The spin dynamics in a broad range of systems can be studied using circularly polarized optical excitation with alternating helicity. The dependence of spin polarization on the frequency of helicity alternation, known as the spin inertia effect, is used here to study the spin dynamics in singly charged (In,Ga)As/GaAs quantum dots (QDs), providing insight into spin generation and accumulation processes. We demonstrate that the dependence of spin polarization in $n$- and $p$-type QDs on the external magnetic field has a characteristic $\mathsf{\text{V}}$- and $\mathsf{\text{M}}$-like shape, respectively. This difference is related to different microscopic mechanisms of the resident carriers' spin orientation. It allows us to determine the parameters of the spin dynamics both for the ground and excited states of singly charged QDs.

As one of the rare ferromagnetic semiconductors, ${\mathrm{CrGeTe}}_3$ has recently attracted a great deal of attention as a potential candidate for next-generation high-performance nano-spintronic devices. In this study, by combining density functional theory calculations and angle-resolved photoemission spectroscopy measurements, we explore the electronic structure of ${\mathrm{CrGeTe}}_3$ directly. The low-lying valence bands are centered around the $\mathrm{\Gamma }$ point and mainly consist of Te $5p$ orbitals. The majority of the bands show almost no $k_z$ dispersion, consistent with its layered crystalline structure. Due to the higher hopping integral along the out-of-plane direction, however, bands comprised of $p_z$ orbitals exhibit significant $k_z$ dispersion. Furthermore, an indirect band gap of 0.38 eV is directly measured by surface electron doping with potassium deposition.

Electrides are exotic compounds that confine anionic electrons in periodically distributed subnanometer-sized spaces. Such trapped electrons are free from onsite electron-nuclear interaction and exhibit unconventional properties. Here, we report that α- and $\beta -\mathrm{Y}{\mathrm{b}}_5\mathrm{S}{\mathrm{b}}_3$ are inorganic electrides exhibiting Mott-insulating features. Anionic electrons are stabilized in the quasi-one- and -zero-dimensional spaces, and give rise to the corresponding electride bands near their Fermi levels. Despite the partially occupied electronic picture, both of these systems exhibit semiconducting conductivity and Curie-type magnetism with $S=1/2$ moments, demonstrating electron localization. These findings show that anionic electrons can serve as magnetic centers, and inorganic electrides have the potential to act as strongly correlated materials even without the presence of localized atomic orbitals.

The spin inertia measurement is a recently emerged tool to study slow spin dynamics, which is based on the excitation of the system by a train of circularly polarized pulses with alternating helicity. Motivated by the experimental results reported by Zhukov et al. [Phys. Rev. B 98, 121304(R) (2018)], we develop the general theory of spin inertia of localized charge carriers. We demonstrate that the spin inertia measurement in longitudinal magnetic field allows one to determine the parameters of the spin dynamics of resident charge carriers and of photoexcited trions, such as the spin relaxation times, longitudinal $g$ factors, parameters of hyperfine interaction, and nuclear spin correlation times.

The process of tunneling injection is known to improve the dynamical characteristics of quantum well and quantum dot lasers; in the latter, it also improves the temperature performance. The advantage of the tunneling injection process stems from the fact that it avoids hot carrier injection, which is a key performance-limiting factor in all semiconductor lasers. The tunneling injection process is not fully understood microscopically and therefore it is difficult to optimize those laser structures. We present here a numerical study of the broadband carrier dynamics in a tunneling injection quantum dot gain medium in the form of an optical amplifier operating at $1.55\mu \mathrm{m}$. Charge carrier tunneling occurs in a hybrid state that joins the quantum dot first excited state and the confined quantum well–injection well states. The hybrid state, which is placed energetically roughly one longitudinal optic phonon above the ground state and has a spectral extent of about $5\text{meV}$, dominates the carrier injection to the ground state. We calculate the dynamical response of the inversion across the entire gain spectrum following a short pulse perturbation at various wavelengths and for two bias currents. At a high bias of $200\text{mA}$, the entire spectrum exhibits gain; at $30\text{mA}$, the system exhibits a mixed gain-absorption spectrum. The carrier dynamics in the injection well is calculated simultaneously. We discuss the role of the pulse excitation wavelengths relative to the gain spectrum peak and demonstrate that the injection well responds to all perturbation wavelengths, even those which are far from the region where the tunneling injection process dominates.

The honeycomb lattice sets the basic arena for numerous ideas to implement electronic, photonic, or phononic topological bands in (meta-)materials. Novel opportunities to manipulate Dirac electrons in graphene through band engineering arise from superlattice potentials as induced by a substrate such as hexagonal boron-nitride. Making use of the general form of a weak substrate potential as dictated by symmetry, we analytically derive the low-energy minibands of the superstructure, including a characteristic 1.5 Dirac cone deriving from a three-band crossing at the Brillouin zone edge. Assuming a large supercell, we focus on a single Dirac cone (or valley) and find all possible arrangements of the low-energy electron and hole bands in a complete six-dimensional parameter space. We identify the various symmetry planes in parameter space inducing gap closures and find the sectors hosting topological minibands, including also complex band crossings that generate a valley Chern number atypically larger than one. Our map provides a starting point for the systematic design of topological bands by substrate engineering.

