The origin of the spin-crossover phenomenon in ${\mathrm{LaCoO}}_3$ is discussed based on the electronic state ${}^{5}D$ realized in a $3d^6$ system subjected to an intermediate crystal field. We calculate the splitting of the ${}^{5}D$ state under a crystal field whose symmetry is predominantly cubic but with a trigonal distortion. We confine ourselves to the ${\mathrm{\Gamma }}_5$ state and use a fictitious orbital angular momentum $\mathit{l}$ of magnitude one. Eigenvalues of a Hamiltonian including the spin-orbit interaction $-{\lambda }^{\prime }\mathit{l}·\mathit{S}$ and the trigonal crystal fields $9B_2^0(l_z^2-2/3)-80B_4^0(l_z^2-9/10)$ are rigorously calculated. A singlet ground state with an energy gap to the excited states is realized for certain values of the parameters in the Hamiltonian. The magnetization calculated rigorously using the energy levels subject to a magnetic field accords with published measurements.

Quantum strongly correlated matter exhibits properties which are not easily explainable in the conventional framework of Fermi liquids. Universal effective field theory tools are applicable in these cases regardless of the microscopic details of the quantum system, since they are based on symmetries. It is necessary, however, to construct these effective tools in full generality, avoiding restrictions coming from particular microscopic descriptions which may inadequately constrain the coefficients that enter in the effective theory. In this work we demonstrate with explicit examples how the hydrodynamic coefficients, which have been recently reinstated in the effective theory of pinned charge density waves (CDWs), can affect the phenomenology of the thermoelectric transport in strongly correlated quantum matter. Our examples, based on two classes of holographic models with pinned CDW, have microscopics which are conceptually different from Fermi liquids. Therefore, the above transport coefficients are nonzero, contrary to the conventional approach. We show how these coefficients allow one to take into account the change of sign of the Seebeck coefficient and the low resistivity of the CDW phase of the cuprate high temperature superconductors, without referring to the effects of Fermi surface reconstruction.

We present a methodology for probing the details of electronic susceptibility through minimally invasive nuclear magnetic resonance techniques. Specifically, we classify electron-mediated long-range interactions in an ensemble of nuclear spins by revealing their effect on simple spin echo experiments. We find that the pulse strength and applied field orientation dependence of these spin echo measurements resolves the spatial extent and anisotropy of electronic spin susceptibility. This Letter provides an alternate explanation to NMR results in superconducting and magnetically ordered systems. The methodology has direct applications for sensing and characterizing emergent electronic phases.

Nonequilibrium dynamics is a paramount scenario for studying quantum systems. The emergence of new features with no equilibrium counterpart, such as dynamical quantum phase transition (DQPT), has attracted wide attention. In this paper, we depart from the well-known Ising model and showcase an experimentally accessible configuration of a negatively charged nitrogen-vacancy center that interacts with nearby carbon-13 nuclear spins. We provide insights into this system in the context of DQPT. We show that nuclear spins undergo DQPT by appropriately choosing the relation between the transverse and longitudinal components of an external magnetic field. Furthermore, we can steer the DQPT via a time-dependent longitudinal magnetic field and apply this control to enhance the estimation of the coupling strength between the nuclear spins. Moreover, we propose a quenched dynamics that originates from the rotation of the central electron spin, which controls the DQPT relying on the anisotropy of the hyperfine coupling.

