The two-dimensional (2D) crystal SiTe is identified to be a 2D topological insulator (TI) with bulk band gap of 220 meV by means of first-principles calculations. The synthesis of 2D SiTe has been reported earlier [Phys. Status Solidi RRL 8, 302 (2014)] as part of a three-dimensional superlattice. The freestanding monolayer is thermally and dynamically stable and only weakly bound within the layered superlattice, offering the possibility of mechanical exfoliation. Our discovery of a topological signature with large band gap raises the expectation that the most apparent showstopper in experimental 2D TI research, the lack of stable materials exposing a quantum spin Hall effect at room temperature, can be overcome. This offers many laboratories an opportunity to participate in investigating exciting new phenomena in condensed matter physics, such as new quasiparticles and dissipationless spin transport.

We introduce a classification of symmetry-protected topological phases applicable to stationary states of open systems based on a generalization of the many-body polarization. The polarization can be used to probe topological properties of noninteracting and interacting closed and open systems and remains a meaningful quantity even in the presence of moderate particle-number fluctuations. As examples, we discuss two open-system versions of a topological Thouless pump in the steady state of one-dimensional lattices driven by Markovian reservoirs. In an analogous unitary system, the Su-Shrieffer-Heeger model, symmetries enforce a quantization of the geometric Zak phase, which acts as a topological invariant. Introducing a further degree of freedom, a nontrivial winding of the phase can be observed upon cyclic variations of parameters. Associated with this is a quantization or, respectively, a winding of the polarization corresponding to a quantized transport (Thouless pump). We here show that in the open system, where the Zak phase loses its meaning, the same symmetries enforce a quantization and more generally a winding of the generalized many-body polarization. These features are shown to be robust against Hamiltonian perturbations as well as homogeneous dephasing and particle losses.

The strongly correlated Kondo insulator ${\mathrm{SmB}}_6$ is known for its peculiar low-temperature residual conduction, which has recently been demonstrated to arise from a robust metallic surface, as predicted by the theory of topological Kondo insulator (TKI). Photoemission, quantum oscillation, and magnetic doping experiments have provided evidence for the Dirac-like dispersion and topological protection. Questions arise as to whether signatures of spin-momentum locking and electron interaction could be resolved in transport measurements. Here we report metallic conduction of surface down to mK temperatures. We observe in the surface the weak-antilocalization (WAL) effect at small perpendicular magnetic fields. At larger perpendicular magnetic fields, the surface exhibits an unusual linear magnetoresistance similar to those found in Bi-based topological insulators and in graphene.

With ${}^6\mathrm{Li}\text{D}$ as an example, we show that the applicable region of the orbital-free molecular dynamics (OFMD) method in a large temperature range is determined by the thermal ionization process of bound electrons in shell structures. The validity boundary of the OFMD method is defined roughly by the balance point of the average thermal energy of an electron and the ionization energy of the lowest localized electronic state. This theoretical proposition is based on the observation that the deviation of the OFMD method originates from its less accurate description to the charge density in partially ionized shells, as compared with the results of the extended first-principles molecular dynamics method, which well reproduces the charge density of shell structures.

The Heusler-type compound of ${\mathrm{Ru}}_2\mathrm{NbGa}$ has been successfully synthesized. X-ray analysis confirms that ${\mathrm{Ru}}_2\mathrm{NbGa}$ crystallizes in a cubic $L{2}_1$ structure. The electronic properties of ${\mathrm{Ru}}_2\mathrm{NbGa}$ have been characterized by means of the transport and nuclear magnetic resonance (NMR) measurements. The temperature dependence of the electrical resistivity exhibits a typical semimetallic behavior. The NMR spin-lattice relaxation rate shows activated behavior at higher temperatures, attributing to the thermally excited carriers across a pseudogap. We have also deduced a low Fermi-level density of states (DOS), being consistent with the semimetallic characteristic for ${\mathrm{Ru}}_2\mathrm{NbGa}$. In addition, we have performed first-principles total-energy calculations including $G_0{W}_0$ and $G{W}_0$ corrections for band gaps to investigate the electronic band structure of ${\mathrm{Ru}}_2\mathrm{NbGa}$. The calculated result reveals an indirect overlap between electron and hole pockets that leads to a residual DOS at the Fermi level, providing a consistent explanation for the experimental observations.

