Here we propose ${\mathrm{BC}}_3$, a graphene derivative that has been synthesized, as a platform to realize exotic quantum phases by introducing a moiré pattern with mismatched stacking. In twisted bilayer ${\mathrm{BC}}_3$, it is shown that a crossover from two-dimensional to quasi-one-dimensional (quasi-1D) band structure takes place with the twist angle as a control parameter. This is a typical manifestation of the guiding principle in van der Waals stacked systems: The quantum interference between Bloch wave functions in adjacent layers has a striking effect on the effective interlayer tunneling. Interestingly, quasi one dimensionalization happens in a valley-dependent manner. Namely, there is interlocking between the valley index and the quasi-1D directionality, which makes ${\mathrm{BC}}_3$ a plausible candidate for valleytronics devices. In addition, the strongly correlated regime of the valley interlocked quasi-1D state reduces to an interesting variant of the Kugel-Khomskii model where intertwined valley and spin degrees of freedom potentially induce exotic quantum phases. Notably, this variant of the Kugel-Khomskii model cannot be realized in conventional solids due to the threefold valley degeneracy.

Starting from ab initio calculations, we derive a five-band Hubbard model to describe the ${\mathrm{CuO}}_2$ chains of ${\mathrm{LiCu}}_2{\mathrm{O}}_2$. This model is further simplified to a low-energy effective Heisenberg model with nearest-neighbor $J_1$ and next-nearest-neighbor $J_2$ interactions, combining perturbation theory, exact diagonalization calculations, and density matrix renormalization group results. For realistic parameters we find the corresponding values of these interactions. The obtained effective model is consistent with a spiral-magnetic ground state as experimentally observed. Using symmetry arguments, the spiral state is a sufficient condition for the ferroelectricity observed in the system.

In modern experiments with hybrid superconducting (SC)/semiconducting nanowires the presence of zero-energy Andreev bound states (ABSs), characterized by a partial overlap of the Majorana wave functions, is a common problem that significantly complicates the detection of a genuine Majorana bound state (MBS). In this article, taking into account spatial inhomogeneity of experimentally investigated nanowire samples, we study interference transport features of a curved heterostructure in which two normal wires (or arms) are separated by a superconducting wire. Since Andreev reflection on the two N/S interfaces with smoothly changing electrostatic and SC pairing potentials results in the emergence of bound states, the low-energy interference transport is described in the framework of the model of two noninteracting Andreev levels or the Andreev double quantum dot. A set of limiting cases is analyzed allowing us to highlight the interference properties that are unique for the different types of ABSs, such as bulk ABS, inhomogeneous ABS, and MBS. In particular, considerable attention is paid to the features of the Aharonov-Bohm (AB) effect. It is shown that the response of each state can be recognized analyzing both AB period and extrema positions of the conductance oscillations which take place without any fine-tuning of the system parameters.

Recent experimental evidence and theoretical simulations have shown that electrical polarization can be tuned and even deterministically controlled in ferroelectrics by applying laser pulses having a frequency that is higher than that of the main polarization (soft) mode. Here, by using ab initio computations, we further discover that such pulses can also create nontrivial topological polar phases in bulk ferroelectric materials. Such a phenomenon is found to originate from nonthermal quenching process in which the applied intense ultrashort laser pulse first squeezes the ferroelectric phase into a cubic-like paraelectric phase at a moderate temperature and then to a renucleation process akin to the Kibble-Zurek mechanism creating nontrivial topological polar defects in the bulk ferroelectric ground-state phase. Our work therefore presents a novel and optical mean of creating topological defects in ferroelectrics.

Simulation of photoluminescence spectroscopy from first principles provides a powerful approach for predicting the experimental spectrum and understanding the origin of the luminescence of materials. We show here that the use of the hybrid-exchange correlation functional combined with first-principles molecular dynamics can simulate the defect-induced photoluminescence spectrum of zinc stannate $\left({\mathrm{Zn}}_2{\mathrm{SnO}}_4\right)$ in good agreement with the experiment. The calculations were carried out for 12 different point defects of ${\mathrm{Zn}}_2{\mathrm{SnO}}_4$, and show that the green-to-red photoluminescence emissions obtained in the experiment are mainly contributed by the oxygen vacancy defects. These defect states play the roles of deep donors and radiative recombination centers during the photoluminescence mechanism. In particular, their electronic properties are significantly affected by temperature, which is related to the strong fluctuation of the nearest-neighbor Sn atoms relative to the vacancy center.