The quasiharmonic approximation (QHA) is the simplest nontrivial approximation for interacting phonons under constant pressure, bringing the effects of anharmonicity into temperature-dependent observables. Nonetheless, the QHA is often implemented with additional approximations due to the complexity of computing phonons under arbitrary strains, and the generalized QHA, which employs constant stress boundary conditions, has not been completely developed. Here we formulate the generalized QHA, providing a practical algorithm for computing the strain state and other observables as a function of temperature and true stress. We circumvent the complexity of computing phonons under arbitrary strains by employing irreducible second-order displacement derivatives of the Born-Oppenheimer potential and their strain dependence, which are efficiently and precisely computed using the lone irreducible derivative approach. We formulate two complementary strain parametrizations: a discretized strain grid interpolation and a Taylor series expansion in symmetrized strain. We illustrate our approach by evaluating the temperature and pressure dependence of select elastic constants and the thermal expansion in thoria (${\mathrm{ThO}}_2$) using density functional theory with three exchange-correlation functionals. The QHA results are compared to our measurements of the elastic constant tensor using time-domain Brillouin scattering and inelastic neutron scattering. Our irreducible derivative approach simplifies the implementation of the generalized QHA, which will facilitate reproducible, data-driven applications.

A three-dimensional version of the electroelastic model allowing the description of spin-crossover (SCO) materials taking into account for the volume change at the transition between the LS and the HS spin states is developed. The investigations are realized on a rectangular parallelepiped lattice with cubic symmetry. The SCO units are modeled by two-states fictitious spins coupled by springs whose equilibrium distances depend on the spin states. We implemented massive parallel simulations using CUDA (Compute Unified Device Architecture) programming where the spin states are updated using the Monte Carlo Metropolis algorithm, while the mechanical relaxation (lattice position) is performed by molecular dynamics. In this work, we investigated (i) the case of the thermal spin transition showing the macroscopic deformation of the parallelepiped accompanying the propagation of single domains and (ii) the isothermal relaxation of the photoinduced metastable HS fraction at low temperature. In both cases, the interplay between the electronic and the structural aspects of these transformations is analyzed and discussed in relation with the model parameters.

Spin ice presents a typical example of a classical spin liquid, where conserved magnetic fluxes emerge from microscopic spin degrees of freedom. In this paper, we investigate the effect of perturbation by magnetic charge disorder in two-dimensional spin ice. To this aim, we develop a cluster update algorithm which enables fast relaxation of magnetic charges. The efficient Monte Carlo calculation reveals a drastic change of the spin structure factor as doping magnetic charges: the pinch point, characterizing the spin ice, is gradually replaced by a diffusive peak. We derive an analytical relation connecting the flux fluctuation and the spin structure factor, and we explain the evolution of the diffusive peak in terms of the roughening of magnetic fluxes.

We study dynamical phase transitions (DPTs) in quantum many-body systems with infinite-range interaction, and present a theory connecting the two kinds of known DPTs (sometimes referred to as DPTs-I and DPTs-II) with the concept of excited-state quantum phase transition (ESQPT), traditionally found in collective models. We show that DPTs-I appear as a manifestation of symmetry restoration after a quench from the broken-symmetry phase, the limits between these two phases being demarcated precisely by an ESQPT. We describe the order parameters of DPTs-I with a generalization of the standard microcanonical ensemble incorporating the information of two additional conserved charges identifying the corresponding phase. We also show that DPTs-I are linked to a mechanism of information erasure brought about by the ESQPT, and quantify this information loss with the statistical ensemble that we propose. Finally, we show analytically the main mechanism for DPTs-II is forbidden in these systems for quenches leading a broken-symmetry initial state to the same broken-symmetry phase, on one side of the ESQPT, and we provide a formulation of DPTs-II depending on the side of the ESQPT where the quench ends. We analyze the connections between various indicators of DPTs-II. Our results are numerically illustrated in the infinite-range transverse-field Ising model and are applicable to a large class of collective quantum systems satisfying a set of conditions.

Recent progress in understanding thermal transport in complex crystals has highlighted the prominent role of heat conduction mediated by interband tunneling processes, which emerge between overlapping phonon bands (i.e., with energy differences smaller than their broadenings). These processes have recently been described in different ways, relying on the Wigner or Green-Kubo formalism, leading to apparently different results, which question the definition of the heat-current operator. Here, we implement a full quantum approach based on the Kubo formula, elucidating analogies and differences with the recently introduced Wigner or Green-Kubo formulations, and extending the description of thermal transport to the overdamped regime of atomic vibrations, where the phonon quasiparticle picture breaks down. We rely on first-principles calculations on complex crystals with ultralow conductivity to compare numerically the thermal conductivity obtained within the aforementioned approaches, showing that at least in the quasiparticle regime the differences are negligible for practical applications.

