}^{3+}$ moments form an antiferromagnetic structure with an easy axis of magnetization lying in the $ab$ plane. The magnetic moments are inclined at an angle of $\sim \pm {28}^{\circ }$ to the $b$ axis. We show that the ground-state Kramers doublet of ${\mathrm{Dy}}^{3+}$ is made up of primarily $|\pm 15/2\rangle$ eigenvectors and well separated by a crystal field from the first excited state at $E_1=24.9$ meV. This leads to an extreme Ising single-ion anisotropy, $M_{\perp }/M_{\parallel }\sim 0.05$. The transverse magnetic fluctuations, which are proportional to $M_{\perp }^2/M_{\parallel }^2$, are suppressed, and only moment fluctuations along the local Ising direction are allowed. We also found that the Dy-Dy dipolar interactions along the crystallographic $c$ axis are two to four times larger than in-plane interactions.

Finite ${\mathrm{MoS}}_2$ nanoparticles are known to support metallic edge states that are responsible for their catalytic activity. In this work we employ time-dependent density-functional theory (TDDFT) to study the influence of such edge states on the optical properties of triangular ${\mathrm{MoS}}_2$ monolayer flakes. We find that the edge states support collective plasmon-like excitations that couple strongly to the optical field leading to pronounced absorption peaks below the onset of interband transitions on the basal plane. Additionally, structural relaxation of the flakes can significantly distort the edge states. Thus, we observe that while an evenly-spaced edge configuration supports one-dimensional (1D) plasmon modes similar to those of an ideal 1D electron gas, the relaxed structures show mixed plasmon and single-electron excitations in the low-energy response. Our findings illustrate the sensitivity of the optical response of ${\mathrm{MoS}}_2$ nanostructures to the details of the edge configuration.

Fracton topological phases host fractionalized excitations that are either completely immobile or only mobile along certain lines or planes. We demonstrate how such phases can be understood in terms of two fundamentally different types of parton constructions, in which physical degrees of freedom are decomposed into clusters of “parton” degrees of freedom subject to emergent gauge constraints. First, we employ noninteracting partons subject to multiple overlapping constraints to describe a fermionic fracton model. Second, we demonstrate how interacting partons can be used to develop new models of bosonic fracton phases, both with string and membrane logical operators (type-I fracton phases) and with fractal logical operators (type-II fracton phases). In particular, we find a new type-II model, which saturates a bound on its information storage capacity. Our parton approach is generic beyond exactly solvable models and provides a variational route to realizing fracton phases in more physically realistic systems.