In this paper, we report temperature-dependent magnetic susceptibility, electrical resistivity, and heat-capacity experiments in the family of intermetallic compounds $R{\mathrm{Ni}}_3{\mathrm{Ga}}_9$ ($R$ = Tb, Dy, Ho, and Er). Single-crystalline samples were grown using Ga self-flux method. These materials crystallize in a trigonal ${\mathrm{ErNi}}_3{\mathrm{Al}}_9$-type structure with space group $R32$. They all order antiferromagnetically with $T_N<20\mathrm{K}$. The anisotropic magnetic susceptibility presents large values of the ratio ${\chi }_{\text{easy}}/{\chi }_{\text{hard}}$ indicating strong crystalline electric-field (CEF) effects. The evolution of the crystal-field scheme for each $R$ was analyzed in detail by using a spin model including anisotropic nearest-neighbor Ruderman-Kittel-Kasuya-Yosida interaction and the trigonal CEF Hamiltonian. Our analysis allows one to understand the distinct direction of the ordered moments along the series—the Tb-, Dy-, and Ho-based compounds have the ordered magnetic moments in the easy $\mathit{ab}$ plane and the Er sample magnetization easy axis is along the $\widehat{c}$ direction.

We model a single layer of heavily electron-doped FeSe by spin-$\frac{1}{2}$ moments over a square lattice of iron atoms that include the $3d_{xz}$ and $3d_{yz}$ orbitals, at strong on-site Coulomb repulsion. Above half filling, we find emergent hole bands below the Fermi level at the center of the one-iron Brillouin zone in a half-metal state characterized by hidden magnetic order and by electron-type Fermi surface pockets at wave numbers that double the unit cell along the principal axes. “Replicas” of the emergent hole bands exist at lower energy in the two-iron Brillouin zone. Exact calculations with two mobile electrons find evidence for isotropic Cooper pairs that alternate in sign between the electron bands and the emergent hole bands.

We study the dynamic response of a superfluid field to a moving edge dislocation line to which the field is minimally coupled. We use a dissipative Gross-Pitaevskii equation, and determine the initial conditions by solving the equilibrium version of the model. We consider the subsequent time evolution of the field for both glide and climb dislocation motion and analyze the results for a range of values of the constant speed $V_D$ of the moving dislocation. We find that the type of motion of the dislocation line is very important in determining the time evolution of the superfluid field distribution associated with it. Climb motion of the dislocation line induces increasing asymmetry, as function of time, in the field profile, with part of the probability being, as it were, left behind. On the other hand, glide motion has no effect on the symmetry properties of the superfluid field distribution. Damping of the superfluid field due to excitations associated with the moving dislocation line occurs in both cases.

Two-dimensional crystals with strong interactions between layers has attracted increasing attention in recent years in a variety of fields. In particular, the growth of a single layer of oxide materials (e.g., MgO, CaO, NiO, and MnO) over metallic substrates were found to display different physical properties than their bulk. In this study, we report on the physical properties of a single layer of metallic oxide materials and compare their properties with their bulk and other two-dimensional (2D) crystals. We found that the planar structure of metallic monoxides are unstable whereas the buckled structures are thermodynamically stable. Also, the 2D-MnO and NiO exhibit different magnetic (ferromagnetic) and optical properties than their bulk, whereas band-gap energy and linear stiffness are found to be decreasing from NiO to MgO. Our findings provide insight into oxide thin-film technology applications.

The representations of a compound, called “descriptors” or “features”, play an essential role in constructing a machine-learning model of its physical properties. In this study, we adopt a procedure for generating a set of descriptors from simple elemental and structural representations. First, it is applied to a large data set composed of the cohesive energy for about 18 000 compounds computed by density functional theory calculation. As a result, we obtain a kernel ridge prediction model with a prediction error of 0.041 eV/atom, which is close to the “chemical accuracy” of 1 kcal/mol (0.043 eV/atom). A prediction model with an error of 0.071 eV/atom of the cohesive energy is obtained for the normalized prototype structures, which can be used for the practical purpose of searching for as-yet-unknown structures. The procedure is also applied to two smaller data sets, i.e., a data set of the lattice thermal conductivity for 110 compounds computed by density functional theory calculation and a data set of the experimental melting temperature for 248 compounds. We examine the effect of the descriptor sets on the efficiency of Bayesian optimization in addition to the accuracy of the kernel ridge regression models. They exhibit good predictive performances.