A surface polariton was predicted in antiferromagnets in a simple geometry. It exhibits interesting characteristics. Its electromagnetic fields and energy-flux density both oscillate and attenuate with the distance from the surface. It is a ghost surface polariton (GSP) and is a magnetic-field-tunable surface polariton. The branch-interference produces a series of rapidly attenuated fringes. It exists only in an external magnetic field along the easy axis. The attenuated total reflection spectra obtained in the Otto configuration accurately demonstrate this GSP and show its excitation or observation path.

We report the structural and magnetic properties of a new spin-$1/2$ antiferromagnet ${\mathrm{NaZnVOPO}}_4({\mathrm{HPO}}_4$) studied via x-ray diffraction, magnetic susceptibility, high-field magnetization, specific heat, and ${}^{31}\mathrm{P}$ nuclear magnetic resonance (NMR) measurements, as well as density-functional band-structure calculations. While thermodynamic properties of this compound are well described by the $J_1\text{−}{J}_2$ square-lattice model, ab initio calculations suggest a significant deformation of the spin lattice. From fits to the magnetic susceptibility we determine the averaged nearest-neighbor and second-neighbor exchange couplings of ${\overline{J}}_1\simeq -1.3\mathrm{K}$ and ${\overline{J}}_2\simeq 5.6\mathrm{K}$, respectively, resulting in the effective frustration ratio $\alpha ={\overline{J}}_2/{\overline{J}}_1\simeq -4.3$ that implies columnar antiferromagnetic order as the ground state. Experimental saturation field of 15.3 T is consistent with these estimates if 20% spatial anisotropy in $J_1$ is taken into account. Specific heat data signal the onset of a magnetic long-range order at $T_{\mathrm{N}}\simeq 2.1\mathrm{K}$, which is further supported by a sharp peak in the NMR spin-lattice relaxation rate. The NMR spectra mark the superposition of two P lines due to two nonequivalent P sites where the broad line with the strong hyperfine coupling and short $T_1$ is identified as the P(1) site located within the magnetic planes, while the narrow line with the weak hyperfine coupling and long $T_1$ is designated as the P(2) site located between the planes.

Density functional theory offers accurate structure prediction at acceptable computational cost, but commonly used approximations suffer from delocalization error; this results in inaccurate predictions of quantities such as energy band gaps of finite and bulk systems, energy level alignments, and electron distributions at interfaces. The localized orbital scaling correction (LOSC) was developed to correct delocalization error by using orbitals localized in space and energy. These localized orbitals span both the occupied and unoccupied spaces and can have fractional occupations in order to correct both the total energy and the one-electron energy eigenvalues. We extend the LOSC method to periodic systems, in which the localized orbitals employed are dually localized Wannier functions. In light of the effect of the bulk environment on the electrostatic interaction between localized orbitals, we modify the LOSC energy correction to include a screened Coulomb kernel. For a test set of semiconductors and large-gap insulators, we show that the screened LOSC method consistently improves the band gap compared to the parent density functional approximation.

