Taking the noncollinear antiferromagnetic hexagonal Heusler compound ${\mathrm{Mn}}_3\mathrm{Ge}$ as a reference system, the contributions to linear response phenomena arising solely from the chiral coplanar and noncoplanar spin configurations are investigated. Orbital moments, x-ray absorption, and anomalous and spin Hall effects, as well as corresponding spin-orbit torques and Edelstein polarizations, are studied depending on a continuous variation of the polar angle relative to the kagome planes of corner-sharing triangles between the noncollinear antiferromagnetic and the ferromagnetic limits. By scaling the speed of light from the relativistic Dirac case to the nonrelativistic limit the chirality-induced or topological contributions can be identified by suppressing the spin-orbit coupling.

It is well known that the critical temperature of multigap superconducting three-dimensional (3D) heterostructures at atomic limit (HAL) made of a superlattice of atomic layers with an electron spectrum made of several quantum subbands can be amplified by a shape resonance driven by the contact exchange interaction between different gaps. The $T_C$ amplification is achieved tuning the Fermi level near the singular nodal point at a Lifshitz transition for opening a neck. Recently high interest has been addressed to the breaking of inversion symmetry which leads to a linear-in-momentum spin-orbit induced spin splitting, universally referred to as Rashba spin-orbit coupling (RSOC) also in 3D layered metals. However the physics of multigap superconductivity near unconventional Lifshitz transitions in 3D HAL with RSOC, being in a non-BCS regime, is not known. The key result of this work getting the superconducting gaps by Bogoliubov theory and the 3D electron wave functions by solution of the Dirac equation is the feasibility of tuning multigap superconductivity by suitably matching the spin-orbit length with the 3D superlattice period. It is found that the presence of the RSOC amplifies both the $k$ dependent anisotropic gap function and the critical temperature when the Fermi energy is tuned near the circular nodal line. Our results suggest a method to effectively vary the effect of RSOC on macroscopic superconductor condensates via the tuning of the superlattice modulation parameter in a way potentially relevant for spintronics functionalities in several existing experimental platforms and tunable materials needed for quantum devices for quantum computing.

Dynamical magnets can pump spin currents into superconductors. To understand such a phenomenon, we develop a method utilizing the generalized Usadel equation to describe time-dependent situations in superconductors in contact with dynamical ferromagnets. Our proof-of-concept theory is valid when there is sufficient dephasing at finite temperatures, and when the ferromagnetic insulators are weakly polarized. We derive the effective equation of motion for the Keldysh Green's function focusing on a thin film superconductor sandwiched between two noncollinear ferromagnetic insulators, one of which is dynamical. In turn, we compute the spin currents in the system as a function of the temperature and the magnetizations' relative orientations. When the induced Zeeman splitting is weak, we find that the spin accumulation in the superconducting state is smaller than in the normal states due to the lack of quasiparticle states inside the gap. This feature gives a lower backflow spin current from the superconductor as compared to a normal metal. Furthermore, in superconductors, we find that the ratio between the backflow spin current in the parallel and antiparallel magnetization configuration depends strongly on temperature, in contrast to the constant ratio in normal metals.

The recently deduced normal and anomalous self-energies from photoemission spectra of cuprate superconductors via the machine learning technique are calling for an explanation. Here the normal and anomalous self-energies in cuprate superconductors are analyzed within the framework of the kinetic-energy-driven superconductivity. It is shown that the exchanged spin excitations give rise to the well-pronounced low-energy peak-structures in both the normal and anomalous self-energies, however, they do not cancel in the total self-energy. In particular, the peak-structure in the normal self-energy is mainly responsible for the peak-dip-hump structure in the single-particle excitation spectrum, and can persist into the normal-state, while the sharp peak in the anomalous self-energy gives rise to a crucial contribution to the superconducting gap, and vanishes in the normal state. Moreover, the evolution of the peak structure with doping and momentum are also analyzed.

We introduce characterizations for many-body phase transitions between delocalized and localized phases based on the system's sensitivity to boundary conditions. In particular, we change boundary conditions from periodic to antiperiodic and calculate a shift in the system's energy and shifts in the single-particle density-matrix eigenvalues in the corresponding energy window. We employ the typical model for studying MBL, a one-dimensional disordered system of fermions with nearest-neighbor repulsive interaction where disorder is introduced as randomness on on-site energies. By calculating numerically the shifts in the system's energy and eigenvalues of the single-particle density matrix, we observe that in the localized regime, both shifts are vanishing; while in the extended regime, both shifts are on the order of the corresponding level spacing. We also applied these characterizations of the phase transition to the case of having next-nearest-neighbor interactions in addition to the nearest-neighbor interactions and studied its effect on the transition.

