We study the spin excitation spectra of the two-dimensional spin-$\frac{1}{2}$ Heisenberg model with a checkerboard structure using stochastic analytic continuation of the imaginary-time correlation function obtained from a quantum Monte Carlo simulation. The checkerboard models have two different antiferromagnetic nearest-neighbor interactions $J_1$ and $J_2$, and the tuning parameter $g$ is defined as $J_2/J_1$. The dynamic spin structure factors are systematically calculated in all phases of the models as well as at the critical points. To give a full understanding of the dynamic spectra, spin wave theory is employed to explain some features of numerical results, especially for the low-energy part. When $g$ is close to 1, the features of the spin excitation spectra of each checkerboard model are roughly the same as those of the original square lattice antiferromagnetic Heisenberg model, and the high-energy continuum among them is discussed. In contrast to the other checkerboard structures investigated in this paper, the $3\times 3$ checkerboard model has distinctive excitation features, such as a gap between a low-energy gapless branch and a gapped high-energy part that exists when $g$ is small. The gapless branch in this case can be regarded as a spin wave in Néel order formed by a “block spin” in each $3\times 3$ plaquette with an effective exchange interaction originating from renormalization. One unexpected finding is that the continuum also appears in this low-energy branch.

Quantum state transfer from a single photon to a single electron following selection rules can only occur for a spin-resolved light-hole excitation in GaAs quantum dots; however, these phenomena have yet to be experimentally realized. Here, we report on single-shot readout of a single electron spin via the Zeeman-resolved light-hole excitation using an optical spin blockade method in a GaAs quantum dot and a Pauli spin blockade method in a double GaAs quantum dot. The observed photoexcitation probability strongly depends on the photon polarization, an indication of angular momentum transfer from a single photon to an electron. Our demonstration will open a pathway to further investigation of fundamental quantum physics and applications of quantum networking technology.

Helical edge modes of 2D topological insulators are supposed to be protected from time-reversal invariant elastic backscattering. Yet substantial deviations from the perfect conductance are typically observed experimentally down to very low temperatures. To resolve this conundrum, we consider the effect of a single magnetic impurity with arbitrary spin on the helical edge transport. We consider the most general structure of the exchange interaction between the impurity and the edge electrons. We take into the account the local anisotropy for the impurity and show that it strongly affects the backscattering current in a wide range of voltages and temperatures. We show that the sensitivity of the backscattering current to the presence of the local anisotropy is different for half-integer and integer values of the impurity spin. In the latter case, the anisotropy can significantly increase the backscattering correction to the current.

We show that complex periodic metasurfaces can be simply represented by conformal transformations from the flat surface of a slab of material to a periodic grating leading to a methodology for computing their properties. Matrix equations are solved to give accurate solutions of Maxwell's equations with detailed derivations given in the Supplemental Material.

We consider a noninteracting disordered 1D quasicrystal in the weak-disorder regime. We show that the critical states of the pure model approach strong localization in strikingly different ways, depending on their renormalization properties. A finite-size scaling analysis of the inverse participation ratios of states (IPR) of the quasicrystal shows that they are described by several kinds of scaling functions. While most states show a progressively increasing IPR as a function of the scaling variable, other states exhibit a nonmonotonic “reentrant” behavior wherein the IPR first decreases, and passes through a minimum, before increasing. This surprising behavior is explained in the framework of perturbation renormalization group treatment, where wave functions can be computed analytically as a function of the hopping amplitude ratio and the disorder, however it is not specific to this model. Our results should help to clarify results of recent studies of localization due to random and quasiperiodic potentials.