We present a calculation of the lattice thermal conductivity of Si-Ge nanowires (NWs), based on solving the Boltzmann transport equation by the Monte Carlo method of sampling the phonon mean free paths. We augment the previous work with the full phonon dispersion and a partially diffuse momentum-dependent specularity model for boundary roughness scattering. We find that phonon flights are comprised of a mix of long free flights over several $\mu \mathrm{m}$ interrupted by bursts of short flights, resulting in a heavy-tailed distribution of flight lengths, typically encountered in Lévy walk dynamics. Consequently, phonon transport in Si-Ge NWs is neither entirely ballistic nor diffusive; instead, it falls into an intermediate regime called superdiffusion where thermal conductivity scales with the length of the NW as $\kappa \propto L^{\alpha }$ with the exponent of length dependence $\alpha \approx 0.33$ over a broad range of wire lengths $10\mathrm{nm}<L<10\mu \mathrm{m}$ regardless of diameter and roughness. We conclude that thermal conductivity in Si-Ge alloy NWs is length dependent up to $10 \mu \mathrm{m}$ and therefore can be tuned for thermoelectric applications.

Engineered isotope variation is a pathway toward modulating lattice thermal conductivity (κ) of a material through changes in phonon-isotope scattering. The effects of isotope variation on intrinsic thermal resistance is little explored, as varying isotopes have relatively small differences in mass and thus do not affect bulk phonon dispersions. However, for light elements, isotope mass variation can be relatively large (e.g., hydrogen and deuterium). Using a first principles Peierls-Boltzmann transport equation approach, the effects of isotope variance on lattice thermal transport in ultra-low-mass compound materials LiH and LiF are characterized. The isotope mass variance modifies the intrinsic thermal resistance via modulation of acoustic and optic phonon frequencies, while phonon-isotope scattering from mass disorder plays only a minor role. This leads to some unusual cases where κ values of isotopically pure systems (${}^6\mathrm{LiH}{,}^7\mathrm{L}{\mathrm{i}}^2\mathrm{H}$, and ${}^6\mathrm{LiF}$) are lower than the values from their counterparts with naturally occurring isotopes and phonon-isotope scattering. However, these κ differences are relatively small. The effects of temperature-driven lattice expansion on phonon dispersions and calculated κ are also discussed. This paper provides insight into lattice thermal conductivity modulation with mass variation and the interplay of intrinsic phonon-phonon and phonon-isotope scattering in interesting light atom systems.

We present a comprehensive powder neutron diffraction study on the heavy-fermion Kondo lattice system ${\mathrm{CeCrGe}}_3$ which orders ferromagnetically below $T_C=70\mathrm{K}$. The neutron diffraction data corroborate the presence of magnetic Bragg peaks below $T_C$ which could be indexed with the propagation vector $\vec{k}=\left(000\right)$. Due to the presence of a very few magnetic reflections at low $T$, we have used ratio of intensities between the temperatures below (10 K) and above $T_C$ (80 K) to analyze magnetic structure, rather than the conventional Rietveld refinement method. Our analysis of the magnetic structure reveals an ordered magnetic moment of 1.1(1) ${\mu }_B$ for the Cr moments and 0.8(2) ${\mu }_B$ for the Ce moments where both are aligned in the $ab$ plane at $T=10\mathrm{K}$. Remarkably, the presence of Ce moment is found to have a strong correlation with Cr moment and sets the Cr moments to order in the $ab$ plane rather than along the $c$ axis which was seen in $4f^0$ counterpart, i.e., ${\mathrm{LaCrGe}}_3$.

