Empirical tight-binding (ETB) methods are widely used in atomistic device simulations. Traditional ways of generating the ETB parameters rely on direct fitting to bulk experiments or theoretical electronic bands. However, ETB calculations based on existing parameters lead to unphysical results in ultrasmall structures like the As-terminated GaAs ultrathin bodies (UTBs). In this work, it is shown that more transferable ETB parameters with a short interaction range can be obtained by a process of mapping ab initio bands and wave functions to ETB models. This process enables the calibration of not only the ETB energy bands but also the ETB wave functions with corresponding ab initio calculations. Based on the mapping process, ETB models of Si and GaAs are parameterized with respect to hybrid functional calculations. Highly localized ETB basis functions are obtained. Both the ETB energy bands and wave functions with subatomic resolution of UTBs show good agreement with the corresponding hybrid functional calculations. The ETB methods can then be used to explain realistically extended devices in nonequilibrium that cannot be tackled with ab initio methods.

Hexagonal $\mathrm{ErFe}{\mathrm{O}}_3$ thin films fabricated by a pulsed laser deposition method were investigated by x-ray diffraction (XRD) and transmission electron microscopy. No structural phase transition was observed between 20 and 900 K. Ferroelectric $D-E$ hysteresis loops were obtained at room temperature using the double-wave method, and the magnitude of spontaneous polarization was determined to be $200\mathrm{nC}/\mathrm{c}{\mathrm{m}}^2$. The existence of superlattice reflections in the XRD indicates that the space group of $\mathrm{ErFe}{\mathrm{O}}_3$ thin film is the polar hexagonal $P{6}_{3}cm$. This result also supports the ferroelectricity at room temperature. Magnetic properties were examined by a superconducting quantum interference device and Mössbauer spectroscopy. Below the Néel temperature of 120 K $\left(T_{\mathrm{N}}\right)$, a weak ferromagnetism appears along the $c$ axis due to the canting of $\mathrm{F}{\mathrm{e}}^{3+}$ magnetic moments. Around $T_{\mathrm{N}}$, the imaginary part of dielectric constant shows an anomaly. These results prove $h-\mathrm{ErFe}{\mathrm{O}}_3$ thin films to be multiferroic below 120 K.

The pressure-induced structural phase transition in the intermediate-valence compound CeNi has been investigated by x-ray and neutron powder diffraction techniques. It is shown that the structure of the pressure-induced CeNi phase (phases) can be described in terms of the $Pnma$ space group. Equations of state for CeNi on both sides of the phase transition are derived and an approximate P-T phase diagram is suggested for $P<8$ GPa and $T<300$ K. The observed $Cmcm\to Pnma$ structural transition is analyzed using density functional theory calculations, which successfully reproduce the ground state volume, the phase transition pressure, and the volume collapse associated with the phase transition.

We present an ab initio framework to calculate anharmonic phonon frequency and phonon lifetime that is applicable to severely anharmonic systems. We employ self-consistent phonon (SCPH) theory with microscopic anharmonic force constants, which are extracted from density functional calculations using the least absolute shrinkage and selection operator technique. We apply the method to the high-temperature phase of ${\mathrm{SrTiO}}_3$ and obtain well-defined phonon quasiparticles that are free from imaginary frequencies. Here we show that the anharmonic phonon frequency of the antiferrodistortive mode depends significantly on the system size near the critical temperature of the cubic-to-tetragonal phase transition. By applying perturbation theory to the SCPH result, phonon lifetimes are calculated for cubic ${\mathrm{SrTiO}}_3$, which are then employed to predict lattice thermal conductivity using the Boltzmann transport equation within the relaxation-time approximation. The presented methodology is efficient and accurate, paving the way toward a reliable description of thermodynamic, dynamic, and transport properties of systems with severe anharmonicity, including thermoelectric, ferroelectric, and superconducting materials.

