A simple invisible structure made of two anisotropic homogeneous layers is analyzed theoretically in temporal regime. The frequency dispersion is introduced and analytic expression of the transient part of the field is derived for large times when the structure is illuminated by a causal excitation. This expression shows that the limiting amplitude principle applies with transient fields decaying as the power $-3/4$ of the time. The quality of the cloak is then reduced at short times and remains preserved at large times. The one-dimensional theoretical analysis is supplemented with full-wave numerical simulations in two-dimensional situations which confirm the effect of dispersion.

We identify the deviation from the straight-line error (DSLE)—i.e., the spurious nonlinearity of the total energy as a function of fractional particle number—as the main source for the discrepancy between experimental vertical ionization energies and theoretical quasiparticle energies, as obtained from the $GW$ and $GW+\mathrm{SOSEX}$ approximations to many-body perturbation theory (MBPT). To check whether a DSLE is present in $GW$, we propose an indicator that only invokes observables at integer particle numbers. For self-consistent calculations, we show that $GW$ suffers from a small DSLE. Conversely, for perturbative $G_0{W}_0$ and $G_0{W}_0+\mathrm{SOSEX}$ calculations the DSLE depends on the starting point. We exploit this starting point dependence to reduce (or completely eliminate) the DSLE. We find that the agreement with experiment increases as the DSLE reduces. DSLE-minimized schemes thus emerge as promising avenues for future developments in MBPT.

The evolution of the atomic structure and the vibrational and electronic transport properties of gold atomic junctions incorporating molecular and atomic hydrogen upon elongation have been investigated with the nonequilibrium Green's function formalism combined with density functional theory. Our calculations show that for the case of gold junctions doped with a single ${\mathrm{H}}_2$ molecule the low-bias conductance drops rapidly with the electrodes’ separation, while it remains almost constant if a single H atom replaces the molecule. In contrast, when one considers two H atoms adsorbed on a gold monatomic chain forming an Au-H-Au-H-Au double-bridge structure, the low-bias conductance increases first and then shows a plateau upon stretching the junction, in perfect agreement with experiments on gold nanocontacts in hydrogen environment. Furthermore, also the distribution of the calculated vibrational energies of the two H atoms is consistent with the experimental result in the low-conductance region, demonstrating clear evidence that hydrogen molecules can dissociate on stretched gold monatomic chains. These findings are helpful to improve our understanding of the structure-property relation of gold nanocontacts and also provide a new prospect for gold nanowires being used as chemical sensors and catalysts.

We present a general review of the projective symmetry group classification of fermionic quantum spin liquids for lattice models of spin $S=1/2$. We then introduce a systematic generalization of the approach for symmetric ${\mathbb{Z}}_2$ quantum spin liquids to the one of chiral phases (i.e., singlet states that break time reversal and lattice reflection, but conserve their product). We apply this framework to classify and discuss possible chiral spin liquids on triangular and kagome lattices. We give a detailed prescription on how to construct quadratic spinon Hamiltonians and microscopic wave functions for each representation class on these lattices. Among the chiral ${\mathbb{Z}}_2$ states, we study the subset of U(1) phases variationally in the antiferromagnetic $J_1\text{−}{J}_2\text{−}{J}_d$ Heisenberg model on the kagome lattice. We discuss static spin structure factors and symmetry constraints on the bulk spectra of these phases.

We present a scaling law for the energy and speed of transition waves in dissipative and diffusive media. By considering uniform discrete lattices and continuous solids, we show that—for arbitrary highly nonlinear many-body interactions and multistable on-site potentials—the kinetic energy per density transported by a planar transition wave front always exhibits linear scaling with wave speed and the ratio of energy difference to interface mobility between the two phases. We confirm that the resulting linear superposition applies to highly nonlinear examples from particle to continuum mechanics.

We study the lowest energy configurations of an equimolar binary mixture of classical pointlike particles with charges $Q_1$ and $Q_2$, such that $q=Q_2/Q_1\in [0,1]$. The particles interact pairwise via three-dimensional (3D) Coulomb potential and are confined to a 2D plane with a homogeneous neutralizing background charge density. In a recent paper by M. Antlanger and G. Kahl [Condens. Matter Phys. 16, 43501 (2013)], using numerical computations based on an evolutionary algorithm, six fully mixed structures were identified for $0\le q\lesssim 0.59$, while the separation of $Q_1$ and $Q_2$ pure hexagonal phases minimizes the energy for $0.59\lesssim q<1$. Here, we introduce a structure which consists of the separation of two phases: a pure hexagonal one formed by a fraction of particles with the larger charge $Q_1$ and the other a mixed one containing different numbers of $Q_1$ and $Q_2$ charges. Using an analytic method based on an expansion of the interaction energy in Misra functions, we show that this structure provides the lowest energy in two intervals of $q$ values, $0<q\lesssim 0.04707$ and $0.58895\lesssim q\lesssim 0.61367$. This fact might inspire numerical methods, for both Coulomb and Yukawa interactions, to test more general separations which go beyond the separation of two pure phases.