I propose the emergence of the spin-orbital-coupled vector chirality in a nonfrustrated Mott insulator with the strong spin-orbit coupling due to $ab$-plane's inversion-symmetry (IS) breaking. I derive the superexchange interactions for a $t_{2g}$-orbital Hubbard model on a square lattice with the strong spin-orbit coupling and the IS-breaking-induced hopping integrals, and explain the microscopic origins of the Dzyaloshinsky-Moriya (DM) -type and the Kitaev-type interactions. Then, by adopting the mean-field approximation to a minimal model including only the Heisenberg-type and the DM-type nearest-neighbor interactions, I show that the IS breaking causes the spin-orbital-coupled chirality as a result of stabilizing the screw state. I also highlight the limit of the hard-pseudospin approximation in discussing the stability of the screw states in the presence of both the DM-type and the Kitaev-type interactions, and discuss its meaning. I finally discuss the effects of tetragonal crystal field and $J_{\mathrm{eff}}=\frac{3}{2}$ states, and the application to the iridates near the $\left[001\right]$ surface of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ and the interface between ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ and ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$.

We show that in ${\mathrm{Pr}}_{0.5}{\mathrm{La}}_{0.2}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3/{\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ (PLCMO/YBCO) multilayers the low temperature state of YBCO is very resistive and resembles that of a granular superconductor or a frustrated Josephson-junction network. Notably, a coherent superconducting response can be restored with a large magnetic field which also suppresses the charge-orbital order in PLCMO. This coincidence suggests that the granular superconducting state of YBCO is induced by the charge-orbital order of PLCMO. The coupling mechanism and the nature of the induced inhomogeneous state in YBCO remain to be understood.

We investigate a long-time asymptotic state of periodically driven open quantum systems analytically. The model we consider in this paper is a free fermionic system coupled to an energy and particle reservoir. We clarify some generic properties of the system which are independent of the details of the reservoir in the high-frequency regime of the external driving. When the frequency of the external driving is much larger than the energy cutoff of the system-reservoir coupling, the low-energy properties of the system are equivalent to those of the Gibbs distribution of a Floquet effective Hamiltonian. Furthermore, we investigate the effect of finite dissipation on the system, and elucidate that we cannot suppress excitations by merely increasing the system-reservoir coupling because the excitations are created through the reservoir by the external driving when the frequency is smaller than the energy cutoff of the system-reservoir coupling.

We derive a $c$-number generalized Langevin equation (GLE) describing the evolution of the expectation values ${〈x_i〉}_t$ of the atomic position operators $x_i$ of an open system. The latter is coupled linearly to a harmonic bath kept at a fixed temperature. The equations of motion contain a non-Markovian friction term with the classical kernel [L. Kantorovich, Phys. Rev. B 78, 094304 (2008)] and a zero mean non-Gaussian random force with correlation functions that depend on the initial preparation of the open system. We used a density operator formalism without assuming that initially the combined system was decoupled. The only approximation made in deriving quantum GLE consists of assuming that the Hamiltonian of the open system at time $t$ can be expanded up to the second order with respect to operators of atomic displacements $u_i=x_i-{〈x_i〉}_t$ in the open system around their exact atomic positions ${〈x_i〉}_t$ (the “harmonization” approximation). The noise is introduced to ensure that sampling many quantum GLE trajectories yields exactly the average one. An explicit expression for the pair correlation function of the noise, consistent with the classical limit, is also proposed. Unlike the usually considered quantum operator GLE, the proposed $c$-number quantum GLE can be used in direct molecular dynamic simulations of open systems under general equilibrium or nonequilibrium conditions.

It was recently found that Coulomb interaction can induce a series of nontrivial spectral and transport properties in a two-dimensional semi-Dirac fermion system. Different from graphene that is basically an ordinary Fermi liquid, the Coulomb interaction in this system invalidates the Fermi liquid description over a wide range of energy scales. We present a systematic renormalization group analysis of the interplay of Coulomb interaction and quenched disorder, and show that they have substantial mutual effects on each other, which leads to a variety of quantum phase transitions and non-Fermi liquid behaviors. The low-energy behaviors of the system depend sensitively on the effective strength of Coulomb interaction and disorder.

We present a microscopic semianalytical theory for the description of organic molecules interacting strongly with a cavity mode. Exciton-vibration coupling within the molecule and exciton-cavity interaction are treated on an equal footing by employing a temperature-dependent variational approach. The interplay between strong exciton-vibration coupling and strong exciton-cavity coupling gives rise to a hybrid ground state, which we refer to as the lower polaron polariton. Explicit expressions for the ground-state wave function, the zero-temperature quasiparticle weight of the lower polaron polariton, the photoluminescence line strength, and the mean number of vibrational quanta are obtained in terms of the optimal variational parameters. The dependence of these quantities upon the exciton-cavity coupling strength reveals that strong cavity coupling leads to an enhanced vibrational dressing of the cavity mode, and at the same time a vibrational decoupling of the dark excitons, which in turn results in a lower polaron polariton resembling a single-mode dressed bare lower polariton in the strong-coupling regime. Thermal effects on several observables are briefly discussed.

