Bulk electronic states of pyrite-type ${\mathrm{CuS}}_2$ and ${\mathrm{CuSe}}_2$ have been investigated by means of hard x-ray photoemission spectroscopy (HAXPES). In the valence-band HAXPES spectra, the Cu $3d$ subshell is basically fully occupied in ${\mathrm{CuS}}_2$ and ${\mathrm{CuSe}}_2$, and the partially occupied anion $p$ orbitals are responsible for the metallic behavior. This situation is opposite to the various conducting transition-metal compounds with open shell transition-metal cations and closed shell anions. The Cu $2p$ core-level HAXPES spectra exhibit satellite peaks, which indicate the presence of charge fluctuations mediated by inverse charge transfer from Cu $3d$ to S $3p/\mathrm{Se}$ $4p$.

The quantum anomalous Hall insulator is characterized by a quantized Hall resistance plateau of $h/\left(C{e}^2\right)$ with $C$ being the Chern number. Previously, high-Chern-number insulators with $C\ge 2$ that intrinsically suppress the energy dissipation in quantized electrical transport have been realized in few-layer $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ nanoflakes under moderate magnetic fields or in Cr-doped topological insulators at extremely low temperatures. It has nevertheless proven elusive to achieve a high-temperature high-Chern-number insulator at zero magnetic field. Here, we demonstrate that the magnetic state in $\mathrm{Mn}{\mathrm{Bi}}_4{\mathrm{Te}}_7$ nanoflakes can be consecutively tuned from an antiferromagnetic (AFM) state to an AFM-ferromagnetic (FM) coexistence state, and finally to a robust ferromagnetic (FM) state with large coercivities at temperatures up to 3 K via a protonic gate. This AFM-FM phase transition is well captured by density functional theory simulations that the FM state develops in $\mathrm{Mn}{\mathrm{Bi}}_4{\mathrm{Te}}_7$ under hole doping. Notably, we find that the Chern number of FM $\mathrm{Mn}{\mathrm{Bi}}_4{\mathrm{Te}}_7$ can be largely tuned not merely by the Zeeman field but also by the sample thickness. Our work demonstrates that gate-tuned $\mathrm{Mn}{\mathrm{Bi}}_4{\mathrm{Te}}_7$ nanoflakes hold high potentials for realizing a high-temperature high-Chern-number insulator at zero magnetic field and promising applications in the field of low-energy electronics.

We describe here a theory of a quantum dot in an electrically tunable ${\mathrm{MoSe}}_2/{\mathrm{WSe}}_2$ heterostructure. van der Waals heterostructures allow for tuning their electronic properties beyond their monolayer counterparts. We start by determining their electronic structure using density functional theory. We obtain the type-II band alignment and close in energy conduction band minima (valleys) at the $K$ and $Q$ points in the Brillouin zone. The valence band maxima, also energetically close, are located at the $K$ and $\mathrm{\Gamma }$ points. By analyzing the Kohn-Sham wave functions, we describe the layer, spin, and orbital contributions. Next, we construct an ab initio–based tight-binding model, which helps us to better understand the complexity of the interlayer interactions. We determine the effect of a vertical electric field, showing that vertical gating enables control of valleys' extrema and their occupancy. Finally, we employ the tight-binding model to investigate laterally gated quantum dots and analyze the influence of a perpendicular electric field on their energy spectrum. Our results demonstrate that tuning the electric field enables control over the valley character of the quantum dot states, selectively localizing them in either the $K$ or $Q$ valleys, as evidenced by their characteristic degeneracies and wave functions.

In this paper, we study self-consistent solutions in one-dimensional lattice models obtained via many-body perturbation theory. The Dyson equation is solved in a fully self-consistent manner via the algorithmic-inversion method based on the sum-over-poles representation (AIM-SOP) of dynamical operators. In particular, we focus on the $\mathit{GW}$ approximation, analyzing the spectral properties and the emergence of possible magnetic- or charge-density-wave broken symmetry solutions. We start by validating our self-consistent AIM-SOP implementation by taking as a test case the one-dimensional Hubbard model. We then move to the study of antiferromagnetic and charge-density-wave solutions in one-dimensional lattice models, taking into account a long-range Coulomb interaction between the electrons. We show that moving from local to nonlocal electronic interactions leads to a competition between antiferromagnetic and charge-density-wave broken symmetry solutions. Complementary, by solving the Sham-Schlüter equation, we can compute the noninteracting Green's function reproducing the same charge density of the interacting system. In turn, this allows for the evaluation of the derivative discontinuity of the Kohn-Sham potential, showing that its contribution to the fundamental gap can become dominating in some of the studied cases.