We study the behavior of exciton polaritons in an optical microcavity with an embedded semiconductor quantum well. We use a two-component exciton-photon approach formulated in terms of path integral formalism. In order to describe spatial distributions of the exciton and photon condensate densities, the two coupled equations of the Gross-Pitaevskii type are derived. For a homogeneous system, we find the noncondensate photon and exciton spectra, calculate the coefficients of transformation from the exciton-photon basis to the lower-upper polariton basis, and obtain the exciton and photon occupation numbers of the lower and upper polariton branches for nonzero temperatures. For an inhomogeneous system, the set of coupled equations of the Bogoliubov–de Gennes type is derived. The equations govern the spectra and spatial distributions of noncondensate photons and excitons.

We study an effective model of two interacting species of bosons in two dimensions, which is amenable to sign problem free Monte Carlo simulations. In addition to conventional ground states, we access a paired superfluid which is also a topological phase, protected by the remaining $U\left(1\right)\times {\mathbb{Z}}_2$ symmetry. This phase arises from the condensation of a composite object, the bound state of vortices and antivortices of one species, to a boson of the second species. We introduce a bulk response function, the Ising analog of the quantized Hall effect, to diagnose the topological phase. The interplay of broken symmetry and topology leads to interesting effects such as fractionally charged vortices in the paired superfluid. Possible extensions towards realistic models of cold atomic bosons are discussed.

Using density functional theory and dynamical mean-field theory, we study the electronic properties of the proposed candidate polar metal ${\mathrm{SrEuMo}}_2{\mathrm{O}}_6$. Its electronic structure shares similarities with centrosymmetric ${\mathrm{SrMoO}}_3$ and ${\mathrm{EuMoO}}_3$, from which it may be considered an ordered derivative, but polar displacements of the divalent cations and oxide anions lift inversion symmetry mediated by an anharmonic lattice interaction in the metallic state. We find that Hund's coupling promotes the effects of electronic correlations owing to the ${\mathrm{Mo}}^{4+} d^2$ electronic configuration, producing a correlated metallic phase far from the Mott state. The contraindication between metallicity and polar distortions is thereby alleviated in part through the renormalized quasiparticles, which are unable to fully screen the ordered local dipoles.

We present a consistent thermodynamic theory for the resonant level model in the wide-band limit, whose level energy is driven slowly by an external force. The problem of defining “system” and “bath” in the strong-coupling regime is circumvented by considering as the system everything that is influenced by the externally driven level. The thermodynamic functions that are obtained to first order beyond the quasistatic limit fulfill the first and second law with a positive entropy production, successfully connect to the forces experienced by the external driving, and reproduce the correct weak-coupling limit of stochastic thermodynamics.

We use the voltage biased tip of a scanning force microscope at a temperature of 35 mK to locally induce the fractional quantum Hall state of $\nu =1/3$ in a split-gate defined constriction. Different tip positions allow us to vary the potential landscape. From the temperature dependence of the conductance plateau at $G=1/3\times e^2/h$ we determine the activation energy of this local $\nu =1/3$ state. We find that at a magnetic field of 6 T the activation energy is between 153 and 194 $\mu \text{eV}$ independent of the shape of the confining potential, but about 50% lower than for bulk samples.

This work reports an experimental and theoretical insight into phenomena of two-color emission and different electron-hole recombination dynamics in InGaN nanodisks, incorporated into pencil-like GaN nanowires. The studied nanodisks consist of one polar (on $c$ facet) and six (nominally) identical semipolar (on $r$ facets) sections, as confirmed by transmission electron microscopy. The combination of cathodoluminescence with scanning electron microscopy spatially resolves the nanodisk two-color emission, the low-energy emission (∼500 nm) originating from the polar section, and the high-energy emission (∼400 nm) originating from the semipolar section. This result has been directly linked to a “facet-dependent” nanodisk composition, the In content being significantly higher in the polar ($\sim 20\%$) vs semipolar ($\sim 10\%$) section (as quantified by energy dispersive x-ray spectroscopy), further leading to a strong facet-dependent strain anisotropy. Time-resolved cathodoluminescence reveals significantly different electron-hole recombination times in the two sections, moderately fast (∼1.3 ns) vs fast (∼0.5 ns) in polar/semipolar sections, respectively, the difference being linked to a strong anisotropy in the nanodisk internal electric fields. To determine the influence of each of the three contributing “facet-related” anisotropies (composition, strain, and electric field) on the two-color emission, a proper simulation [relying on virtual crystal approximation and involving three-dimensional (3D) continuum mechanical modeling, a 3D Poisson equation, and a one-dimensional Schrödinger equation] has been performed. The theoretical simulations allow the three effects to be quantitatively disentangled, revealing a clear hierarchy among their contributing weights, the facet-dependent composition inhomogeneity being identified as the dominant one (and the strain inhomogeneity being identified as the least significant one). As for different recombination times, while it is mainly linked to the internal electric field anisotropy, we also suggest that it is, very likely, influenced by gradually increasing In content along the nanodisk growth direction (lattice-pulling effect); the latter mechanism keeps electrons and holes in (relative) proximity within the polar section, enabling their relatively fast and efficient radiative recombination.

