The crystal structure and magnetic correlations in triangular antiferromagnet $\mathrm{Fe}\mathrm{Ga}{}_2\mathrm{S}{}_4$ are studied by x-ray diffraction, magnetic susceptibility, neutron diffraction, and neutron inelastic scattering. We report significant mixing at the cation sites and disentangle magnetic properties dominated by major and minor magnetic sites. The magnetic short-range correlations at $0.77{\AA }^{-1}$ correspond to the major sites and being static at base temperature they evolve into dynamic correlations around 30–50 K. The minor sites contribute to the magnetic peak at $0.6{\AA }^{-1}$, which vanishes at 5.5 K. Our analytical studies of triangular lattice models with bilinear and biquadratic terms provide the ratios between exchanges for the proposed ordering vectors. The modeling of the inelastic neutron spectrum within linear spin-wave theory results in the set of exchange couplings $J_1=1.7,J_2=0.9,J_3=0.8\mathrm{meV}$ for the bilinear Heisenberg Hamiltonian. However, not all features of the excitation spectrum are explained with this model.

The behavior of magnetic materials can be simulated at the macroscale using the micromagnetic model whose key parameters, such as exchange stiffness constants and magnetic anisotropies, can be derived from first-principles electronic structure calculations. In this work we employed the Korringa-Kohn-Rostoker (KKR) Green's function method with the coherent potential approximation (CPA) to investigate the dependence of the spin-wave stiffness on the Si concentration for the three magnetic phases of FeSi, namely A2, B2, and $\mathrm{D}{0}_3$. Based on the structural, magnetic, and electronic structure analysis using the KKR-CPA methodology, the changes in the spin-wave stiffness caused by the addition of Si are primarily governed by the variations in the electronic structure.

The pairing mechanism in $A_3{\mathrm{C}}_{60}$ is investigated by studying the properties of a three-orbital Hubbard model with antiferromagnetic Hund coupling in the normal and superconducting phases. Local orbital fluctuations are shown to be substantially enhanced in the superconducting state, with a fluctuation energy scale that matches the low-energy peak in the spectral weight of the order parameter. Our results demonstrate that local orbital fluctuations provide the pairing glue in strongly correlated fulleride superconductors and support the spin/orbital freezing theory of unconventional superconductivity. They are also consistent with the experimentally observed universal relation between the gap energy and local susceptibility in a broad range of unconventional superconductors.

Here we study the single-particle, electronic transport, and optical properties of a gapped system described by a simple two-band Hamiltonian with inverted valence bands. We analyze its properties in the three-dimensional (3D) and the two-dimensional (2D) case. The insulating phase changes into a metallic phase when the band gap is set to zero. The metallic phase in the 3D case is characterized by a nodal surface. This nodal surface is equivalent to a nodal ring in two dimensions. Within a simple theoretical framework, we calculate the density of states, the total and effective charge carrier concentration, the Hall concentration, and the Hall coefficient, for both 2D and 3D cases. The main result is that the three concentrations always differ from one another in the present model. These concentrations can then be used to resolve the nature of the electronic ground state. Similarly, the optical conductivity is calculated and discussed for the insulating phase. We show that there are no optical excitations in the metallic phase. Finally, we compare the calculated optical conductivity with the rule-of-thumb derivation using the joint density of states.

Cobalt oxides with multiple local Co-O coordination environments such as octahedral, tetrahedral, and tetragonal networks display versatile electronic and magnetic properties which have attracted great interest in many fields. Understanding the ground-state properties and determining the fundamental band gap remain challenges in cobalt-based compounds and thin films, which have been investigated here using synchrotron-based Co $L_{23}$-edge x-ray absorption measurements followed by configuration interaction cluster calculations. We focus on the detailed Co $L_{23}$-edge absorption spectral variations in different octahedral crystal fields as well as in the less investigated tetrahedral and tetragonal systems, taking into account Co ions with different valence states. From a quantitative comparison between the simulated spectrum and an accurately measured absorption spectrum of a specified compound, the crystal field value can be extracted from the Co $L_{23}$-edge absorption spectrum, which is complementary to the results obtained in optical measurements and other calculations. Furthermore, Co $L_{23}$-edge x-ray linear dichroism shows the same spectral evolutions as a result of either local ${\mathrm{CoO}}_6$ cluster with tetragonal symmetry or the magnetic exchange field, whereas both coexist in most antiferromagnetic cobalt oxide thin films. Detailed temperature and polarization-dependent Co $L_{23}$-edge absorption measurements have been proposed to distinguish both contributions, which show different spectral variations due to the specified modifications of the ground and final states at different temperatures. Our results offer theoretical guidance for understanding the multiplet structure of Co $L_{23}$-edge absorption spectrum, obtaining the precise crystal field value for cobalt oxides with versatile coordinations, and explaining the underlying mechanism of x-ray linear dichroism, as well as understanding the fundamental physical properties and their potential applicability of cobalt oxides and their thin films.

