A local thickness-dependent permittivity is derived in closed form for bounded nonlocal wire-medium structures. The model takes into account spatial dispersion (as an average per length of the wires) and the effect of the boundary, and shows that an effective local model of a nonlocal medium is thickness dependent. The permittivity is comprised of a local bulk (Drude) term and a boundary- and thickness-dependent term, the latter including the effect of spatial nonlocality. Results are obtained for different wire-medium topologies which possess strong spatial dispersion, demonstrating good agreement with a nonlocal homogenization model and full-wave simulation results.

The calculations of electronic transport coefficients and optical properties require a very dense interpolation of the electronic band structure in reciprocal space that is computationally expensive and may have issues with band crossing and degeneracies. Capitalizing on a recently developed pseudoatomic orbital projection technique, we exploit the exact tight-binding representation of the first-principles electronic structure for the purposes of (i) providing an efficient strategy to explore the full band structure $E_n\left(\mathbf{k}\right)$, (ii) computing the momentum operator differentiating directly the Hamiltonian, and (iii) calculating the imaginary part of the dielectric function. This enables us to determine the Boltzmann transport coefficients and the optical properties within the independent particle approximation. In addition, the local nature of the tight-binding representation facilitates the calculation of the ballistic transport within the Landauer theory for systems with hundreds of atoms. In order to validate our approach we study the multivalley band structure of ${\mathrm{CoSb}}_3$ and a large core-shell nanowire using the ACBN0 functional. In ${\mathrm{CoSb}}_3$ we point the many band minima contributing to the electronic transport that enhance the thermoelectric properties; for the core-shell nanowire we identify possible mechanisms for photo-current generation and justify the presence of protected transport channels in the wire.

We investigate the entanglement spectrum of the ground state of the Su-Schrieffer-Heeger-Hubbard model. The topological phases of the model can be identified by degeneracy of the largest eigenvalues of entanglement spectrum. The study of the periodic boundary condition is enough to obtain the phase diagram of the model, without the consideration of the open boundary condition case. By analyzing the states corresponding to the degenerate entanglement spectrum and showing the related edge states in real space, physical interpretation about the relation between bulk-edge correspondence and the entanglement spectrum is presented. The method of the entanglement spectrum can be applicable in studying other topological phases of matter.

We explore the effect of multiorbital electron-electron interactions in a two-dimensional monolayer made of elemental carbon. Using density functional dynamical mean-field theory (DFDMFT), we show that the interplay between one-particle band narrowing and sizable on-site interactions naturally stabilizes the Mott insulating state in isotropically superstrained graphene. Our theory is expected to be a key step to understanding both the ability of graphene to afford large strain deformations and the changes in electronic degrees of freedom of $p$-band Coulomb interacting electrons for the next generation of flexible electronics made of semiconductive graphene.

We study the Kane-Mele-Hubbard model with an additional inversion-symmetry-breaking term. Using the topological Hamiltonian approach, we calculate the ${\mathbb{Z}}_2$ invariant of the system as function of spin-orbit coupling, Hubbard interaction $U$, and inversion-symmetry-breaking onsite potential. The phase diagram calculated in that way shows that, on the one hand, a large term of the latter kind destroys the topological nontrivial state. On the other hand, however, this inversion-symmetry-breaking field can enhance the topological state since for moderate values the transition from the nontrivial topological to the trivial Mott insulator is pushed to larger values of interaction $U$. This feature of an enhanced topological state is also found on honeycomb ribbons. With inversion symmetry, the edge of the zigzag ribbon is magnetic for any value of $U$. This magnetic moment destroys the gapless edge mode. Lifting inversion symmetry allows for a finite region in interaction strength $U$ below which gapless edge modes exist.