The transient processes of electron transport in nano-scale devices exhibit special phenomena that exist only in the transient regime. Besides how fast the steady states are approached, one interesting aspect of transient transport arises from its strong dependence on the initial state of the system. Here we address the issue of how the symmetries embedded in the initial state interplay with those of the system structure in the course of transient transports. We explicitly explore the transient currents arising from various initial occupations in a double-quantum-dot Aharonov-Bohm interferometer. We find symmetry relations between the transient in-tunneling and out-tunneling dynamics for initially empty or full quantum dots when the energy levels in the electrodes are symmetrically distributed with respect to the energy levels in the quantum dots. This is true for whatever applied fluxes. We also find the flux-even components of the currents and the flux-odd components of the currents exhibit distinct cross-lead symmetric relations.

In the present work, we study the magnetic properties of the ${\mathrm{NbS}}_2$ monolayer by first-principles calculations. The transition metal dichalcogenides (TMDCs) are a family of laminar materials presenting exciting properties such as charge density waves (CDWs), superconductivity, and metal-insulating transitions. $2H\text{−}{\mathrm{NbS}}_2$ is a particular case within the family, because it is the only one that is a superconductor without exhibiting a CDW order. Although no long-range magnetic order was experimentally observed in the TMDCs, we show here that the single monolayer of ${\mathrm{NbS}}_2$ is on the verge of a spin density wave (SDW) phase. Our calculations indicate that a wavelike magnetic order is stabilized in the ${\mathrm{NbS}}_2$ monolayer in the presence of magnetic defects or within zigzag nanoribbons, due to the presence of unpaired electrons. We calculate the real part of the bare electronic susceptibility and the corresponding nesting function of the clean ${\mathrm{NbS}}_2$ monolayer, showing that there are strong electronic instabilities at the same wave vector associated with the calculated SDWs, also corresponding with one of the main nesting vectors of the Fermi surface. We conclude that the physical mechanism behind the spin-wave instabilities are the nesting properties, accentuated by the quasi-2D character of this system, and the rather strong Coulomb interactions of the $4d$ band of the Nb atom. We also estimate the amplitude of the spin fluctuations and find that they are rather large, as expected for a system on the verge of a quantum critical transition.

In this article the mechanism of the linear magnetoelectric (ME) effect in the rhombohedral multiferroic ${\mathrm{BiFeO}}_3$ is considered. The study is based on the symmetry approach of the Ginzburg-Landau type, in which polarization, antiferrodistortion, and antiferromagnetic momentum vectors are viewed as ordering parameters. We demonstrate that the linear ME effect in BFO is caused by reorientation of the antiferrodistortion vector in either electric or magnetic field. The numerical estimations, which show quantitative agreement with the results of the recent measurements in film samples, have been performed. A possibility of significant enhancement of the magnetoelectric effect by applying an external static electric field has been investigated. The considered approach is promising for explaining the high values of the ME effect in composite films and heterostructures with BFO.

Motivated by recent quantum Monte Carlo (QMC) simulations of the quantum Kagome ice model by Juan Carrasquilla et al., [Nat. Commun., 6, 7421 (2015)], we study the ground-state properties of this model on the triangular lattice. In the presence of a magnetic field $h$, the Hamiltonian possesses competing interactions between a $Z_2$-invariant easy-axis ferromagnetic interaction $J_{\pm \pm }$ and a frustrated Ising term $J_z$. As in the U(1)-invariant model, we obtain four classical distinctive phases, however, the classical phases in the $Z_2$-invariant model are different. They are as follows: a fully polarized (FP) ferromagnet for large $h$, an easy-axis canted ferromagnet (CFM) with broken $Z_2$ symmetry for small $h$ and dominant $J_{\pm \pm }$, a ferrosolid phase with broken translational and $Z_2$ symmetries for small $h$ and dominant $J_z$, and two lobes with $m=\langle S_z\rangle =\pm 1/6$ for small $h$ and dominant $J_z$. We show that quantum fluctuations are suppressed in this model, hence the large-$S$ expansion gives an accurate picture of the ground-state properties. When quantum fluctuations are introduced, we show that the ferrosolid state is the ground state in the dominant Ising limit at zero magnetic field. It remains robust for $J_z\to \infty$. With nonzero magnetic field the classical lobes acquire a finite magnetic susceptibility with no $S_z$ order. We present the trends of the ground-state energy and the magnetizations. We also present a detail analysis of the CFM.