The origin of the spin-glass state in ${\left(\mathrm{Fe},\mathrm{Sn}\right)}_4\mathrm{N}$ alloys is studied on the basis of a Heisenberg Hamiltonian with parameters derived from first principles within the magnetic force theorem applied in the framework of the disordered local moments method and local spin-density approximation. We show that in the alloy concentration range where the spin-glass state is stable only one Fe sublattice is intrinsically magnetic and the interatomic exchange magnetic interactions are essentially short ranged due to effects of chemical and magnetic disorder. The magnetic Fe atoms with well-localized spin moments are randomly distributed over the nongeometrically frustrated simple cubic lattice. The magnetic frustration, which generally is believed to be an essential ingredient of the spin-glass state formation condition, may occur only due to the competition of the two nearest-neighbor interactions. We thus argue that ${\left(\mathrm{Fe},\mathrm{Sn}\right)}_4\mathrm{N}$ is a rare example of a spin-glass system where the mechanism of spin-glass state formation might be studied in the framework of the minimal random-site model on a simple cubic lattice with competing interactions, while the effects of the geometrical frustration can be excluded.

The family of the endohedral gallide cluster compounds recently emerged as a new family of superconductors which is expected to host systems displaying unconventional physics. ${\mathrm{Mo}}_8{\mathrm{Ga}}_{41}$ is an important member of this family which shows relatively large $T_c\sim 10$ K and has shown indications of strong electron-phonon coupling and multiband superconductivity. Here, through direct measurement of superconducting energy gap by scanning tunneling spectroscopy (STS), we demonstrate the existence of two distinct superconducting gaps of magnitude 0.85 and 1.6 meV, respectively, in ${\mathrm{Mo}}_8{\mathrm{Ga}}_{41}$. Both gaps are seen to be conventional in nature as they evolve systematically with temperature as per the predictions of BCS theory. Our band structure calculations reveal that only two specific Mo sites in a unit cell contribute to superconductivity where only $d_{xz}/d_{yz}$ and $d_{x^2-y^2}$ orbitals have strong contributions. Our analysis indicates that the site-elective contribution governs the two-gap nature of superconductivity in ${\mathrm{Mo}}_8{\mathrm{Ga}}_{41}$.

The influence of a staggered molecular field in frustrated rare-earth pyrochlores, produced via the magnetic iridium occupying the transition metal site, can generate exotic ground states, such as the fragmentation of the magnetization in the Ho compound. At variance with the Ising ${\mathrm{Ho}}^{3+}$ moment, we focus on the behavior of the quasi-isotropic magnetic moment of the ${\mathrm{Gd}}^{3+}$ ion at the rare-earth site. By means of macroscopic measurements and neutron scattering, we find a complex situation where different components of the magnetic moment are decoupled and contribute to two antiferromagnetic noncollinear arrangements: a high-temperature all in–all out order induced by the Ir molecular field, and Palmer and Chalker correlations that tend to order at much lower temperatures. This is enabled by the anisotropic nature of the Gd-Gd interactions and requires a weak easy-plane anisotropy of the ${\mathrm{Gd}}^{3+}$ moment due to the mixing of the ground state with multiplets of higher spectral terms.

Calcium silicate perovskite (${\mathrm{CaSiO}}_3$) is one of the major mineral components of the lower mantle, but has been the subject of relatively little work compared to the more abundant Mg-based materials. One of the major problems related to ${\mathrm{CaSiO}}_3$ that is still the subject of research is its crystal structure under lower mantle conditions—a cubic $Pm\overline{3}m$ structure is accepted in general, but some have suggested that lower-symmetry structures may be relevant. In this paper, we use a fully first-principles vibrational self-consistent field method to perform high accuracy anharmonic vibrational calculations on several candidate structures at a variety of points along the geotherm near the base of the lower mantle to investigate the stability of the cubic structure and related distorted structures. Our results show that the cubic structure is the most stable throughout the lower mantle, and that this result is robust against the effects of thermal expansion.