At low temperature, a fermionic system with perfectly nested Fermi surface orders into a spin-density wave. Upon doping, however, the latter state becomes unstable, and several spatially inhomogeneous phases emerge competing against each other to become the true ground state. We investigate this competition using the anisotropic Hubbard model on a three-dimensional cubic lattice in a weak-coupling regime as a convenient study case. For this model it is known that, at half-filling (one electron per site), the Fermi surface nesting is perfect, and the ground state is the commensurate spin-density wave. Away from the half-filling, various types of spatially inhomogeneous phases, such as phase-separated states and the state with domain walls (“soliton lattice”), emerge. Using the mean-field theory, we evaluate the free energies of these phases to determine which of them could become the true ground state. Our study demonstrates that the free energies of all discussed states are very close to each other. Smallness of these energy differences suggests that, for a real material, numerous factors, unaccounted by the model, may arbitrary shift the relative stability of the competing phases. We further argue that purely theoretical prediction of a structure of inhomogeneous phase in a particular doped system is unreliable.

Paramagnetic substitutional carbon (${\mathrm{C}}_{\text{B}},{\mathrm{C}}_{\text{N}}$) defects in hexagonal boron nitride are discussed as candidates for quantum bits. Their identification and suitability are approached by means of photoluminescence (PL), charge transitions, electron paramagnetic resonance, and optically detected magnetic resonance (ODMR) spectra. Several clear trends in these are revealed by means of an efficient plane wave periodic supercell ab initio density functional theory approach. In particular, this yields insight into the role of the separation between ${\mathrm{C}}_{\text{B}}$ and ${\mathrm{C}}_{\text{N}}$. In most of the cases, the charge transition between the neutral and a singly charged ground state of a defect is predicted to be experimentally accessible, since the charge transition level position lies within the band gap. A posteriori charge corrections are also discussed. A near-identification of an experimentally isolated single spin center as the neutral ${\mathrm{C}}_{\text{B}}$ point defect was found via comparison of results to recently observed PL and ODMR spectra.

Conducting interfaces between polar and nonpolar insulating oxides, e.g., ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$, have generated interest for both fundamental physics and oxide-electronics applications. Current understanding is based on an amalgamation of a classical electrostatic model (polar catastrophe model) that was originally derived for semi-infinite solids and quantum density-functional-theory (DFT) results on ultrathin films. Here we report comprehensive DFT calculations that unveil a very different purely quantum physical reality. We show that, for ultrathin polar films, the interfacial dipole does not control the electrostatic potential in the polar film—the surface and interface play equal roles, and the absence or presence of centrosymmetry in the physical LAO film results in different, purely quantum mechanisms for the generation of a conducting interface, neither involving physical-charged transfer. Predictions are made that can be tested and can guide technology development.

Collective orders and photoinduced phase transitions in quantum matter can evolve on timescales which are orders of magnitude slower than the femtosecond processes related to electronic motion in the solid. Quantum Boltzmann equations can potentially resolve this separation of timescales, but are often constructed by assuming the existence of quasiparticles. Here we derive a quantum Boltzmann equation which only assumes a separation of timescales (taken into account through the gradient approximation for convolutions in time), but is based on a nonperturbative scattering integral, and makes no assumption on the spectral function such as the quasiparticle approximation. In particular, a scattering integral corresponding to nonequilibrium dynamical mean-field theory is evaluated in terms of an Anderson impurity model in a nonequilibrium steady state with prescribed distribution functions. This opens the possibility to investigate dynamical processes in correlated solids with quantum impurity solvers designed for the study of nonequilibrium steady states.