The enhancement of the Néel temperature ($T_N$) of low-$T_N$ antiferromagnets (AFs) in AF bilayers AF1/AF2, where the $T_N$ of AF1 is larger than AF2 (for example, enhancement of $T_N$ of CoO in NiO/CoO or FeO in CoO/FeO), is a subject of considerable interest. One essential question needs to be answered in these bilayers: Is the interfacial coupling a short- or long-range one that mediates the effect of the AF1 layers on the magnetic properties of the AF2 layer? To understand the systematics of the magnetic coupling across the interface, we investigate the plane-resolved magnetotransport properties of AF bilayers using an electron-hole symmetric one-band Hubbard model at half-filling, employing a semiclassical Monte Carlo method. In our model Hamiltonian calculations, we set Coulomb repulsion $U_1=8$ to mimic the high-$T_N$ AF1 layer, whereas we use $U_2=2U_1$ to mimic the low-$T_N$ AF2 layer. Our calculations show that the $T_N$ of the low-$T_N$ AF enhances substantially when its thickness is small, like experiments, giving rise to the single magnetic transition temperature of the bilayer system. These findings are well supported by a single peak in temperature-dependent specific heat. However, for larger thicknesses, the $T_N$ of the low-$T_N$ AF approaches its bulk value, constituent AF layers align antiferromagnetically at two separate temperatures, and two maxima are observed in specific heat data. Our calculations also show that the delocalization of moments is more or less confined near the interface, indicating the short-ranged nature of the proximity effect. In addition, the density of states of the interfacial plane of a low-$T_N$ AF gets modified only due to the proximity effect. Our obtained results are consistent with the experimental observations and provide insights into the phenomena of the enhancement of Néel temperatures of a low-$T_N$ AF layer in AF bilayer systems. At the end, a detailed discussion of the modifications that will occur if we use $U_1=8$ and $U_2=0.5U_1$ is also addressed.

The $U\left(1\right)$ Dirac spin liquid might realize an exotic phase of matter whose low-energy properties are described by quantum electrodynamics in $2+1$ dimensions, where gapless modes exist but spinons and gauge fields are strongly coupled. Its existence was proposed in frustrated Heisenberg models in the presence of frustrating superexchange interactions by the (Abrikosov) fermionic representation of the spin operators [Wen, Phys. Rev. B 65, 165113 (2002)], supplemented by the Gutzwiller projection. Here, we construct charge-$Q$ monopole excitations in the Heisenberg model on the triangular lattice with nearest-neighbor ($J_1$) and next-nearest-neighbor ($J_2$) couplings. In the highly frustrated regime, singlet and triplet monopoles with $Q=1$ become gapless in the thermodynamic limit; in addition, the energies for generic $Q$ agree with field-theoretical predictions, obtained for a large number of gapless fermion modes. Finally, we consider localized gauge excitations, in which magnetic $\pi$ fluxes are concentrated in the triangular plaquettes (in analogy with ${\mathbb{Z}}_2$ visons), showing that these kinds of states do not play a relevant role at low energies. All our findings lend support to a stable $U\left(1\right)$ Dirac spin liquid in the $J_1\text{−}{J}_2$ Heisenberg model on the triangular lattice.

We consider systems in which a continuous symmetry $G$, which may be anomalous, is spontaneously broken to an anomaly-free subgroup $H$ such that the effective action for the Goldstone modes contains topologically nontrivial terms. If the original system has trivial $G$ anomaly, then it is known that the possible topological terms are fully determined by SPT or SET invariants of the residual $H$ symmetry. Here we address the more general setting in which the $G$ symmetry has an anomaly. We argue that in general, the appropriate concept to consider is the “compatibility relation” between the Goldstone invariants and the $G$ anomaly. In the case where the Goldstone modes can be gapped out to obtain invertible families (i.e., without any topological order), we give an explicit mathematical scheme to construct the desired compatibility relation. We also address the case where gapping out the Goldstone modes leads to a family of topologically ordered states. We discuss several examples including the canonical Thouless pump, the quantum Hall ferromagnet, pumps arising from breaking $\mathrm{U}\left(1\right)$ symmetry at the boundary of topological insulators in two and three dimensions, and pumps classified by the higher Chern number.