We have investigated the spin wave modes in perpendicularly magnetized permalloy nanodots as a function of film thickness using field modulated perpendicular ferromagnetic resonance spectroscopy. We observed mode intensity inversion with an increase in dot thickness. In thicker dots, one ellusive antisymmetric spin wave mode was also detected due to the asymmetric nature of the excitation signal. In the case of the thickest nanodot, a circulating chiral mode near the dot surface was also observed. Our experimental results are in good qualitative agreement with the dynamic micromagnetic simulations.

We have identified anomalous behavior of the escape rate out of the zero-voltage state in Josephson junctions with a high critical current density $J_c$. For this study we have employed ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ grain boundary junctions, which span a wide range of $J_c$ and have appropriate electrodynamical parameters. Such high $J_c$ junctions, when hysteretic, do not switch from the superconducting to the normal state following the expected stochastic Josephson distribution, despite having standard Josephson properties such as a Fraunhofer magnetic field pattern. The switching current distributions (SCDs) are consistent with nonequilibrium dynamics taking place on a local rather than a global scale. This means that macroscopic quantum phenomena seem to be practically unattainable for high $J_c$ junctions. We argue that SCDs are an accurate means to measure nonequilibrium effects. This transition from global to local dynamics is of relevance for all kinds of weak links, including the emergent family of nanohybrid Josephson junctions. Therefore caution should be applied in the use of such junctions in, for instance, the search for Majorana fermions.

Superconducting quantum interference devices (SQUIDs) and other superconducting circuits are limited by intrinsic flux noise with spectral density $1/f^{\alpha }$ with $\alpha <1$ whose origin is believed to be due to spin impurities. Here, we present a theory of flux noise that takes into account the vectorial nature of the coupling of spins to superconducting wires. We present explicit numerical calculations of the flux-noise power (spectral density integrated over all frequencies) for electron impurities and lattice nuclear spins under several different assumptions. The noise power is shown to be dominated by surface electron spins near the wire edges, with bulk lattice nuclear spins contributing $\sim 5\%$ of the noise power in aluminum and niobium wires. We consider the role of electron spin phase transitions, showing that the spin-spin correlation length (describing, e.g., the average size of ferromagnetic spin clusters) greatly impacts the scaling of flux noise with wire geometry. Remarkably, the flux-noise power is exactly equal to zero when the spins are polarized along the flux-vector direction, forming what we call a poloidal state. Flux noise is nonzero for other spin textures, but gets reduced in the presence of correlated ferromagnetic fluctuations between the top and bottom wire surfaces, where the flux vectors are antiparallel. This demonstrates that engineering spin textures and/or intersurface correlation provides a method to reduce flux noise in superconducting devices.

It was recently shown that conventional phonon-electron interactions may induce a triplet pairing state in time-reversal invariant three-dimensional Dirac semimetals. Starting from the microscopic model of the isotropic Dirac semimetal, the Ginzburg-Landau equations for the vector order parameter is derived using the Gor'kov technique. The collective modes including gapless Goldstone modes and gapped Higgs modes of various polarizations are identified. They are somewhat analogous to the modes in the B phase of ${\text{He}}^3$, although in the present case quantitatively there is a pronounced difference between longitudinal and transverse components. The difference is caused by the vector nature of the order parameter leading to two different coherence lengths or penetration depths. The system is predicted to be highly dissipative due to the Goldstone modes. The time-dependent Ginzburg-Landau model in the presence of external fields is used to investigate some optical and magnetic properties of such superconductors. The ac conductivity of a clean sample depends on the orientation of the order parameter. It is demonstrated that the difference between the penetration depths results in rotation of the polarization vector of microwave passing a slab made of this material. The upper critical magnetic field $H_{c2}$ was found. It turns out that at fields close to $H_{c2}$ the order parameter orients itself perpendicular to the field direction. In certain range of parameters the triplet superconducting phase persists at arbitrarily high magnetic field like in some $p$-wave superconductors.