The effect of electronic correlations on the orbital magnetization in real materials has not been explored beyond a static mean-field level. Based on the dynamical mean-field theory, the effect of electronic correlations on the orbital magnetization in layered ferromagnet ${\mathrm{VI}}_3$ has been studied. A comparison drawn with the results obtained from density-functional theory calculations robustly establishes the crucial role of dynamical correlations in this case. In contrast to the density-functional theory that leads to negligible orbital magnetization in ${\mathrm{VI}}_3$, in dynamical mean-field approach the orbital magnetization is greatly enhanced. Further analysis shows that this enhancement is mainly due to the enhanced local circulations of electrons, which can be attributed to a better description of the localization behavior of correlated electrons in ${\mathrm{VI}}_3$. The conclusion drawn in our paper could be applicable to a wide range of layered materials in this class.

Neural networks have been used as variational wave functions for quantum many-particle problems. It has been shown that the correct sign structure is crucial to obtain highly-accurate ground state energies. In this paper, we propose a hybrid wave function combining the convolutional neural network (CNN) and projected entangled pair states (PEPS), in which the sign structures are determined by the PEPS, and the amplitudes of the wave functions are provided by CNN. We benchmark the ansatz on the highly frustrated spin-1/2 $J_1\text{−}{J}_2$ model. We show that the achieved ground energies are competitive with state-of-the-art results.

We measured the optical signature of charge density waves (CDWs) in the multiband conductor $\mathrm{TTF}{\left[\mathrm{Ni}{\left(\mathrm{dmit}\right)}_2\right]}_2$ by electronic Raman scattering. At low energies, a hump develops below 60 K. This hump is associated to the amplitude mode of the CDW with an energy around 9 meV. Raman symmetry-resolved measurements show that the CDW amplitude mode is anisotropic and that the CDW can be associated to the band nesting of $\mathrm{Ni}{\left(\mathrm{dmit}\right)}_2$ chains.

Nitride-based van der Waals magnetic heterostructures can simultaneously utilize both charge and spin degrees of electrons, which are important materials for low-dimensional spintronic devices. In this work, we study the stabilities and the electronic and magnetic properties of GaN/VN and VN/GaN/VN van der Waals heterostructures via first-principles calculations. The GaN/VN van der Waals heterostructure is a half-metal with 100% spin polarization, which can be applied in a spin filter and spin injection. Although monolayer GaN is a nonmagnetic semiconductor, it can introduce half-metallic characters via the magnetic proximity effect in both GaN/VN and VN/GaN/VN van der Waals heterostructures. The easy magnetization axis of hexagonal VN transits from the out-of-plane to the in-plane direction after combining with hexagonal GaN due to the different orbital occupations. Moreover, the electronic properties of the VN/GaN/VN van der Waals heterostructure depend largely on the magnetic configurations of the VN layers. By applying parallel or antiparallel magnetic configurations, the VN/GaN/VN van der Waals heterostructure presents half-metallic and semiconducting characters. The transformation indicates that the VN/GaN/VN van der Waals heterostructure is a promising candidate for room-temperature van der Waals spin valve devices.

Optical absorption and emission spectra of doped two-dimensional (2D) materials exhibit sharp peaks that are often mistakenly identified with pure excitons and pure trions (or charged excitons), but both peaks have been recently attributed to superpositions of two-body exciton and four-body trion states and correspond to the approximate energy eigenstates in doped 2D materials. In this paper, we present the radiative lifetimes of these exciton-trion superposition energy eigenstates using a many-body formalism that is appropriate given the many-body nature of the strongly coupled exciton and trion states in doped 2D materials. Whereas the exciton component of these superposition eigenstates are optically coupled to the material ground state and can emit a photon and decay into the material ground state provided the momentum of the eigenstate is within the light cone, the trion component is optically coupled only to the excited states of the material and can emit a photon even when the momentum of the eigenstate is outside the light cone. In an electron-doped 2D material, when a four-body trion state with momentum outside the light cone recombines radiatively, and a photon is emitted with a momentum inside the light cone, the excess momentum is taken by an electron-hole pair left behind in the conduction band. The radiative lifetimes of the exciton-trion superposition states with momenta inside the light cone are found to be in the few hundred femtoseconds to a few picoseconds range and are strong functions of the doping density. The radiative lifetimes of exciton-trion superposition states with momenta outside the light cone are in the few hundred picoseconds to a few nanoseconds range and are again strongly dependent on the doping density. The doping density dependence of the radiative lifetimes of the two peaks in the optical emission spectra follows the doping density dependence of the spectral weights of the same two peaks observed in the optical absorption spectra, as both have their origins in the Coulomb coupling between the excitons and trions in doped 2D materials.