We investigate the hcp to bcc phase transition in beryllium (Be) at high-pressure and -temperature ($PT$) conditions. A recently developed hybrid approach that combines first-principles molecular dynamics and lattice dynamics is used to account for anharmonic contributions to the free energy. Anharmonic effects are shown to be strong at high $T$ in both hcp and bcc Be. They are stronger in hcp Be than in bcc Be, as evidenced by the larger anharmonic vibrational entropy of hcp Be. We find that anharmonicity has a significant influence on the hcp to bcc transition at high-$PT$ conditions. It substantially enlarges the stability domain of hcp Be at high $T$ compared with that calculated under the quasiharmonic approximation (QHA), as a result bringing theoretical predictions into good consistency with recent experimental observations. After considering anharmonic effects, the calculated pressure and temperature of the hcp/bcc/liquid triple point increase from about 85 to 165 GPa, and from about 3300 to 4200 K, respectively, and the predicted Clapeyron slope at the triple point takes a value of $-7.4\pm 0.7\mathrm{K}/\mathrm{GPa}$, noticeably larger in magnitude than previous QHA results in the range of $-3$ to 2 K/GPa.

In order to develop force fields (FF) for molecular dynamics simulations that retain the accuracy of ab initio density functional theory (DFT), we developed a machine learning protocol based on an energy decomposition scheme that extracts atomic energies from DFT calculations. Our DFT to FF (DFT2FF) approach provides almost hundreds of times more data for the DFT energies, which dramatically improves accuracy with less DFT calculations. In addition, we use piecewise cosine basis functions to systematically construct symmetry invariant features into the neural network model. We illustrate this DFT2FF approach for amorphous silicon where only 800 DFT configurations are sufficient to achieve an accuracy of 1 meV/atom for energy and 0.1 eV/A for forces. We then use the resulting FF model to calculate the thermal conductivity of amorphous Si based on long molecular dynamics simulations. The dramatic speedup in training in our DFT2FF protocol allows the adoption of a simulation paradigm where an accurate and problem specific FF for a given physics phenomenon is trained on-the-spot through a quick DFT precalculation and FF training.

The Hubbard-Holstein model describes fermions on a discrete lattice, with on-site repulsion between fermions and a coupling to phonons that are localized on sites. Generally, at half-filling, increasing the coupling $g$ to the phonons drives the system towards a Peierls charge density wave state, whereas increasing the electron-electron interaction $U$ drives the fermions into a Mott antiferromagnet. At low $g$ and $U$, or when doped, the system is metallic. In one dimension, using quantum Monte Carlo simulations, we study the case where fermions have a long-range coupling to phonons, with characteristic range $\xi$, interpolating between the Holstein and Fröhlich limits. Without electron-electron interaction, the fermions adopt a Peierls state when the coupling to the phonons is strong enough. This state is destabilized by a small coupling range $\xi$ and leads to a collapse of the fermions, and, consequently, phase separation. Increasing interaction $U$ will drive any of these three phases (metallic, Peierls, phase separation) into a Mott insulator phase. The phase separation region is once again present in the $U\ne 0$ case, even for small values of the coupling range.

We consider the simplest variant of a Kondo insulator where a doublet of localized $f$ electrons hybridizes with spin-degenerate conduction electrons. We analyze the symmetries of $f$ orbitals involved in the hybridization and point out that the effective four-band model of such systems provides further descriptions of clean Kondo insulators; namely the spin texture of the surface states is described by an integer winding number. We discuss general conditions for the appearance of topological nontrivial states and implications for rare-earth-based compounds. As an example, we derive the full phase diagram of tetragonal Kondo insulators. In particular, our findings describe the spin texture in the physically interesting nontrivial topological phase, i.e., when the bandwidth of conduction electrons sets the largest energy scale, and a new weak topological phase appears as a function of the normalized distance between the bands' center.