Exceptional points (EPs) are degeneracies in open wave systems where at least two energy levels and their corresponding eigenstates coalesce. We report evidence of the existence of EPs in three-dimensional (3D) plasmonic nanostructures. The systems are composed of coupled plasmonic nanoresonators and can be judiciously and systematically driven to EPs by controlling symmetry-compatible modes via their near-field and far-field interactions. The proposed platform opens the way to the investigation of EPs for enhanced light-matter interactions and applications in communication, sensing, and imaging.

We report on a detailed investigation of the spin-$1 J_1\text{−}{J}_2\text{−}{J}_3$ Heisenberg model, a frustrated model with nearest-neighbor coupling $J_1$, next-nearest neighbor coupling $J_2$, and a three-site interaction $J_3\left[({\mathbf{S}}_{i-1}·{\mathbf{S}}_i)({\mathbf{S}}_i·{\mathbf{S}}_{i+1})+\mathrm{H}.\mathrm{c}.\right]$ previously studied in [Phys. Rev. B 93, 241108(R) (2016)]. Using density matrix renormalization group (DMRG) and exact diagonalizations, we show that the phase boundaries between the Haldane phase, the next-nearest neighbor Haldane phase, and the dimerized phase can be very accurately determined by combining the information deduced from the dimerization, the ground-state energy, the entanglement spectrum and the Berry phase. By a careful investigation of the finite-size spectrum, we also show that the transition between the next-nearest neighbor Haldane phase and the dimerized phase is in the Ising universality class all along the critical line. Furthermore, we justify the conformal embedding of the $\mathrm{SU}{\left(2\right)}_2$ Wess-Zumino-Witten conformal field theory in terms of a boson and an Ising field, and we explicitly derive a number of consequences of this embedding for the spectrum along the $\mathrm{SU}{\left(2\right)}_2$ transition line between the Haldane phase and the dimerized phase. We also show that the solitons along the first-order transition line between the Haldane phase and the dimerized phase carry a spin-1/2, while the domain walls between different dimerization domains inside the dimerized phase carry a spin 1. Finally, we show that short-range correlations change character in the Haldane and dimerized phases through disorder and Lifshitz lines, as well as through the development of short-range dimer correlations in the Haldane phase, leading to a remarkably rich phase diagram.

We present a comprehensive study of the low-temperature heat capacity and thermal expansion of single crystals of the hole-doped ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ series $\left(0<x<1\right)$ and the end-members ${\mathrm{RbFe}}_2{\mathrm{As}}_2$ and ${\mathrm{CsFe}}_2{\mathrm{As}}_2$. A large increase of the Sommerfeld coefficient ${\gamma }_n$ is observed with both decreasing band filling and isovalent substitution (K, Rb, and Cs) revealing a strong enhancement of electron correlations and the possible proximity of these materials to a Mott insulator. This trend is well reproduced theoretically by our density functional theory + slave-spin (DFT+SS) calculations, confirming that 122-iron pnictides are effectively Hund metals, in which sizable Hund's coupling and orbital selectivity are the key ingredients for tuning correlations. We also find direct evidence for the existence of a coherence-incoherence crossover between a low-temperature heavy Fermi liquid and a highly incoherent high-temperature regime similar to heavy fermion systems. In the superconducting state, clear signatures of multiband superconductivity are observed with no evidence for nodes in the energy gaps, ruling out the existence of a doping-induced change of symmetry (from $s$ to $d$ wave). We argue that the disappearance of the electron band in the range $0.4<x<1.0$ is accompanied by a strong-to-weak coupling crossover and that this shallow band remains involved in the superconducting pairing, although its contribution to the normal state fades away. Differences between hole- and electron-doped ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ series are emphasized and discussed in terms of strong pair breaking by potential scatterers beyond the Born limit.