We report a systematic analysis of the performance of a widely used set of Dirac-Fock pseudopotentials for quantum Monte Carlo (QMC) calculations. We study each atom in the periodic table from hydrogen ($Z=1$) to mercury ($Z=80$), with the exception of the $4f$ elements ($57\le Z\le 70$). We demonstrate that ghost states are a potentially serious problem when plane-wave basis sets are used in density functional theory (DFT) orbital-generation calculations, but that this problem can be almost entirely eliminated by choosing the $s$ channel to be local in the DFT calculation; the $d$ channel can then be chosen to be local in subsequent QMC calculations, which generally leads to more accurate results. We investigate the achievable energy variance per electron with different levels of trial wave function and we determine appropriate plane-wave cutoff energies for DFT calculations for each pseudopotential. We demonstrate that the so-called “T-move” scheme in diffusion Monte Carlo is essential for many elements. We investigate the optimal choice of spherical integration rule for pseudopotential projectors in QMC calculations. The information reported here will prove crucial in the planning and execution of QMC projects involving beyond-first-row elements.

Two-dimensional (2D) metallic states formed on the $\mathrm{ZnO}(10\overline{1}0)$ surface by hydrogen adsorption have been investigated using angle-resolved photoelectron spectroscopy (ARPES). The observed metallic state is characterized by a peak-dip-hump structure at just below the Fermi level and a long tail structure extending up to 600 meV in binding energy. The peak and hump positions are separated by about 70 meV, a value close to the excitation energy of longitudinal optical (LO) phonons. Spectral functions formulated on the basis of the 2D electron-phonon coupling well reproduce the ARPES intensity distribution of the metallic states. This spectral analysis suggests that the 2D electrons accumulated on the ZnO surface couple to the LO phonons and that this coupling is the origin of the anomalous long tail. Our results indicate that the 2D electrons at the ZnO surface are described as the electron liquid model.

Two-dimensional (2D) phosphorene, which possesses fascinating physical and chemical properties distinctively different from other 2D materials, calls for a fundamental understanding of thermal transport properties for its rapidly growing applications in nano- and optoelectronics and thermoelectrics. However, even the basic phonon property, for example, the exact value of the lattice thermal conductivity ($\kappa$) of phosphorene reported in the literature, can differ unacceptably by one order of magnitude. More importantly, the fundamental physics underlying its unique properties such as strong phonon anharmonicity and unusual anisotropy remains largely unknown. In this paper, based on the analysis of electronic structure and lattice dynamics from first principles, we report that the giant phonon anharmonicity in phosphorene is associated with the soft transverse optical (TO) phonon modes and arises from the long-range interactions driven by the orbital governed resonant bonding. We also provide a microscopic picture connecting the anisotropic and low $\kappa$ of phosphorene to the giant directional phonon anharmonicity and long-range interactions, which are further traced back to the asymmetric resonant orbital occupations of electrons and characteristics of the hinge-like structure. The unambiguously low $\kappa$ of phosphorene obtained consistently by three independent ab initio methods confirms the phonon anharmonicity to a large extent and is expected to end the confusing huge deviations in previous studies. This work further pinpoints the necessity of including van der Waals interactions to accurately describe the interatomic interactions in phosphorene. We propose in 2D material that resonant bonding leads to low thermal conductivity, despite that it is originally found in three-dimensional (3D) thermoelectric and phase-change materials. Our study offers insights into phonon transport from the view of orbital states, which would be of great significance to the design of emerging phosphorene-based nanodevices.