The electronic structure of one of the nickelates ($\mathrm{La}\mathrm{Ni}{\mathrm{O}}_2$) and one of the cuprates ($\mathrm{Ca}\mathrm{Cu}{\mathrm{O}}_2$) is studied with three self-consistent $GW$-based methods: $\mathrm{sc}GW$, $\mathrm{sc}(GW+\text{vertex})$, and quasiparticle self-consistent $GW$. Low-energy features obtained in our study are in many respects similar to the features reported in previous density functional theory plus dynamical mean-field theory ($\mathrm{DFT}+\mathrm{DMFT}$) studies. Consistent with the $\mathrm{DFT}+\mathrm{DMFT}$ conclusion, we find $\mathrm{La}\mathrm{Ni}{\mathrm{O}}_2$ to be more correlated than $\mathrm{Ca}\mathrm{Cu}{\mathrm{O}}_2$. However, correlation effects included in our study change the DFT Fermi surface near the $\mathrm{\Gamma }$ point differently from that reported in DMFT studies. Features that are a few electronvolts away from the Fermi level are broader in our calculations than in the $\mathrm{DFT}+\mathrm{DMFT}$, which reflects the differences between the DFT and the $GW$ methods. Our results are in qualitative agreement with previous $G0W0$ results, but the self-consistency results in quantitative differences. Generally, correlation effects are found to be sufficiently weak in both materials, which allows one to use totally ab initio diagrammatic approaches such as $\mathrm{sc}(GW+\text{vertex})$ and to avoid the methods with adjustable parameters ($\mathrm{DFT}+\mathrm{U}$ or $\mathrm{DFT}+\mathrm{DMFT}$). However, the possibility of some strong correlations at low energy that cannot be captured by perturbative methods cannot be completely excluded. For instance, differences in the Fermi surface should be resolved, thus experimental studies are necessary.

We present an approach based on density functional theory for the calculation of fundamental gaps of both finite and periodic two-dimensional (2D) electronic systems. The computational cost of our approach is comparable to that of total energy calculations performed via standard semilocal forms. We achieve this by replacing the 2D local density approximation with a more sophisticated—yet computationally simple—orbital-dependent modeling of the exchange potential within the procedure by Guandalini et al. [Phys. Rev. B 99, 125140 (2019)]. We showcase promising results for semiconductor 2D quantum dots and artificial graphene systems, where the band structure can be tuned through, e.g., Kekulé distortion.

The interplay between the spin-orbit and Zeeman interactions acting on a spinful Su-Schrieffer-Heeger model is studied based on an InAs nanowire subjected to a periodic gate potential along the axial direction. It is shown that a nontrivial topological phase can be achieved by regulating the confining-potential configuration. In the absence of the Zeeman field, we prove that the topology of the chain is not affected by the Rashba spin-orbit interaction due to the persisting chiral symmetry. The energies of the edge modes can be manipulated by varying the magnitude and direction of the external magnetic field. Remarkably, the joint effect of the two spin-related interactions leads to novel edge states that appear in the gap formed by the anticrossing of the bands of a finite spinful dimerized chain, and can be merged into the bulk states by tilting the magnetic-field direction.

Silicon-germanium heterostructures have successfully hosted quantum dot qubits, but the intrinsic near-degeneracy of the two lowest valley states poses an obstacle to high-fidelity quantum computing. We present a modification to the Si/SiGe heterostructure by the inclusion of a spike in germanium concentration within the quantum well in order to increase the valley splitting. The heterostructure is grown by chemical vapor deposition and magnetospectroscopy is performed on gate-defined quantum dots to measure the excited state spectrum. We demonstrate a large and widely tunable valley splitting as a function of applied vertical electric field and lateral dot confinement. We further investigate the role of the germanium spike by means of tight-binding simulations in single-electron dots and show a robust doubling of the valley splitting when the spike is present, as compared to a standard (spike-free) heterostructure. This doubling effect is nearly independent of the electric field, germanium content of the spike, and spike location. This experimental evidence of a stable, tunable quantum dot, despite a drastic change to the heterostructure, provides a foundation for future heterostructure modifications.

We predict the emergence of turbulent scaling in the quench dynamics of the two-dimensional Heisenberg model for a wide range of initial conditions and model parameters. In the isotropic Heisenberg model, we find that the spin-spin correlation function exhibits universal scaling consistent with a turbulent energy cascade. When the spin rotational symmetry is broken by an easy-plane exchange anisotropy, we find a dual cascade of energy and an emergent conserved charge associated to transverse magnetization fluctuations. The scaling exponents are estimated analytically and agree with numerical simulations using phase space methods. We also define the range of initial conditions and model parameters (energy, magnetization, and spin number $S$) that lead to a turbulent cascade. The universal character of the cascade, insensitive to microscopic details or initial conditions, suggests that turbulence in spin systems can be broadly realized in cold atom and solid-state experiments.