The linear behavior of thermal transport has been widely explored, both theoretically and experimentally. On the other hand, the nonlinear thermal response has not been fully discussed. In light of the thermal vector potential theory [Tatara, Phys. Rev. Lett. 114, 196601 (2015)], we develop a general formulation to calculate the linear and nonlinear dynamic thermal responses. In the DC limit, we recover the well-known Mott relation and the Wiedemann-Franz (WF) law at the linear order response, which link the thermoelectric conductivity $\eta$, thermal conductivity $\kappa$, and electric conductivity $\sigma$ together. To be specific, the linear Mott relation describes the linear $\eta$ is proportional to the first derivative of $\sigma$ with respect to Fermi energy (for brevity we call the first derivative, the others are similar); and the linear WF law shows the linear $\kappa$ is proportional to the zero derivative (i.e., the $\sigma$ itself). We found there are higher-order Mott relations and WF laws which follow an order-dependent relation. At the second order, the Mott relation indicates that the second order $\sigma$ is proportional to the zero derivative of the second order $\eta$; but the second WF law shows that the second $\sigma$ is proportional to the first derivative of $\kappa$. At the third order, the derivative order increases once. Although we only did explicit calculations up to the third-order response, we can deduce that the $n\text{th-order}$ electric conductivity is proportional to the ($n-2$)th derivative of the $n\text{th-order}$ thermoelectric conductivity for the nonlinear Mott relation; and the $n\text{th-order}$ electric conductivity is proportional to the ($n-1$)th derivative of the $n\text{th-order}$ thermal conductivity for the nonlinear WF law. Since the second-order Hall effect has been studied in experiment, our theory may be tested by measuring the second-order Mott and WF as well. Our theory is presented explicitly for fermions, and it can also be applied to bosons. As an example, we calculate the second-order thermal conductivity of magnons in a strained collinear antiferromagnet on a honeycomb, in which the linear response disappears.

Parity anomaly is a quantum mechanical effect that the parity symmetry in a two-dimensional classical action is failed to be restored in any regularization of the full quantum theory and is characterized by a half-quantized Hall conductivity. Here we propose a scheme to explore the experimental signature from parity anomaly in the measurement of optical Hall conductivity, in which the optical Hall conductivity is nearly half-quantized for a proper range of frequency. The behaviors of optical Hall conductivity are studied for several models, which reveal the appearance of half-quantized Hall conductivity in low- or high-frequency regimes. The optical Hall conductivity can be extracted from the measurement of Kerr and Faraday rotations and the absorption rate of the circularly polarized light. This proposal provides a practical method to explore the signature of parity anomaly in topological quantum materials.

To realize novel topological phases and to pursue potential applications in low-energy consumption spintronics, the study of magnetic topological materials is of great interest. Starting from the theory of nonmagnetic topological quantum chemistry [Bradlyn et al., Nature 547, 298 (2017)], we have obtained irreducible (co)representations and compatibility relations (CRs) in momentum space, and we constructed a complete list of magnetic band (co)representations (MBRs) in real space for other magnetic space groups (MSGs) with antiunitary symmetries (i.e.,type-III and type-IV MSGs). The results are consistent with the magnetic topological quantum chemistry [Elcoro et al., Nat. Commun. 12, 5965 (2021)]. Using the CRs and MBRs, we reproduce the symmetry-based classifications for MSGs, and we obtain a set of Fu-Kane-like formulas of symmetry indicators (SIs) in both spinless (bosonic) and spinful (fermionic) systems, which are implemented in an automatic code—TopMat—to diagnose topological magnetic materials. The magnetic topological materials, whose occupied states cannot be decomposed into a sum of MBRs, are consistent with nonzero SIs. Lastly, using our online code, we have performed spin-polarized calculations for magnetic compounds in the materials database, and we find many magnetic topological candidates.

Topological insulators with intrinsic magnetic order are emerging as an exciting platform to realize fundamentally new excitations from topological quantum states of matter. To study these systems and their physics, people have proposed a variety of magnetic topological insulator systems, including HoSbTe, an antiferromagnetic weak topological insulator candidate. In this work, we use scanning tunneling microscopy to probe the electronic structure of HoSbTe with antiferromagnetic and ferromagnetic orders that are tuned by applying an external magnetic field. Although around the Fermi energy we find minor differences between the quasiparticle interferences under the ferromagnetic and antiferromagnetic orders, deep inside the valance region, a quasiparticle interference signal emerges with ferromagnetism. This observation is consistent with our first-principles calculations indicating the magnetism-driven transition of the electronic states in this spin-orbit coupled topological magnet.