The propagator of the induced dynamically screened Coulomb interaction $W^{\mathrm{ind}}(\mathbf{Q},\omega ,z,z^{\prime })$ is calculated for the ${\mathrm{MoS}}_2$ monolayer. The energy-loss rate of a point charge placed near the ${\mathrm{MoS}}_2$ monolayer is calculated (using the spatial resolution of the $W^{\mathrm{ind}}$ in the direction perpendicular to the ${\mathrm{MoS}}_2$ plane) and successfully compared with very recent electron-energy-loss spectroscopy measurements of Hong et al. [J. Hong et al., Phys. Rev. B 93, 075440 (2016)]. The induced propagator $W^{\mathrm{ind}}$, compared with its classical analogous propagator, is used to determine the effective dielectric function ${\epsilon }_{\mathrm{eff}}\left(\omega \right)$ of atomically thick crystals. It is shown that ${\epsilon }_{\mathrm{eff}}\left(\omega \right)$ extracted from $W^{\mathrm{ind}}$ is in good agreement with the dielectric function of the ${\mathrm{MoS}}_2$ extracted from the ellipsometry measurements. A recently proposed method for calculation of the dielectric tensor in quasi-two-dimensional crystals is used to calculate the parallel ${\epsilon }_{\parallel }\left(\omega \right)$ and perpendicular ${\epsilon }_{\perp }\left(\omega \right)$ dielectric functions in ${\mathrm{MoS}}_2$, which are compared with some previous measurements and calculations.

A Langevin model accounting for all six molecular degrees of freedom is applied to femtosecond-laser induced, hot-electron driven dynamics of Ru(0001)($2\times 2$):CO. In our molecular dynamics with electronic friction approach, a recently developed potential energy surface based on gradient-corrected density functional theory accounting for van der Waals interactions is adopted. Electronic friction due to the coupling of molecular degrees of freedom to electron-hole pairs in the metal are included via a local density friction approximation, and surface phonons by a generalized Langevin oscillator model. The action of ultrashort laser pulses enters through a substrate-mediated, hot-electron mechanism via a time-dependent electronic temperature (derived from a two-temperature model), causing random forces acting on the molecule. The model is applied to laser induced lateral diffusion of CO on the surface, “hot adsorbate” formation, and laser induced desorption. Reaction probabilities are strongly enhanced compared to purely thermal processes, both for diffusion and desorption. Reaction yields depend in a characteristic (nonlinear) fashion on the applied laser fluence, as well as branching ratios for various reaction channels. Computed two-pulse correlation traces for desorption and other indicators suggest that aside from electron-hole pairs, phonons play a non-negligible role for laser induced dynamics in this system, acting on a surprisingly short time scale. Our simulations on precomputed potentials allow for good statistics and the treatment of long-time dynamics (300 ps), giving insight into this system which hitherto has not been reached. We find generally good agreement with experimental data where available and make predictions in addition. A recently proposed laser induced population of physisorbed precursor states could not be observed with the present low-coverage model.

We study the dynamics of exciton-polariton condensates in reconfigurable potentials created by an incoherent optical pump. The mode selection in the effective optically induced trap is shown to be determined by the largest growth rates of the linear eigenmodes of the effective complex potential created by the pump. We demonstrate that selection of higher order nonlinear modes with nonzero orbital angular momenta in such traps leads to formation of vortices in a single-component condensate and spin textures in exciton-polariton condensates consisting of two polarization components. We also show that cross-polarization coupling can lead to the synchronization of the modes belonging to different polarizations. We highlight clear signatures of the various spontaneously formed trapped states in the cavity photoluminescence spectrum, which should aid in their experimental observations.

We investigate the effects of fluctuations on the dynamics of an isolated quantum system represented by a ${\left({\mathbf{\varphi }}^2\right)}^2$ field theory with $O\left(N\right)$ symmetry after a quench in $d>2$ spatial dimensions. A perturbative renormalization-group approach involving a dimensional expansion in $\epsilon =4-d$ is employed in order to study the evolution within a prethermal regime controlled by elastic dephasing. In particular, we focus on a quench from a disordered initial state to the critical point, which introduces an effective temporal boundary in the evolution. At this boundary, the relevant fields acquire an anomalous scaling dimension, while the evolution of both the order parameter and its correlation and response functions display universal aging. Since the relevant excitations propagate ballistically, a light cone in real space emerges. At longer times, the onset of inelastic scattering appears as secularly growing self-generated dissipation in the effective Keldysh field theory, with the strength of the dissipative vertices providing an estimate for the time needed to leave the prethermal regime.