We present a detailed theoretical description of the electron transport through a canted quantum dot (QD) spin valve consisting of QD weakly coupled to ferromagnetic leads with noncollinear magnetization directions. In such a system, we can observe the nonequilibrium accumulated spin on the QD and virtual particle exchange processes between the QD and the ferromagnetic electrodes resulting in an effective exchange field that can be controlled by the gate or bias voltages. The exchange field affects the spin dynamics in the QD and can be used for spin manipulation. We analyze a general Bloch equation, which describes the complex spin dynamics on the QD, and we find its solutions for various useful and important limits. We obtain analytical relations between the spin accumulated on the QD and the DC current flowing in the analyzed system for the linear and nonlinear transport regimes. Surprisingly, the DC current is related to three distinct projections of the QD spin that also can be controlled by the bias and gate voltages and are independent of the electrode coupling asymmetry. Thus the ferromagnetic electrodes can act effectively as spin detectors that translate spin information into a charge signal, while the readout direction can be controlled electrically. We also predict a new type of zero-bias anomaly that is related to both the switching of the spin detection direction at the zero bias and to the spin dynamics due to the exchange field. These findings allow us to explain the tunnel magnetoresistance (TMR) characteristics from the recent experiment, where the nonequilibrium spin transport in the canted quantum dot spin valve was studied and signatures of out of equilibrium spin precession, that are electrically tunable, were observed. Additionally, we demonstrate various experimental schemes for determination of the effect of the local exchange field from the TMR characteristics also at relatively higher temperatures as compared to other technologies.

We study prethermal time-crystalline order in periodically driven quantum Ising models on disorder-free decorated lattices. Using a tensor network ansatz for the state which reflects the geometry of a unit cell of the lattice, we show through finite entanglement scaling that the system has an exponentially long-lived subharmonic response in the thermodynamic limit, which decays nonperturbatively in deviations from a perfect periodic drive. The resulting prethermal discrete time crystal is not only stable to imperfections in the transverse field, but also exhibits a bipartite rigidity to generic perturbations in the longitudinal field. We call this state a bipartite discrete time crystal and reveal a rich prethermal phase diagram, including multiple regions of bipartite time-crystalline order, uniform time-crystalline order, and thermalization, with boundaries depending delicately on the topology of the decorated lattice. Our results thus uncover a variety of time crystals which may be realized on current digital quantum processors and analog quantum simulators.

We show that the driving field of circularly polarized light (CPL) can be used to enhance and reverse the spin current generated in the spin Hall effect for some transition metal dichalcogenides. This is demonstrated by analyzing the time-averaged spin Hall conductivities in the nonequilibrium steady states of monolayers ${\mathrm{WS}}_2$, ${\mathrm{MoS}}_2$, ${\mathrm{MoTe}}_2$, and ${\mathrm{WTe}}_2$ driven by CPL with the Floquet linear-response theory. We argue that the enhancement and reversals of the spin current come from a combination of the non-Rashba effect of broken inversion symmetry and the nonperturbative effect of CPL beyond the dynamical localization. This work allows optical control of the magnitude and direction of the pure spin current.

Over the past decades, our understanding of thermal transport in amorphous materials has predominantly relied on the inherently harmonic Allen-Feldman theory, which has been found to be insufficient. In this study, the Wigner transport formalism is adopted to explicitly account for anharmonicity. In studying the thermal transport in amorphous silicon, the results highlight that amorphous materials are not generally computationally equivalent to crystals with disordered primitive cells. A method that leverages the properties of the two-mode terms in the Wigner transport formalism is proposed to predict the bulk thermal conductivity of amorphous materials using finite-size models. In doing so, the need for mode classification schemes required in the Allen-Feldman theory is eliminated, and similarities are discovered between the two-mode terms and the carriers commonly used to describe thermal transport in amorphous materials, i.e., propagons, diffusons, and locons. Two competing trends are identified that shed light on the recently discovered anomalous decrease in the high-temperature thermal conductivity in some amorphous materials.