We have observed that the Rh substitution for Co in $\mathrm{LaC}{\mathrm{o}}_{0.5}\mathrm{R}{\mathrm{h}}_{0.5}{\mathrm{O}}_3$ leads to the spectral feature at the $\mathrm{Co}-L_{2,3}$ absorption edge taken at 300 K similar to that in $\mathrm{LaCo}{\mathrm{O}}_3$ taken at 650 K, in which the spin state of magnetic ${\mathrm{Co}}^{3+}$ ions has been controversially discussed in the past decades and can be easily clarified by studying $\mathrm{LaC}{\mathrm{o}}_{0.5}\mathrm{R}{\mathrm{h}}_{0.5}{\mathrm{O}}_3$ without worrying about oxygen loss induced at high temperature. Our combined experimental and theoretical x-ray absorption spectroscopy (XAS) at the $\mathrm{Co}-L_{2,3}$ and the experimental $\mathrm{Rh}-L_{2,3}$ edges indicated a nearly 1:1 mixture of high-spin (HS) and low-spin (LS) $\mathrm{C}{\mathrm{o}}^{3+}\left(3d^6\right)$ and a LS $\mathrm{R}{\mathrm{h}}^{3+}\left(4d^6\right)$ in contrast to the $\mathrm{C}{\mathrm{o}}^{2+}/\mathrm{R}{\mathrm{h}}^{4+}$ state found in $\mathrm{C}{\mathrm{a}}_3\mathrm{CoRh}{\mathrm{O}}_6$ at room temperature. Upon cooling only a small portion of the HS $\mathrm{C}{\mathrm{o}}^{3+}$ ions was converted to a LS state until 10 K in $\mathrm{LaC}{\mathrm{o}}_{0.5}\mathrm{R}{\mathrm{h}}_{0.5}{\mathrm{O}}_3$. The Co-Kβ x-ray emission spectra revealed a gradual spin-state transition from a mixed LS/HS at ambient pressure to a complete LS state of $\mathrm{C}{\mathrm{o}}^{3+}$ ions up to 14 GPa. The theoretical and experimental intensity ratio $I\left(L_3\right)/I\left(L_2\right)$ on the $\mathrm{Co}-L_{2,3}$ edges and a comparison between the difference spectrum of Kβ x-ray emission of $\mathrm{LaC}{\mathrm{o}}_{0.5}\mathrm{R}{\mathrm{h}}_{0.5}{\mathrm{O}}_3$ taken at ambient pressure (AP) and 14 GPa and that of ${\mathrm{Sr}}_2{\mathrm{CoRuO}}_6$ taken at AP and 39.6 GPa exclude the intermediate spin state of $\mathrm{C}{\mathrm{o}}^{3+}$ in $\mathrm{LaC}{\mathrm{o}}_{0.5}\mathrm{R}{\mathrm{h}}_{0.5}{\mathrm{O}}_3$.

We study numerically the confinement of light in a three-dimensional (3D) photonic crystal cavity in a diamondlike inverse-woodpile structure. We present a versatile field-field cross-correlation method to identify resonances in the finite-support crystal with defect states in the 3D band gap of the infinite crystal. We argue that the five eigenstates of our 3D photonic band gap cavity have quadrupolar symmetry, in analogy to $d$-like orbitals of transition metals. It is remarkable that quality factors up to $Q=1000$ appear in such thin structures of only three unit cells, which is attributed to the relatively small Bragg length of the perfect crystal. We find that the optical energy density is remarkably enhanced at the cavity resonances by up to $2400\times$ the incident energy density in free space or up to $1200\times$ the energy density of the equivalent effective medium. We find that an inverse woodpile photonic band gap cavity with a suitably adapted lattice parameter reveals substantial absorption in the visible range. Below the 3D band gap, Fano resonances arise due to interference between the discrete fundamental cavity mode and the continuum light scattered by the photonic crystal. Hence, our study concludes that inverse woodpile cavities offer interesting perspectives for applications in optical sensing and photovoltaics.

Machine learning is becoming widely used in condensed matter physics. Inspired by the concept of image super-resolution, we propose a method to increase the size of lattice spin configurations using deep convolutional neural networks. Through supervised learning on Monte Carlo (MC) generated spin configurations, we train networks that invert real-space renormalization decimations. We demonstrate that super-resolution can reproduce thermodynamic observables that agree with MC calculations for the one- and two-dimensional Ising model at various temperatures. We find that it is possible to predict thermodynamic quantities for lattice sizes larger than those used in training by extrapolating the parameters of the network. We use this method to compute the critical exponents of the 2D Ising model, finding good agreement with theory.