Several Tl and Pb based halides and chalcohalides have recently been discovered as promising optoelectronic materials [i.e., photovoltaic (PV) and gamma-ray detection materials]. Efficient carrier transport in these materials is attributed partly to the special chemistry of $n{s}^2$ ions (e.g., $\mathrm{T}{\mathrm{l}}^{+}, \mathrm{P}{\mathrm{b}}^{2+}$, and $\mathrm{B}{\mathrm{i}}^{3+}$). However, the toxicity of Tl and Pb is challenging to the development and the wide use of Tl and Pb based materials. In this paper, we investigate materials that contain $\mathrm{B}{\mathrm{i}}^{3+}$, which is also an $n{s}^2$ ion. By combining Bi halides with Bi chalcogenides or oxides, the resulting ternary compounds exhibit a wide range of band gaps, offering opportunities in various optoelectronic applications. Density functional calculations of electronic structure, dielectric properties, optical properties, and defect properties are performed on selected $\mathrm{B}{\mathrm{i}}^{3+}$ based chalcohalides and oxyhalides, i.e., BiSeBr, BiSI, BiSeI, and BiOBr. We propose different applications for these Bi compounds based on calculated properties, i.e., $n$-BiSeBr, $p$-BiSI, and $p$-BiSeI as PV materials, BiSeBr and BiSI as room-temperature radiation detection materials, and BiOBr as a $p$-type transparent conducting material. BiSeBr, BiSI, and BiSeBr have chain structures while BiOBr has a layered structure. However, in BiSI, BiSeI, and BiOBr, significant valence-band dispersion is found in the directions perpendicular to the atomic chain or layer because the valence-band edge states are dominated by the halogen states that have strong interchain or interlayer coupling. We find significantly enhanced Born effective charges and anomalously large static dielectric constants of the Bi compounds, which should reduce carrier scattering and trapping and promote efficient carrier transport in these materials. The strong screening and the small anion coordination numbers in Bi chalcohalides should lead to weak potentials for electron localization at anion vacancies. Defect calculations indeed show that the anion vacancies (Se and Br vacancies) in BiSeBr are shallow, which is beneficial to efficient electron transport.

The Talbot effect, employing graphene as a grating and using electron matter waves, is simulated using density functional theory and solving the Helmholtz equation. Talbot fractional images and long wavelength images are calculated. The results show focusing effects that suggest possible applications for wavepacket reshaping and interferometry.

Compounds of Fe, Ti, and Sb were prepared using arc melting and vacuum annealing. ${\mathrm{Fe}}_2\mathrm{TiSb}$, expected to be a full Heusler compound crystallizing in the $\mathrm{L}{2}_1$ structure, was shown by XRD and SEM analyses to be composed of weakly magnetic grains of nominal composition ${\mathrm{Fe}}_{1.5}\mathrm{TiSb}$ with iron-rich precipitates in the grain boundaries. FeTiSb, a composition consistent with the formation of a half-Heusler compound, also decomposed into ${\mathrm{Fe}}_{1.5}\mathrm{TiSb}$ grains with Ti-Sb rich precipitates and was weakly magnetic. The dominant ${\mathrm{Fe}}_{1.5}\mathrm{TiSb}$ phase appears to crystallize in a defective $\mathrm{L}{2}_1$-like structure with iron vacancies. Based on this finding, a first-principles DFT-based binary cluster expansion of Fe and vacancies on the Fe sublattice of the $\mathrm{L}{2}_1$ structure was performed. Using the cluster expansion, we computationally scanned $>{10}^3$ configurations and predict a novel, stable, nonmagnetic semiconductor phase to be the zero-temperature ground state. This new structure is an ordered arrangement of Fe and vacancies, belonging to the space group $R3m$, with composition ${\mathrm{Fe}}_{1.5}\mathrm{TiSb}$, i.e., between the full- and half-Heusler compositions. This phase can be visualized as alternate layers of $\mathrm{L}{2}_1$ phase ${\mathrm{Fe}}_2\mathrm{TiSb}$ and $\mathrm{C}{1}_b$ phase FeTiSb, with layering along the [111] direction of the original cubic phases. Our experimental results on annealed samples support this predicted ground-state composition, but further work is required to confirm that the $R3m$ structure is the ground state.