The dynamics of disordered two-dimensional systems is much less understood than the dynamics of disordered chains, mainly due to the lack of appropriate numerical methods. We demonstrate that a single-trajectory version of the fermionic truncated Wigner approximation (fTWA) gives unexpectedly accurate results for the dynamics of one-dimensional (1D) systems with moderate or strong disorder. Additionally, the computational complexity of calculations carried out within this approximation is small enough to enable studies of two-dimensional (2D) systems larger than standard fTWA. Using this method, we analyze the dynamics of interacting spinless fermions propagating on disordered 1D and 2D lattices. We find for both spatial dimensions that the imbalance exhibits a universal dependence on the rescaled time $t/{\xi }_W$, where in two dimensions the timescale ${\xi }_W$ follows a stretched-exponential dependence on disorder strength.

$\beta {\text{-Ga}}_2{\text{O}}_3$ is a promising material candidate for next-generation high-power devices even as its low thermal conductivity ($\kappa$) limits utilization due to an inability to sufficiently dissipate heat. Despite the importance of this inherent thermal challenge, a significant discrepancy persists between experimental results and computational models regarding the anisotropic thermal conductivity of $\beta {\text{-Ga}}_2{\text{O}}_3$. Specifically, computational results are within experimental error bounds for ${\kappa }_{100}$ and ${\kappa }_{001}$ while underpredicting ${\kappa }_{010}$, suggesting that the bare phonon models used in the literature are missing essential physics related to the anisotropic thermal transport. In response, we compute the anisotropic $\kappa$ using first principles and the Peirels-Boltzmann transport equation under different approximations. For the simplest model, we consider the heat carriers to be harmonic phonons with scattering rates obtained perturbatively. These results are then compared with those obtained by including phonon renormalization and four-phonon scattering. Our results show that accounting for phonon renormalization resolves the discrepancy between experiment and theory. This is because phonon renormalization leads to an anisotropic $\kappa$ enhancement caused by directionally dependent changes in the phonon group velocities accompanied by a general increase in phonon lifetime. Owing to the crucial role of these anharmonic interactions in accurately describing anisotropic thermal transport, we also explore the anharmonicity of individual atoms and show that the octahedrally coordinated gallium atom is the most anharmonic and thus most likely responsible for the failure of the harmonic phonon model to describe thermal transport in this material. Finally, we demonstrate that atomic anharmonicities could be used as a useful metric to guide the tailoring of vibrational properties.

In this paper, results of experimental studies and modeling of standing spin-wave spectra and their temperature evolution in graded Pd-Fe alloy thin magnetic films are presented. Studied sample series include 200-nm thick epitaxial ${\mathrm{Pd}}_{1-x}{\mathrm{Fe}}_x$ films on MgO (001): two with the linear ($0.02\cdots 0.10$ and $0.12\cdots 0.18x$-ranges), one with the sine and one with the cosine (both with $0.02\cdots 0.10x$-range) iron distribution profiles. Versatile spin-wave excitation patterns were obtained, and their pronounced evolution with temperature was observed. Importantly, temperature affects spectra not only in a trivial way via the temperature dependence of magnetization and related quantities, but also through modification of magnetic phase structure of a film due to the Curie temperature crossing by its fraction(s). Continuous at low temperatures magnetization profile becomes discontinuous or significantly altered at elevated ones. Standing spin-wave resonance spectra were modeled using a semiclassical Landau-Lifshitz approach. Based on the modeling, temperature dependences of the spin-wave stiffness $D$ and magnitudes of the interface ${\alpha }_{\mathrm{inter}}$ and surface ${\alpha }_{\mathrm{surf}}$ pinning constants were obtained. It is shown that, besides a compositional grading of magnetic thin films, the temperature variation is a powerful tool for modifying the magnetization profile and thus, for a flexile tuning of spin-wave spectra.

Breakthroughs in near-room-temperature superconductivity within clathrate hydrides underscore the effectiveness of theory-driven approaches to designing high-$T_c$ superconductors. However, superconductivity in these clathrate hydrides has not been realized at low or ambient pressures. To explore phonon-mediated superconductors with $T_c$ exceeding the liquid-nitrogen temperature under ambient pressure, this study investigates the crystal structure, metallization, electron-phonon coupling, and anisotropic superconducting behavior in hole-doped ${\left(\mathrm{BN}\right)}_6$ and ${\left(\mathrm{BN}\right)}_5$ clathrates, with configurations akin to ${\mathrm{CaH}}_6$ and ${\mathrm{LaH}}_{10}$, utilizing first-principles calculations in junction with Wannier interpolation technique. By solving the fully anisotropic Migdal-Eliashberg equations, this work reveals a positive correlation between $T_c$ and hole-doping levels, with the highest $T_c$ approximately 32 K and 96 K in hole-doped ${\left(\mathrm{BN}\right)}_6$ and ${\left(\mathrm{BN}\right)}_5$, respectively, and both with single anisotropic energy gap nature. The high-$T_c$ superconductivity in B-N clathrates arises from two primary factors: the enhanced density of states at the Fermi level and the presence of strong B-N $s{p}^3$-hybridized covalent $\sigma$-bonding bands. These insights provide a new perspective for revealing the superconductivity of B-N clathrates from an anisotropic perspective and offer valuable avenues for inducing high-$T_c$ superconductors at ambient conditions.