Chemical bonding is the central concept of chemistry that has been used to explain the properties of molecules and solids as well as chemical processes. In recent years, considerable progress has been made toward a simple and yet fundamental understanding of this concept for isolated systems. Here we propose the quantum pressure to study electron localization in molecules and solids as well as the influence of an external magnetic field. A high pressure indicates chemical bonding and electron localization, while a low pressure indicates intershell region and electron delocalization. We find that electrons become more localized between nuclei when exposed to a magnetic field. We demonstrate that our quantum pressure not only can reveal electronic shell structure of atoms, but also can be used to visualize chemical bonding in molecules and solids, significantly extending the applicability of this tool to wide-ranging problems.

We investigate a single-particle density of states (DOS) in strongly disordered two-dimensional high dielectric permittivity systems with logarithmic Coulomb interaction between particles. We derive self-consistent DOS at zero temperature and show that it is appreciably suppressed as compared to the DOS expected from the Efros-Shklovskii approach. We carry out zero- and finite-temperature Monte Carlo numerical studies of the DOS and find the perfect agreement between the numerical and analytical results at zero temperature, observing, in particular, a hardening of the Coulomb gap with the increasing electrostatic screening length. At finite temperatures, we reveal a striking scaling of the DOS as a function of energy normalized to the temperature of the system.

We use determinant quantum Monte Carlo to study the single-particle properties of quasiparticles and phonons in a variant of the two-dimensional Holstein model that includes an additional nonlinear electron-phonon (e-ph) interaction. We find that a small positive nonlinear interaction reduces the effective coupling between the electrons and the lattice, suppresses charge-density-wave (CDW) correlations, and hardens the effective phonon frequency. Conversely, a small negative nonlinear interaction can enhance the e-ph coupling resulting in heavier quasiparticles, an increased tendency towards a CDW phase at all fillings, and a softened phonon frequency. An effective linear model with a renormalized interaction strength and phonon frequency can qualitatively capture this physics; however, the quantitative effects of the nonlinearity on both the electronic and phononic degrees of freedom cannot be captured by such a model. These results are significant for typical nonlinear coupling strengths found in real materials, indicating that nonlinearity can have an important influence on the physics of many e-ph coupled systems.

Thin films of the ferromagnetic metal ${\mathrm{SrRuO}}_3$ (SRO) show a varying easy magnetization axis depending on the epitaxial strain, and undergo a metal-to-insulator transition with decreasing film thickness. We have investigated the magnetic properties of SRO thin films with varying thicknesses fabricated on ${\mathrm{SrTiO}}_3$(001) substrates by soft x-ray magnetic circular dichroism at the Ru $M_{2,3}$ edge. Results have shown that, with decreasing film thickness, the film changes from ferromagnetic to nonmagnetic at around 3 monolayer thickness, consistent with previous magnetization and magneto-optical Kerr effect measurements. The orbital magnetic moment perpendicular to the film was found to be $\sim 0.1{\mu }_{\text{B}}/\mathrm{Ru}$, and remained nearly unchanged with decreasing film thickness while the spin magnetic moment decreases. A mechanism for the formation of the orbital magnetic moment is discussed based on the electronic structure of the compressively strained SRO film.

We provide an approach for the identification of the electronic and magnetic configurations of ferromagnetic Fe after an ultrafast decrease or increase of the magnetization. The model is based on the well-grounded assumption that, after an ultrafast variation of the magnetization, the system achieves a partial thermal equilibrium. With statistical arguments we show that the magnetic configurations are qualitatively different in the case of reduced or increased magnetization. The predicted magnetic configurations are then used to compute the dielectric response at the $3p \left(M\right)$ absorption edge, which is directly related to the changes observed in the experimental T-MOKE data. The good qualitative agreement between theory and experiment offers a substantial support for the validity of the model, and to the very existence of an ultrafast increase of the magnetization.