Based on the first-principles total-energy calculation, we have studied the shear-strain gradient effect on the polarization reversal of ferroelectric ${\mathrm{BaTiO}}_3$ thin films. By calculating the energies of double-domain supercells for different electric polarization, shear-strain gradients, and domain-wall displacement, we extracted, in addition to the domain-wall energy, the polarization energy, elastic energy, and flexoelectric coefficient of a single domain. The constructed Landau-Devonshire phenomenological theory yields a critical shear-strain gradient of $9.091\times {10}^7/\text{m}$ (or a curvature radius ($R$) of 110 Å) for reversing the ${180}^{\circ }$ domain at room temperature, which is on the same order of the experimentally estimated value of $3.333\times {10}^7/\text{m}$ ($R=300\AA$). In contrast to the commonly used linear response theory, the flexoelectric coefficient derived from fitting the total energy to a Landau-Devonshire energy functional does not depend on the specific pseudopotential. Thus, our method offers an alternative numerical approach to study the flexoelectric effect.

We investigate the propagation dynamics of initially localized excitations in spin-$\frac{1}{2}$ Heisenberg ladders. We consider initial states with two overturned spins, either on neighboring sites on the same leg or on the two sites of a single rung, in an otherwise polarized (ferromagnetic) background. Compared to the corresponding dynamics in a chain (single leg), we observe several additional modes of propagation. We connect these propagation modes to features of the spectrum of the ladder system, and to different effective models corresponding to different segments of the spectrum. In addition to the regular propagation modes, we observe for one mode a peculiar “jamming” dynamics where components of the excitations remain localized in an unusual manner. A comparison with the spin-1 bilinear-biquadratic chain is developed and explored, where a similar phenomenon is shown to occur.

Two-dimensional materials have unusual phonon spectra due to the presence of flexural (out-of-plane) modes. Although molecular dynamics simulations have been extensively used to study heat transport in such materials, conventional formalisms treat the phonon dynamics isotropically. Here, we decompose the microscopic heat current in atomistic simulations into in-plane and out-of-plane components, corresponding to in-plane and out-of-plane phonon dynamics, respectively. This decomposition allows for direct computation of the corresponding thermal conductivity components in two-dimensional materials. We apply this decomposition to study heat transport in suspended graphene, using both equilibrium and nonequilibrium molecular dynamics simulations. We show that the flexural component is responsible for about two-thirds of the total thermal conductivity in unstrained graphene, and the acoustic flexural component is responsible for the logarithmic divergence of the conductivity when a sufficiently large tensile strain is applied.

We investigate relaxation in the recently discovered “fracton” models and discover that these models naturally host glassy quantum dynamics in the absence of quenched disorder. We begin with a discussion of “type I” fracton models, in the taxonomy of Vijay, Haah, and Fu. We demonstrate that in these systems, the mobility of charges is suppressed exponentially in the inverse temperature. We further demonstrate that when a zero-temperature type I fracton model is placed in contact with a finite-temperature heat bath, the approach to equilibrium is a logarithmic function of time over an exponentially wide window of time scales. Generalizing to the more complex “type II” fracton models, we find that the charges exhibit subdiffusion up to a relaxation time that diverges at low temperatures as a superexponential function of inverse temperature. This behavior is reminiscent of “nearly localized” disordered systems, but occurs with a translation invariant three-dimensional Hamiltonian. We also conjecture that fracton models with conserved charge may support a phase which is a thermal metal but a charge insulator.

Rashba spin splitting in two-dimensional (2D) semiconductor systems is generally calculated in a $\mathbf{k}·\mathbf{p}$ Luttinger-Kohn approach where the spin splitting due to asymmetry emerges naturally from the bulk band structure. In recent years, several new classes of 2D systems have been discovered where electronic correlations are believed to have an important role. In these correlated systems, the effects of asymmetry leading to Rashba splitting have typically been treated phenomenologically. We compare these two approaches for the case of 2D electron systems in ${\mathrm{SrTiO}}_3$-based heterostructures, and find that the two models produce fundamentally different behavior in regions of the Brillouin zone that are particularly relevant for magnetotransport. Our results demonstrate the importance of identifying the correct approach in the quantitative interpretation of experimental data, and are likely to be relevant to a range of 2D systems in correlated materials.