Topological superconductors (TSCs) are an ideal platform for realizing Majorana fermions to implement fault-tolerant topological quantum computation. However, the low transition temperature ($T_c$) of TSCs hinders experimental measurements and practical applications. Here, we propose that metal-bonded perovskite ${\mathrm{Ag}}_4\mathrm{H}$ is a TSC, characterized by nontrivial topological surface states, a bulk $s$-wave superconducting gap, and a $T_c$ reaching up to 63 K at ambient pressure. The structural stability, synthesis routes, band topology, and superconductivity are well investigated by first-principles calculations, Wannier interpolation method, effective k $·$ p model, and Migdal-Eliashberg theory. The ambient-pressure phase ${\mathrm{Ag}}_4\mathrm{H}$ can be realized through kinetic processes of cooling and depressurization from its pressure state. Topological nodal lines and Dirac points with nontrivial $Z_2$ index are identified in ${\mathrm{Ag}}_4\mathrm{H}$. Our in-depth analysis reveals an unconventional band inversion mechanism, in which the usually low-lying H-$1s$ band is inverted partially to the Fermi level by intermediate metallization of hydrogen resulting from the metallic bonding of the H sublattice with the Ag matrix. Gap anisotropy is related to Fermi surface nesting and $\mathbf{q}$-dependent electron-phonon coupling. The large H interstitial space, more $s/p$ orbital electrons at the Fermi level, and high H concentration promote high-temperature superconductivity under ambient pressure. Last but not least, ${\mathrm{Ag}}_4\mathrm{H}$ is the first few-hydrogen metal-bonded intrinsic Eliashberg TSC, exhibiting high $T_c$ near the liquid-nitrogen temperature region. This work may pave a different way to realize topological superconductivity at higher temperature under ambient pressure.

As a layered ferroelectric material, $\mathrm{CuIn}{\mathrm{P}}_2{\mathrm{S}}_6$ has garnered significant attention for its robust ferroelectric state and potential applications in memory devices. In this work, we demonstrate that with short laser pulses ultrafast reversible polarization switching within hundreds of femtoseconds can be achieved in ferroelectric $\mathrm{CuIn}{\mathrm{P}}_2{\mathrm{S}}_6$. Specifically, photoexcitation triggers collective ionic migration and ferroelectricity reversal in $\mathrm{CuIn}{\mathrm{P}}_2{\mathrm{S}}_6$, revealing a different pathway to access different ferroelectric phases through optical excitation. Our findings indicate that laser pulses substantially alter the transition barriers, promoting ionic transport facilitated by the photodoping effect. This laser-induced ionic migration proves critical for enabling polarization transitions, offering a pathway to explore and control exotic quantum phases. These insights open exciting possibilities for manipulating ferroelectric states and electronic properties on an ultrafast timescale.

Gallium nitride (GaN) is a crucial material for high power density electronic devices, and its self-heating effects under high power conditions have attracted considerable attention. The study of intrinsic point defects in GaN and their impact on thermal conductivity is a crucial research focus. The influence of these defects on the thermal conductivity is dependent on their type, concentration, and charge state. However, existing experimental and computational methods, such as density functional theory and molecular dynamics, encounter limitations, particularly when dealing with defects of low concentrations, various types, and charged states. To overcome these challenges, we have developed machine-learning potentials for point defects at various concentrations, charge states, and temperatures. Predictive capabilities for the mechanical properties and migration pathways of point defects are demonstrated. We investigate the temperature dependence of thermal conductivity in defect-free structures and analyze how point defect concentration affects thermal conductivity. These studies provide insights for optimizing the performance of GaN materials.