Charge transport in AgI subject to an external magnetic field is studied via computer simulations. We demonstrate that a recently developed algorithm can effectively complement problematic experiments to detect the ionic Hall effect, and identify previously unreported effects such as ionic magnetoresistance. We focus first on the charge transport properties of superionic AgI, showing that the magnetic field induces a considerable reduction in the diagonal elements of the conductivity tensor (magnetoresistance). Within the limits of the signal-to-noise ratio, calculation of the off-diagonal elements of this tensor also shows the onset of the Hall effect in this material. We then present numerical evidence supporting the use of the Nerst-Einstein approximation to obtain the Hall mobility of the system. This approximation enables very efficient detection of the Hall signal, with values of the mobility and of the migration activation energy in very good agreement with experiments. Having validated our simulation approach, we consider melted AgI to investigate if the Hall signal persists in this nonsuperionic case, finding a detectable signal. Comparison of the diffusion tensor of this system with that of molten NaCl also indicates why the Hall signal is absent in the latter. This work paves the way for the routine use of simulations to study transport, and in particular the ionic Hall effect, in ionic systems under external magnetic field.

To investigate the barrier effect of grain boundaries on the propagation of avalanchelike plasticity at the atomic scale, we perform three-dimensional molecular dynamics simulations by using simplified polycrystal models including symmetric-tilt grain boundaries. The cutoffs of the stress-drop distributions following power-law distributions decrease as the size of the crystal grains decreases. We show that some deformation avalanches are confined by grain boundaries; on the other hand, unignorable avalanches penetrate all the grain boundaries included in the models. The blocking probability that one grain boundary hinders this system-spanning avalanche is evaluated by using an elemental probabilistic model.

We present a high-energy-resolution inelastic x-ray scattering data investigation of the charge-density-wave (CDW) soft phonon mode upon entering the superconducting state in $2H\text{−}\mathrm{NbS}{\mathrm{e}}_2$. Measurements were done close to the CDW ordering wave vector ${\mathit{q}}_{\mathrm{CDW}}$ at $\mathit{q}={\mathit{q}}_{\mathrm{CDW}}+\left(0,0,l\right),0.15\le l\le 0.5$, for $T=10\mathrm{K}$ (CDW order) and $3.8\mathrm{K}$ (CDW order+superconductivity). We observe changes of the phonon line shape that are characteristic for systems with strong electron-phonon coupling in the presence of a superconducting energy gap $2{\mathrm{\Delta }}_c$ and from which we can demonstrate an $l$ dependence of the superconducting gap. Reversely, our data imply that the CDW energy gap is strongly localized along the $c$* direction. The confinement of the CDW gap to a very small momentum region explains the rather low competition and easy coexistence of CDW order and superconductivity in $2H\text{−}\mathrm{NbS}{\mathrm{e}}_2$. However, the energy gained by opening ${\mathrm{\Delta }}_{\mathrm{CDW}}$ seems to be too small to be the driving force of the phase transition at $T_{\mathrm{CDW}}=33\mathrm{K}$, which is better described as an electron-phonon coupling driven structural phase transition.