Many modern functional materials exhibit microscopic degrees of freedom that are difficult to describe mathematically or do not conform to conventional models in solid state physics. In this paper, we consider crystals containing orientable molecular species, which encompass the class of promising photovoltaic materials ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbX}}_3$. Abstracting these materials as crystals of orientable rigid rotors, we develop an effective Hamiltonian that expresses the crystal potential energy in terms of collective orientations of interacting rigid rotors. The approach is motivated by the cluster expansion framework from alloy theory and is appropriate for describing both chiral and achiral molecules. This framework utilizes a quaternion parametrization of molecular orientation and makes full use of the symmetry both of the rotors and of the crystal in order to constrain the functional form of the final Hamiltonian expression. The resulting Hamiltonian is compact, systematically improvable, and is suitable for Monte Carlo or molecular dynamics simulation. We apply this formalism to ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ and report symmetry-adapted basis functions that can be used to construct order parameters or Hamiltonians that describe orientational correlation of methylammonium ions in hybrid organic-inorganic halide perovskite materials.

Based on ab initio calculations and spin dynamics simulations, we perform a detailed study on the magnetic properties of bulk MnN and the MnN/Fe interface. We determine the spin model parameters for the $\theta$-phase of bulk MnN, and we find that the competition between the nearest and the next-nearest-neighbor interactions leads to antiferromagnetic ordering of the Mn spins, in agreement with previous theoretical and experimental results. At the MnN/Fe interface, a sizable Dzyaloshinskii-Moriya interaction appears leading to a stable exchange-bias effect. We study the dependences of the exchange-bias effect on the thicknesses of the ferromagnetic and the antiferromagnetic layers, and we compare them to experimentally obtained results [Meinert et al., Phys. Rev. B 92, 144408 (2015)].

We use polarization-resolved Raman scattering to study lattice dynamics in ${\mathrm{NaFe}}_{0.53}{\mathrm{Cu}}_{0.47}\mathrm{As}$ single crystals. We identify four $A_{1g}$ phonon modes, at 126, 172, 183, and 197 ${\mathrm{cm}}^{-1}$, and four $B_{3g}$ phonon modes at 101, 139, 173, and 226 ${\mathrm{cm}}^{-1}$($D_{4h}$ point group). The phonon spectra are consistent with the $Ibam$ space group, which confirms that the Cu and Fe atoms form a stripe order. The temperature dependence of the phonon spectra suggests weak electron-phonon and magnetoelastic interactions.

The domain boundaries of ferroelastic $\mathrm{LaAl}{\mathrm{O}}_3$ (LAO) are examined using a nonlinear optical second harmonic generation (SHG) microscope. Although bulk crystalline LAO possesses centro-symmetry and is inherently non-SH active, our three-dimensional observations reveal that the domain boundaries of LAO exhibit SH activity with almost the same magnitude within a boundary. The polarization dependence of the SH intensities shows strong anisotropy, which is explained by the polar trigonal point group, $3m$. The polar direction is found to be along the ${\left[111\right]}_{pc}$ axis and inclined from the domain boundary plane. It implies that the biquadratic coupling scheme of two order parameters could be the main origin of the emergence of polar nature at the domain boundary. Our experimental results together with those of $\mathrm{CaTi}{\mathrm{O}}_3$ suggest that the polar nature of the domain boundary in ferroelastic materials may be a ubiquitous phenomenon.

A theoretical framework is proposed for the spin-current driven synchronized self-oscillations in ferromagnets in the spin Hall geometry. The spin current generated by the spin Hall effect in a bottom nonmagnetic heavy metal excites a self-oscillation of the magnetization in an attached ferromagnet through spin-transfer effect. The spin current simultaneously creates spin accumulation inside the ferromagnet. Therefore, when the top surfaces of two ferromagnets are connected by a nonmagnetic material having a long spin diffusion length, another spin current flows according to the gradient of the spin accumulations between the ferromagnets. This additional spin current excites an additional spin torque leading to a coupled motion of the magnetizations. This coupling mechanism comes purely from spin degree of freedom in the system without using electric and/or magnetic interactions. The additional spin torque acts as a repulsive force between the magnetizations, and prefers an antiphase synchronization between the oscillators. The phase difference in a synchronized state is determined by the competition between this additional spin torque and spin pumping. Eventually, either an in-phase or antiphase synchronization is spontaneously excited in the individual ferromagnets, depending on the current magnitude. These conclusions are obtained by deriving the theoretical formula of the additional spin torque from the diffusive spin transport theory and solving the equation of motion of the magnetizations both numerically and analytically.