Discovering the physical requirements for meeting the indefinite permittivity in natural material, as well as proposing a new natural hyperbolic media, offer a possible route to significantly improving our knowledge and ability to confine and control light in optoelectronic devices. We demonstrate the hyperbolicity in a class of materials with hexagonal $P6/mmm$ and $P{6}_3/mmc$ layered crystal structures, and its physical origin is thoroughly investigated. By utilizing density functional theory and solving the Bethe-Salpeter equation, we find that the layered crystal structure and symmetry-imposed constraints in ${\mathrm{Li}}_3\mathrm{N}$ give rise to an exceedingly strong anisotropy in optical responses along in- and out-of-plane directions of the crystals, making it a natural hyperbolic in a broad spectral range from the visible spectrum to the ultraviolet. More excitingly, the hyperbolicity relation to anisotropic interband absorption, in addition to the impressive dependency of the conduction band on the lattice constant along the out-of-plane direction, provide the hyperbolicity tunability in these hexagonal structures under strain, doping, and alloying. Our findings not only suggest a large family of real hexagonal compounds as a unique class of materials for realization of the highly tunable broadband hyperbolicity, but they also offer an approach to search for new hyperbolic materials.

Atomic collapse in graphene nanoribbons behaves in a fundamentally different way as compared to monolayer graphene due to the presence of multiple energy bands and the effect of edges. For armchair nanoribbons we find that bound states gradually transform into atomic collapse states with increasing impurity charge. This is very different in zigzag nanoribbons where multiple quasi-one-dimensional bound states are found that originates from the zero-energy zigzag edge states. They are a consequence of the flat band and the electron distribution of these bound states exhibits two peaks. The lowest-energy edge state transforms from a bound state into an atomic collapse resonance and shows a distinct relocalization from the edge to the impurity position with increasing impurity charge.

We report an analytical representation of the correlation energy $e_{\mathrm{c}}\left(r_s,\zeta \right)$ for a uniform electron gas (UEG), where $r_s$ is the Seitz radius or density parameter and ζ is the relative spin polarization. The functional, called W20, is constructed to capture the known high-density ($r_s\to 0$) and low-density ($r_s\to \infty$) limits (for $\zeta =0$ and 1) without any fitting parameters. The comparative assessment against the recent quantum Monte Carlo (QMC) results shows that the performance of the W20 functional is comparable to the popular parametrized UEG correlation functionals. On average, W20 agrees with QMC and PW92 [Perdew and Wang, Phys. Rev. B 45, 13244 (1992)] within 0.01 eV for $0.5\le r_s\le 20$, and W20 recovers the correct high- and low-density limits, whereas the QMC-data fitted UEG correlation functionals do not recover the low-density limit. W20 shows better accuracy than the popular QMC-data fitted UEG correlation functionals for the low-density region.

A tensor network renormalization algorithm with global optimization based on the corner transfer matrix is proposed. Since the environment is updated by the corner transfer matrix renormalization group method, the forward-backward iteration is unnecessary, which is a time-consuming part of other methods with global optimization. In addition, a further approximation reducing the order of the computational cost of contraction for the calculation of the coarse-grained tensor is proposed. The computational time of our algorithm in two dimensions scales as the sixth power of the bond dimension, while the higher-order tensor renormalization group and the higher-order second renormalization group methods have the seventh power. We perform benchmark calculations in the Ising model on the square lattice and show that the time-to-solution of the proposed algorithm is faster than that of other methods.

We consider the dynamical properties of a gapped quantum spin system coupled to the electric field of a laser, which drives the resonant excitation of specific phonon modes that modulate the magnetic interactions. We deduce the quantum master equations governing the time-evolution of both the lattice and spin sectors, by developing a Lindblad formalism with bath operators providing an explicit description of their respective phonon-mediated damping terms. We investigate the nonequilibrium steady states (NESS) of the spin system established by a continuous driving, delineating parameter regimes in driving frequency, damping, and spin-phonon coupling for the establishment of physically meaningful NESS and their related nontrivial properties. Focusing on the regime of generic weak spin-phonon coupling, we characterize the NESS by their frequency and wave-vector content, explore their transient and relaxation behavior, and discuss the energy flow, the system temperature, and the critical role of the type of bath adopted. Our study lays a foundation for the quantitative modeling of experiments currently being designed to control coherent many-body spin states in quantum magnetic materials.