Optical absorption spectra and excitonic properties of hydrogenated anatase titania have been calculated by ab initio calculations based on density functional theory, GW approximation, and the Bethe-Salpeter equation. Neutral hydrogen impurities lead to enhanced absorption in the infrared and visible regions, giving spectra compatible with experimental observations. Beside a broad signal in the infrared part, a sharp peak appears in the visible region that is due to excitonic effects. The wave functions of excitons in the infrared and in the visible are very localized, while those in the UV region are more delocalized than their counterparts in pure titania. It is argued that this could lead to a decreased electron-hole recombination of excitons in the UV, possibly leading to an increase in the photocatalytic activity of the material. On the contrary, positively charged hydrogen impurities barely modify the absorption spectrum, are probably difficult to be detected by optical absorption, and are unlikely to have a big effect on the photocatalytic properties of titania. These results contribute to understand the experimentally observed variability in the optical properties of hydrogenated titania and to obtain insight into their relation with the photocatalytic activity.

Photoluminescence polarization is experimentally studied for samples with (In,Ga)As/GaAs self-assembled quantum dots in transverse magnetic field (Hanle effect) under slow modulation of the excitation light polarization from fractions of Hz to tens of kHz. The polarization reflects the evolution of strongly coupled electron-nuclear spin systems in the quantum dots. Strong modification of the Hanle curves under variation of the modulation period is attributed to the peculiarities of the spin dynamics of quadrupole nuclei, which states are split due to deformation of the crystal lattice in the quantum dots. Analysis of the Hanle curves is fulfilled in the framework of a phenomenological model considering a separate dynamics of a nuclear field $B_{Nd}$ determined by polarization of the $\pm 1/2$ nuclear spin states and of a nuclear field $B_{Nq}$ determined by polarization of the split-off states $\pm 3/2, \pm 5/2$, etc. It is found that the characteristic relaxation time for the nuclear field $B_{Nd}$ is of order a fraction of a second, while the relaxation of the field $B_{Nq}$ is faster by about two orders of magnitude.

We report the preparation of an interface between graphene and a strong Rashba-split ${\mathrm{BiAg}}_2$ surface alloy and an investigation of its structure as well as the electronic properties by means of scanning tunneling microscopy/spectroscopy and density functional theory calculations. Upon evaluation of the quasiparticle interference patterns, an unperturbed linear dispersion for the $\pi$ band of $n$-doped graphene is observed. Our results also reveal the intact nature of the giant Rashba-split surface states of the ${\mathrm{BiAg}}_2$ alloy, which demonstrate only a moderate downward energy shift due to the presence of graphene. This effect is explained in the framework of density functional theory by an inward relaxation of the Bi atoms at the interface and subsequent delocalization of the wave function of the surface states. Our findings demonstrate a realistic pathway to prepare a graphene-protected giant Rashba-split ${\mathrm{BiAg}}_2$ for possible spintronic applications.

We discuss how one-dimensional interacting fermion systems, which in the low-energy approximation are described by Luttinger liquid theory, can be reformulated as systems of weakly interacting particles with fractional charge and statistics. Our approach is to use Landau's phenomenological approach to Fermi liquid theory, where the quasiparticles are interpreted as adiabatically dressed fermions. In an earlier publication, the local charge carried by these excitations has been shown to be fractional. We focus here on the statistics of the quasiparticles and show that by a change of momentum variables the Landau parameters of the generalized Fermi fluid can be transformed to zero. This change in interaction is compensated by a change of the entropy function, which is consistent with the interpretation of the quasiparticles as satisfying generalized exclusion statistics.

We study the interface physics of bipartite magnetic materials deposited on a topological insulator. Our study comprises antiferromagnets as well as ferrimagnets and ferromagnets with multiple magnetic moments per unit cell. If an energy gap is induced in the Dirac states on the topological surface, a topological magnetoelectric effect has been predicted. Here, we show that this effect can act in opposite directions on the two components of the magnet in a certain parameter region. Consequently, an electric field will mainly generate a staggered field rather than a net magnetization in the plane. This result is relevant for current attempts to detect the magnetoelectric effect experimentally, as well as for possible applications. We take a field-theoretic approach that includes the quantum fluctuations of both the Dirac fermions on the topological surface and the fermions in the surface layer of the magnet in an analytically solvable model. The effective Lagrangian and the Landau-Lifshitz equation describing the interfacial magnetization dynamics are derived.