Magnetic-field-angle-resolved specific heat and magnetization measurements were conducted on a ferromagnetic superconductor UCoGe with remarkable anisotropic upper critical field $H_{\mathrm{c}2}$. Although $H_{\mathrm{c}2}$ reaches a high magnetic field ($\sim 20\mathrm{T}$) along the $b$ axis, it is small ($\sim 0.6\mathrm{T}$) when a magnetic field is applied along the magnetic easy $c$ axis. This study indicates that the specific heat is abruptly suppressed when the magnetic field is applied toward the $c$ axis from the $a$ and $b$ axes in the ferromagnetic state. The field response of density of states (DOS) is anisotropic, relative to the $c$ axis, and its angle dependence is slightly singular. The Ising-type magnetic anisotropy of the ferromagnetic state is dominant even in the anisotropic reinforced superconducting state. These facts indicate that the suppression of DOS may closely relate to the superconducting state. We theoretically analyze these findings together with URhGe and ${\mathrm{UTe}}_2$ by highlighting the common and distinctive features among three compounds.

The ${\mathrm{Ca}}_{\mathrm{Ga}}$ acceptor in GaN was studied using photoluminescence (PL) experiments and first-principles calculations. The experimentally found $\text{–}/0$ transition level of the ${\mathrm{Ca}}_{\mathrm{Ga}}$ at 0.50 ± 0.02 eV above the valence band reasonably agrees with the value calculated using the Heyd-Scuseria-Ernzerhof functional tuned to fulfill the generalized Koopmans condition (0.70 eV). The ${\mathrm{Ca}}_{\mathrm{Ga}}$ acceptor is responsible for the green luminescence band (labeled ${\mathrm{GL}}_{\mathrm{Ca}}$) with a maximum at 2.5 eV. The electron- and hole-capture coefficients for the ${\mathrm{Ca}}_{\mathrm{Ga}}$ acceptor are obtained from PL experiments: $C_n=9\times {10}^{\text{–}14}\mathrm{c}{\mathrm{m}}^3/\mathrm{s}$ and $C_p=6\times {10}^{\text{–}7}\mathrm{c}{\mathrm{m}}^3/\mathrm{s}$. The ${\mathrm{GL}}_{\mathrm{Ca}}$ band is an efficient radiative recombination channel in GaN samples implanted with Ca and in unintentionally doped GaN grown by molecular-beam epitaxy at relatively low temperatures, where Ca is a common contaminant.

We present a two-fluid plasma model to describe localized interaction, at submicrometer scale, of highly focused ultrashort laser pulses within dielectrics and semiconductors. This model includes transport of the electron-hole plasma, electron-hole scattering, photoionization, impact ionization, and interactions between charge carriers and phonons. We start with a plasma kinetic description, and we consistently derive the two-fluid plasma model equations. Our numerical results highlight that transport of the electron-hole plasma can strongly affect the plasma density profile in the case of extremely localized interaction.

A giant built-in electric field in the growth direction makes excitons in wide GaN/(Al, Ga)N quantum wells spatially indirect even in the absence of any external bias. Significant densities of indirect excitons can accumulate in electrostatic traps imprinted in the quantum well plane by a thin metal layer deposited on top of the heterostructure. By jointly measuring spatially resolved photoluminescence and photo-induced current, we demonstrate that exciton density in the trap can be controlled via an external electric bias, which is capable of altering the trap depth. Application of a negative bias deepens the trapping potential, but does not lead to any additional accumulation of excitons in the trap. This is due to exciton dissociation instigated by the lateral electric field at the electrode edges. The resulting carrier losses are detected as an increased photocurrent and reduced photoluminescence intensity. By contrast, application of a positive bias washes out the electrode-induced trapping potential. Thus, excitons get released from the trap and recover free propagation in the plane that we reveal by spatially resolved photoluminescence.