Noninteracting spinless electrons in one-dimensional quasicrystals, described by the Aubry-André-Harper (AAH) Hamiltonian with nearest-neighbor hopping, undergo a metal-to-insulator transition at a critical strength of the quasiperiodic potential. The AAH Hamiltonian is also known to be self-dual. Interestingly, the critical point and the self-dual point are identical in this case. In this work, we have studied the one-dimensional quasiperiodic AAH Hamiltonian in the presence of spin-orbit coupling of Rashba-type, which introduces an additional spin-conserving complex hopping and a spin-flip hopping. We have found that the AAH Hamiltonian remains self-dual in the presence of Rashba spin-orbit coupling, and the self-dual point follows a simple rescaled relationship among the parameters of the Hamiltonian. This system also undergoes a metal-to-insulator transition, and the nature of this transition has been found to be identical with the original AAH Hamiltonian. However, the critical points do not follow the same relationship as the self-dual points in general. In fact, the metal-to-insulator transition happens earlier than the self-dual point, except in some special cases in which they are observed to coincide.

We suggest coherent switching of canted antiferromagnetic (AFM) spins using spin-orbit torque (SOT) in a small magnet. The magnetic system of orthoferrite features biaxial easy anisotropy and the Dzyaloshinskii-Moriya interaction, which is perpendicular to the easy axes and therefore creates weak magnetization $\left(\mathit{m}\right)$. A dampinglike component of the SOT induces Néel reorientation along one of the easy axes and then exerts torque on $\mathit{m}$, leading to tilting of the Néel order $\mathit{l}$. The torque on the magnetization becomes stronger due to coupling with the induced Oersted field or the fieldlike component of the SOT, enhancing the tilting of $\mathit{l}$. Therefore, $\mathit{l}$ is found to experience deterministic switching after the SOT is turned off. Based upon both numerical and analytical analysis of the coherent switching, exclusive OR (XOR) logic gates are also found to be implemented in a single magnetic layer. In addition, we investigate how magnetic parameters affect the critical reorientation angle and current density in a simple layered structure of platinum and a canted AFM. Our findings are expected to provide an alternative spin-switching mechanism for ultrafast applications such as spin logic and electronic devices.

An $A$-site ordered perovskite $\mathrm{Sm}{\mathrm{Mn}}_3{\mathrm{Cr}}_4{\mathrm{O}}_{12}$ with a cubic $Im\text{−}3$ space group was prepared at high pressure and temperature conditions. Two antiferromagnetic phase transitions are found to occur with decreasing temperature to $T_{\mathrm{N}1}\approx 145\mathrm{K}$ and $T_{\mathrm{N}2}\approx 41\mathrm{K}$, due to the spin orderings of the $B$-site ${\mathrm{Cr}}^{3+}$ and $A$′-site ${\mathrm{Mn}}^{3+}$ magnetic sublattices, respectively. The total spin structure composed of these two magnetic sublattices can break the space inversion symmetry and therefore induce a spontaneous ferroelectric phase transition at $T_{\mathrm{N}2}$. More interestingly, the magnetoelectric multiferroicity of the polycrystalline $\mathrm{Sm}{\mathrm{Mn}}_3{\mathrm{Cr}}_4{\mathrm{O}}_{12}$ exhibits remarkable magnetic and electric field dependent anisotropic quadratic behaviors upon a proper magnetoelectric poling procedure. When the applied magnetic field is parallel to the electric field, the magnetic field can significantly enhance the electric polarization. In contrast, the polarization is sharply suppressed if the magnetic field is perpendicular to the electric field, irrespective of the sign of the magnetic field.

$\mathrm{Pr}{\mathrm{Mn}}_{1-x}{\mathrm{Sb}}_2$ exhibits two antiferromagnetic orders that are attributed to the two magnetic elements Mn and Pr. Both magnetic phases have an intriguing magnetic structure as the magnetic moments are tilted from the principal crystal axes, but the magnetic properties had not been studied in detail. We grew single crystals of $\mathrm{Pr}{\mathrm{Mn}}_{1-x}{\mathrm{Sb}}_2$ and measured the temperature and magnetic field dependencies of magnetization for different field directions. We found that both Mn and Pr moments undergo a metamagnetic transition at different temperatures and different field directions. A spin-flop transition was observed prior to the metamagnetic transition of the Mn sublattice when the magnetic field ($H$) was applied parallel to the crystallographic $a$ axis, but not for $H\left|\right|\left[110\right]$, reflecting the magnetic anisotropy. The destruction of the antiferromagnetic order of the Mn sublattice via metamagnetic transition promoted the ordering of the Pr moments, indicating that the two antiferromagnetic phases are in competition. The magnetic phase diagrams determined from our measurements indicate that the present compound is strongly anisotropic and exhibits a far richer variety of magnetic states than the other compounds in the same material family $A\mathrm{Mn}{X}_2$ ($A$: alkaline earth or rare earth element; $X$: Sb or Bi).