We set up a general density-operator approach to geometric steady-state pumping through slowly driven open quantum systems. This approach applies to strongly interacting systems that are weakly coupled to multiple reservoirs at high temperature, illustrated by an Anderson quantum dot. Pumping gives rise to a nonadiabatic geometric phase that can be described by a framework originally developed for classical dissipative systems by Landsberg. This geometric phase is accumulated by the transported observable (charge, spin, energy) and not by the quantum state. It thus differs radically from the adiabatic Berry-Simon phase, even when generalizing it to mixed states, following Sarandy and Lidar. As a key feature, our geometric formulation of pumping stays close to a direct physical intuition (i) by tying gauge transformations to calibration of the meter registering the transported observable and (ii) by deriving a geometric connection from a driving-frequency expansion of the current. Furthermore, our approach provides a systematic and efficient way to compute the geometric pumping of various observables, including charge, spin, energy, and heat. These insights seem to be generalizable beyond the present paper's working assumptions (e.g., Born-Markov limit) to more general open-system evolutions involving memory and strong-coupling effects due to low-temperature reservoirs as well. Our geometric curvature formula reveals a general experimental scheme for performing geometric transport spectroscopy that enhances standard nonlinear spectroscopies based on measurements for static parameters. We indicate measurement strategies for separating the useful geometric pumping contribution to transport from nongeometric effects. A large part of the paper is devoted to an explicit comparison with the Sinitsyn-Nemenmann full-counting-statistics (FCS) approach to geometric pumping, restricting attention to the first moments of the pumped observable. Covering all key aspects, gauge freedom, pumping connection, curvature, and gap condition, we argue that our approach is physically more transparent and, importantly, simpler for practical calculations. In particular, this comparison allows us to clarify how in the FCS approach an “adiabatic” approximation leads to a manifestly nonadiabatic result involving a finite retardation time of the response to parameter driving.

Single-atom gating, achieved by manipulation of adatoms on a surface, has been shown in experiments to allow precise control over superposition of electronic states in quantum corrals. Using a Green's function approach, we demonstrate theoretically that such atom gating can also be used to control the coupling between magnetic degrees of freedom in these systems. Atomic gating enables control not only on the direct interaction between magnetic adatoms, but also over superpositions of many-body states which can then control long distance interactions. We illustrate this effect by considering the competition between direct exchange between magnetic impurities and the Kondo screening mediated by the host electrons, and how this is affected by gating. These results suggest that both magnetic and nonmagnetic single-atom gating may be used to investigate magnetic impurity systems with tailored interactions, and may allow the control of entanglement of different spin states.

Free energies of bulk materials are nowadays routinely computed by density functional theory. In particular for metals, electronic excitations can significantly contribute to the free energy. For an ideal static lattice, this contribution can be obtained at low computational cost, e.g., from the electronic density of states derived at $T=0$ K or by utilizing the Sommerfeld approximation. The error introduced by these approximations at elevated temperatures is rarely known. The error arising from the ideal lattice approximation is likewise unexplored but computationally much more challenging to overcome. In order to shed light on these issues we have computed the electronic free energies for all $3d,4d$, and $5d$ transition elements on the ideal lattices of the bcc, fcc, and hcp structures using finite-temperature density-functional theory. For a subset of elements we have explored the impact of explicit thermal vibrations on the electronic free energies by using ab initio molecular dynamics simulations. We provide an analysis of the observed chemical trends in terms of the electronic density of states and the canonical $d$ band model and quantify the errors in the approximate methods. The electronic contribution to the heat capacities and the corresponding errors due to the different approximations are studied as well.

The charge dynamical response function of the Hubbard model is investigated on the square lattice in the thermodynamic limit. The obtained charge-excitation spectra consist of a continuum, a gapless collective mode with anisotropic zero-sound velocity, and a correlation-induced high-frequency mode at $\omega \approx U$. The correlation function is calculated from Gaussian fluctuations around the paramagnetic saddle point within the Kotliar and Ruckenstein slave-boson representation. Its dependence on the on-site Coulomb repulsion $U$ and density is studied in detail. An approximate analytical expression of the high-frequency mode, which holds for any lattice with one atom in the unit cell, is derived. Comparison with numerical simulations, perturbation theory, and the polarization potential theory is carried out. We also show that magnetic instabilities tend to vanish for $T\gtrsim t/6$, and finite-temperature phase diagrams are established.

The presence of nonconventional bonding features is an intriguing part of elemental boron. The recent addition of τ boron to the family of three-dimensional boron allotropes is no exception. We provide an understanding of the electronic structure of τ boron using a fragment molecular approach, where the effect of symmetry reduction on skeletal bands of ${\mathrm{B}}_{12}$ and the ${\mathrm{B}}_{57}$ fragments are examined qualitatively by analyzing the projected density of states of these fragments. In spite of the structural resemblance to β boron, the reduction of symmetry from a rhombohedral space group to the orthorhombic one destabilizes the bands and reduces the electronic requirements. This suggests the presence of the partially occupied boron sites, as seen for a β boron unit cell, and draws the possibility for the existence of different energetically similar polymorphs. τ boron has a lower binding energy than β boron.