The effects of femtosecond laser excitation on the anisotropy parameter in orthoferrites $\mathrm{ErFe}{\mathrm{O}}_3$ and $\mathrm{DyFe}{\mathrm{O}}_3$ were probed through Faraday rotation induced by coherent spin precession, which was triggered by a terahertz pulse. Through the delayed frequency shift of the precession, gradual change in the anisotropic energy of the iron spins is evidenced. This is attributed to the slow energy transfer from the $\mathrm{F}{\mathrm{e}}^{3+}3d$ electron system to the rare-earth $4f$ electron system, which in turn alters the anisotropy parameter of the $\mathrm{F}{\mathrm{e}}^{3+}$ spins. The result presented here reveals the time dependent spin alignment mechanism in cant-type antiferromagnetic orthoferrites and enables deeper understanding of spin orientation dynamics triggered by electronic excitation.

Thermal vector potential formulation is applied to study the thermal dynamics of magnetic structures for insulating ferromagnets. By separating the variables of the magnetic structure and the magnons, the equation of motion for the structure, including spin-transfer effect because of thermal magnons, is derived for the cases of a domain wall and a vortex. The magnon current is evaluated based on the linear response theory with the thermal vector potential representing the temperature gradient. The velocity of a domain wall when driven by thermal magnons exhibits a strong temperature dependence unlike the case of an electrically driven domain wall in metals.

We show how the dynamical modulation of the qubit-field coupling strength in a circuit quantum electrodynamics architecture mimics the motion of the qubit at relativistic speeds. This allows us to propose a realistic experiment to detect microwave photons coming from simulated acceleration radiation. Moreover, by combining this technique with the dynamical Casimir physics, we enhance the toolbox for studying relativistic phenomena in quantum field theory with superconducting circuits.

We investigate a transition between a two-dimensional topological insulator conduction state, characterized by a conductance $G=2$ (in fundamental units $e^2/h$) and a Chern insulator with $G=1$, induced by polarized magnetic impurities. Two kinds of coupling, ferromagnetic and antiferromagnetic, are considered with the electron and hole subbands. We demonstrate that for strong disorder, a phase $G=1$ exists even for ferromagnetic order, in contrast with the prediction of the mean field approximation. This result is supported by direct numerical computations using Landauer transport formula, and by analytical calculations of the chemical potential and mass renormalization as a function of the disorder strength, in the self-consistent Born approximation. The transition is related to the suppression of one of the spin conduction channels, for strong enough disorder, by selective spin scattering and localization.

Band offsets between different semiconductors are important parameters that determine the electronic transport properties near the interface in the heterostructure devices. The computation of the natural band offset is a well-known challenge. In this paper, we propose a new method, which is called the three-step method, to accurately predict the natural band offset. Compared to previous methods, the present method is more direct and can be easily applied to systems with larger lattice mismatch and to systems with lower symmetry. Using the present method, we successfully calculate the natural band offset between the inorganic halide perovskites $AB{X}_3(A=\mathrm{Cs};B=\mathrm{Sn},\mathrm{Pb};X=\mathrm{Cl},\mathrm{B},\mathrm{I})$ in the cubic and orthorhombic phase. We show that the valence band maximum shifts down as the atomic number of the $X$ site ion increases, while the valence band maximum shifts up as $B$ site ion varies from Sn to Pb or as the compound transforms from the cubic phase to the orthorhombic phase. It is found that the band gap differences between these compounds can be attributed primarily to the valence band offsets, with a much smaller contribution from the conduction band offsets.

We demonstrate the existence of metallic spin density waves (SDWs) in the Kondo lattice model on a square lattice for a wide range of parameters by means of real space dynamical mean-field theory. In these SDWs, the spin polarization as well as the charge density depend on the lattice site and are modulated along one direction of the square lattice. We show that within this phase of metallic SDWs the Fermi surface changes from small to large, when the coupling strength is increased. Furthermore, the transition between the large Fermi-surface SDW phase and the paramagnetic phase is of second order, while the transition between the small Fermi-surface SDW phase and the paramagnetic phase is of first order. A local quantum critical point is thus avoided in our calculations by undergoing a first-order phase transition.