Photodoped states are widely observed in laser-excited Mott insulators, in which charge excitations are quickly created and can exist beyond the duration of the external driving. Despite the fruitful experimental explorations, theoretical studies on the microscopic models face the challenge to simultaneously deal with exponentially separated time scales, especially in multiband systems, where the longtime behaviors are often well beyond the reach of state-of-the-art numerical tools. Here, we address this difficulty by introducing a steady-state description of photodoped Mott insulators using an open-system setup, where the photodoped system is stabilized as a nonequilibrium steady state by a weak external driving. Taking advantage of the stationarity, we implement and discuss the details of an efficient numerical tool using the steady-state dynamical mean-field theory combined with the noncrossing approximation. We demonstrate that these stationary photodoped states exhibit the same properties of their transient counterparts, while being solvable with reasonable computational efforts. Furthermore, they can be parametrized by just a few physical quantities, including the effective temperature and the density of charge excitations, which confirms the universal nature of photodoped states indeed independent of the excitation protocols. As a first application, we consider the stationary photodoped states in a two-band Hubbard model with intertwined spin-and-orbital ordering and find a family of hidden phases unknown from the previous studies, implying an apparently unexplored time regime of the relaxation of the intertwined orders.

Semiconductor spin lasers are distinguished from their conventional counterparts by the presence of spin-polarized carriers. The transfer of angular momentum of the spin-polarized carriers to photons provides important opportunities for the operation of lasers. With the injection of spin-polarized carriers, which lead to the circularly polarized light, the polarization of the emitted light can be changed an order of magnitude faster than its intensity. This ultrafast operation of spin lasers relies on a large birefringence, usually viewed as detrimental in spin and conventional lasers. We introduce a transparent description of spin lasers using intensity equations which elucidate the influence of birefringence on the intensity and polarization modulation of lasers. Unlike commonly describing the role of birefringence on laser dynamics by employing complex quantities, our approach is simpler, because it is relying on real quantities and allowing analytical solutions. While intensity modulation is independent of birefringence, for polarization modulation an increase in birefringence directly increases the resonant frequency. Our results for dynamical operation of lasers provide a guide for their spin-dependent response and spintronic applications beyond magnetoresistance.

We study the transport of noninteracting fermions through a periodically driven quantum point contact (QPC) connecting two tight-binding chains. Initially, each chain is prepared in its own equilibrium state, generally with a bias in chemical potentials and temperatures. We examine the heating rate (or, alternatively, energy increase per cycle) in the nonequilibrium time-periodic steady state established after initial transient dynamics. We find that the heating rate vanishes identically when the driving frequency exceeds the bandwidth of the chain. We first establish this fact for a particular type of QPCs where the heating rate can be calculated analytically. Then we verify numerically that this nonequilibrium phase transition is present for a generic QPC. Finally, we derive this effect perturbatively in leading order for cases when the QPC Hamiltonian can be considered a small perturbation. Strikingly, we discover that for certain QPCs the current averaged over the driving cycle also vanishes above the critical frequency, despite a persistent bias. This shows that a driven QPC can act as a frequency-controlled quantum switch.

Thermal rectification based on a new strategy of nanostructuration has been assessed with nonequilibrium molecular dynamics and wave-packet propagation simulations. The use of asymmetric crystalline-core/amorphous-conical-shell nanowires creates a direction-dependent thermal conductivity due to variable axial and radial phonon propagation/confinement. The origin of this effect is related to the combination of a crystalline/amorphous interface parallel to the heat flux, together with the variable amount of amorphous coating due to the conical shell of the nanowire. Physical insights of the rectification are given by the mean free path of phonons, the axial and radial energy transfer, the energy diffusivity, and the vibrational density of states restricted to different constitutive elements of the nanowire.

The optical and optoelectronic properties of two-dimensional (2D) semiconductors are dominated by excitons, which usually appear as well-defined peaks in the spectral domain. Thus, detailed behaviors of excitons can be understood by tracking transient changes of the fundamental spectral observables, i.e., the resonance energy and the spectral linewidth. Rhenium disulfide ($\mathrm{Re}{\mathrm{S}}_2$) is a 2D semiconductor that has recently attracted attention due to its excellent exciton properties, such as light-polarization selectivity and anisotropic coherent effects. However, an understanding of exciton dynamics and spectral behavior of excitons in $\mathrm{Re}{\mathrm{S}}_2$ is lacking. Here, we used time- and spectrally resolved pump-probe spectroscopy to investigate ultrafast exciton dynamics in bilayer $\mathrm{Re}{\mathrm{S}}_2$. Upon photoexcitation, the exciton resonance undergoes linewidth broadening and redshift, but they exhibit different dynamics, and an opposite pump fluence dependence on the approximately hundreds of picoseconds timescale. This is because the spectral broadening and the red-shift both have different origins (the exciton-carrier scattering and exciton-exciton attractive interaction, respectively) and different decay mechanisms (the trapping of carriers and exciton-exciton annihilation, respectively). On a longer timescale of ∼100 ps, both the spectral broadening and the redshift are well explained by the indirect recombination of carriers and lattice heating. This work provides in-depth insight into exciton dynamics in 2D rhenium dichalcogenides.