Hidden order in ${\mathrm{URu}}_2{\mathrm{Si}}_2$ has remained a mystery that is now entering its fourth decade. The importance of resolving the nature of the hidden order has stimulated extensive research. Here we present a detailed characterization of different surface terminations in ${\mathrm{URu}}_2{\mathrm{Si}}_2$ by angle-resolved photoemission spectroscopy, in conjunction with scanning tunneling spectroscopy and dynamical mean-field theory calculations that may unveil a piece of this puzzle. The U-terminated surface is characterized by an electronlike band around the $\overline{X}$ point, while a holelike band characterizes the Si-terminated surface. We also investigate the temperature evolution of the electronic structure around the $\overline{X}$ point from 11 up to 70 K, and do not observe any abrupt change of the electronic structure around the coherence temperature (55 K). The $f$ spectral weight gradually weakens upon increasing temperature; still some $f$ spectral weight can be found above this temperature. Our results suggest that surface terminations in ${\mathrm{URu}}_2{\mathrm{Si}}_2$ are an important issue to be taken into account in future work.

The effective low energy description of interacting Dirac and Weyl semimetals is that of massless quantum electrodynamics with several Lorentz breaking material parameters. We perform a renormalization group analysis of Coulomb interaction in anisotropic Dirac and Weyl semimetals and show that the anisotropy persists in the material systems at the infrared fixed point. In addition, a tilt of the fermion cones breaking inversion symmetry induces a magnetoelectric term in the electrodynamics of the material whose magnitude runs to match that of the electronic tilt at the fixed point.

We reconsider the theory of Hall effect in the systems with hopping conduction. The purpose of this study is to compare the percolation approach based on the optimal triad model with numerical simulations and recent experimental results. We show that, in the nearest-neighbor hopping regime, the results of the percolation theory agree to the simulation. However, in the variable range hopping (VRH) regime, the optimal triad model fails to describe the numerical results. It is related to the extremely small probability to find the optimal triad of sites in the percolation cluster in the VRH regime. The contribution of these triads to the Hall effect appears to be small. We describe the Hall mobility in the VRH regime with the empirical law obtained from the numerical results. The law is in agreement with our recent experimental data in two-dimensional quantum dot arrays with the hopping transport.

We study two-electron states confined in two coupled quantum dots formed by a short-range potential in a two-dimensional topological insulator. It is shown that there is a fairly wide range of the system parameters where the ground state is a tripletlike state formed by a superposition of two spin-polarized states. Outside this range, the ground state is a singlet. A transition between the singlet and triplet states can be realized by changing the potential of the quantum dots. The effect is caused by a significant change in the energies of the Coulomb repulsion and the exchange interaction of electrons due to the presence of the pseudospin components of the wave function when the band spectrum is inverted.

We report on a nanoscale quantum sensing protocol which tracks a free precession of a single nuclear spin and is capable of estimating an azimuthal angle—a parameter which standard multipulse protocols cannot determine—of the target nucleus. Our protocol combines pulsed dynamic nuclear polarization, a phase-controlled radio-frequency pulse, and a multipulse ac sensing sequence with a modified readout pulse. Using a single nitrogen-vacancy center as a solid-state quantum sensor, we experimentally demonstrate this protocol on a single ${}^{13}\mathrm{C}$ nuclear spin in diamond and uniquely determine the lattice site of the target nucleus. Our result paves the way for magnetic resonance imaging at the single-molecular level.

We report on ground state phases of a doped one-dimensional Hubbard model, which for large onsite interactions is governed by the $t\text{−}J$ Hamiltonian, where the extant entanglement is immutable under perturbative or sudden changes of system parameters, a phenomenon termed as adiabatic freezing. We observe that in the metallic Luttinger liquid phase of the model bipartite entanglement decays polynomially and is adiabatically frozen, in contrast to the variable, exponential decay in the phase-separation and superconducting spin-gap phases. Significantly, at low fixed electron densities, the spin-gap phase shows remarkable affinity to doped resonating valence bond gas, with multipartite entanglement frozen across all parameter space. We note that entanglement, in general, is sensitive to external perturbation, as observed in several systems, and hitherto, no such invariance or freezing behavior has been reported.

We perform a charge density functional theory plus $U$ calculation of ${\mathrm{LaMnO}}_3$. While all the previous calculations were based on spin density functionals, our results and analysis show that the use of spin-unpolarized charge-only density is crucial to correctly describe the phase diagram, electronic structure, and magnetic property. Using magnetic force linear response calculation, a long-standing issue is further addressed regarding the second-neighbor out-of-plane interaction strength. We also estimate the orbital-resolved magnetic couplings. Remarkably, the interorbital $e_g-t_{2g}$ interaction is quite significant due to the Jahn-Teller distortion and orbital ordering.