We report the structural, magnetic, and thermodynamic properties of the double perovskite compound ${\mathrm{La}}_2{\mathrm{CuIrO}}_6$ from x-ray, neutron diffraction, neutron depolarization, $\mathit{dc}$ magnetization, $\mathit{ac}$ susceptibility, specific heat, muon-spin-relaxation $\left(\mu \mathrm{SR}\right)$, electron-spin-resonance (ESR) and nuclear magnetic resonance (NMR) measurements. Below $\sim 113$ K, short-range spin-spin correlations occur within the ${\mathrm{Cu}}^{2+}$ sublattice. With decreasing temperature, the ${\mathrm{Ir}}^{4+}$ sublattice is progressively involved in the correlation process. Below $T=74$ K, the magnetic sublattices of Cu (spin $\mathit{s}=\frac{1}{2}$) and Ir (pseudospin $\mathit{j}=\frac{1}{2}$) in ${\mathrm{La}}_2{\mathrm{CuIrO}}_6$ are strongly coupled and exhibit an antiferromagnetic phase transition into a noncollinear magnetic structure accompanied by a small uncompensated transverse moment. A weak anomaly in $\mathit{ac}$ susceptibility as well as in the NMR and $\mu \mathrm{SR}$ spin lattice relaxation rates at 54 K is interpreted as a cooperative ordering of the transverse moments which is influenced by the strong spin-orbit coupled $5\mathit{d}$ ion ${\mathrm{Ir}}^{4+}$. We argue that the rich magnetic behavior observed in ${\mathrm{La}}_2{\mathrm{CuIrO}}_6$ is related to complex magnetic interactions between the strongly correlated spin-only $3\mathit{d}$ ions with the strongly spin-orbit coupled $5\mathit{d}$ transition ions where a combination of the spin-orbit coupling and the low symmetry of the crystal lattice plays a special role for the spin structure in the magnetically ordered state.

Order by disorder is a decision making process for frustrated systems but often leads to simple answers. We study order by disorder in the kagome Kondo model known for its complexity seeking rich decision making capabilities. At half-filling and large Kondo coupling to hopping ratio $J_K/t$, the full manifold of ${120}^{\circ }$ kagome ground states are degenerate at second order in $t/J_K$. We show this degeneracy lifts at sixth order when a fermion can hop around a hexagon and feel the Berry flux induced by a given spin texture. Using the Monte Carlo method, we then seek the ground state of this sixth-order Hamiltonian and find in a $4\times 4$ unit cell system that a coplanar 12-site unit cell order is selected over the $\sqrt{3}\times \sqrt{3},q=0$ and cuboc1 states, a result that survives even in the thermodynamic limit. This state is selected for its SU(2) flux properties induced by the spin texture. Given the existence of numerous quantum Hall plateaus for electrons in a magnetic field, the existence of this 12-site unit cell state suggests that complex decision making is possible on the manifold of ${120}^{\circ }$ states and achievable in different kagome Kondo models.

Diverse phenomena emerge at the interface between band insulators $\mathrm{LaAl}{\mathrm{O}}_3$ and $\mathrm{SrTi}{\mathrm{O}}_3$, such as superconductivity and ferromagnetism, showing an opportunity for potential applications as well as contributing to fundamental research interests. Here, we report the superconductor-metal transition driven by a perpendicular magnetic field in superconducting two-dimensional electron gas formed at the $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3\left(110\right)$ interface, which offers an appealing platform for quantum phase transition from a superconductor to a weakly localized metal. Interestingly, when approaching the quantum critical point, the dynamic critical exponent is not a constant but a diverging value, which is direct evidence of a quantum Griffiths singularity arising from quenched disorder at ultralow temperatures. Furthermore, the hysteretic property of magnetoresistance is observed at the $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3\left(110\right)$ interface, which suggests the potential coexistence of superconductivity and ferromagnetism.

We compute the ground-state correlation functions of an exactly solvable chain of integer spins, recently introduced in [R. Movassagh and P. W. Shor, arXiv:1408.1657], whose ground state can be expressed in terms of a uniform superposition of all colored Motzkin paths. Our analytical results show that for spin $s\ge 2$ there is a violation of the cluster decomposition property. This has to be contrasted with $s=1$, where the cluster property holds. Correspondingly, for $s=1$ one gets a light-cone profile in the propagation of excitations after a local quench, while the cone is absent for $s=2$, as shown by time dependent density-matrix renormalization group. Moreover, we introduce an original solvable model of half-integer spins, which we refer to as Fredkin spin chain, whose ground state can be expressed in terms of superposition of all Dyck paths. For this model we exactly calculate the magnetization and correlation functions, finding that for $s=1/2$, a conelike propagation occurs, while for higher spins, $s\ge 3/2$, the colors prevent any cone formation and clustering is violated, together with square root deviation from the area law for the entanglement entropy.