A quarter century ago, a semiconductorlike resistivity was observed in the ternary ${\mathrm{Fe}}_2\mathrm{VAl}$ Heusler alloy, sparking great interest in this material. Here, we reinvestigate the origin of this temperature-dependent behavior by simultaneously analyzing experimental resistivity and Seebeck and Hall effect data in the framework of a two-band model with different energy-dependent electron relaxation times. We reveal the pivotal role of an anomalous carrier scattering mechanism off localized antisite defect states in a nominally stoichiometric and seemingly well-ordered sample, which is crucial for comprehending the semiconductorlike transport. Our work demonstrates the benefit of concurrently probing complementary transport properties such as Seebeck coefficient, Hall mobility, and electrical resistivity for understanding electronic transport phenomena in complex materials.

Coherent magnon-photon coupling under the Floquet drive has been demonstrated in recent experimental measurements and theoretical calculations. In this work, we studied the Floquet-driven dissipative cavity magnonics theoretically. Our results show that the Floquet states $d^{\left(n\right)}$ ($n$ is integer) with dissipative coupling give rise to many level attractions in the transmission spectrum. As driving amplitude increases or driving frequency decreases, the coupling for the $n=0$ mode becomes weak while for the sideband $(n=\pm 1)$ becomes strong due to renormalized coupling strength. When the coherent and the dissipative couplings are simultaneously present, a series of sharp dips originating from the interference of two couplings occur, in contrast to a single dip with zero damping in previous work. The modified zero-damping condition is derived for the Floquet system. Moreover, we give a generalized selection rule for coupling between Floquet states by considering the multiphoton process that has not yet been studied in previous work. Our results open up promising roads for exploring the dissipative magnon-photon coupling with the Floquet driving technology.

One of the key problems in valleytronics is to realize valley polarization. Ferrovalley semiconductors and half-valley metals (HVM) have been proposed, which possess intrinsic spontaneous valley polarization. Here, we propose the concept of a quasi-HVM (QHVM), including electron and hole carriers with only a type of carriers being valley polarized. The QHVM may realize the separation function of the electron and hole. A concrete example of the ${\mathrm{VGe}}_2{\mathrm{P}}_4$ monolayer is used to illustrate our proposal through first-principle calculations. To better realize QHVM, the electric field is applied to tune related valley properties of ${\mathrm{VGe}}_2{\mathrm{P}}_4$. Within the considered electric field range, ${\mathrm{VGe}}_2{\mathrm{P}}_4$ adopts a ferromagnetic (FM) ground state, which possesses out-of-plane magnetization, as confirmed by calculating magnetic anisotropy energy including magnetic shape anisotropy and magnetocrystalline anisotropy energies. These out-of-plane FM properties guarantee intrinsic spontaneous valley polarization in ${\mathrm{VGe}}_2{\mathrm{P}}_4$. Within a certain range of electric field, the QHVM can be maintained, and the related polarization properties can be effectively tuned. In this paper, we pave the way toward two-dimensional functional materials design of valleytronics.

The discovery of new mechanisms of controlling magnetic properties by electric fields or currents furthers the fundamental understanding of magnetism and has important implications for practical use. Here, we present an approach of utilizing resistive switching to control magnetic anisotropy. We study a ferromagnetic oxide that exhibits an electrically triggered metal-to-insulator phase transition producing a volatile resistive switching. The switching occurs in a characteristic spatial pattern, the formation of an insulating barrier perpendicular to the current flow, which results in an unusual ferromagnetic/paramagnetic/ferromagnetic configuration. The formation of this voltage-driven paramagnetic insulating barrier is accompanied by the emergence of a strong uniaxial magnetic anisotropy that overpowers the intrinsic material anisotropy. Our results demonstrate that resistive switching is an effective tool for manipulating magnetic properties. Because resistive switching can be induced in a broad range of materials, our findings could enable a new class of voltage-controlled magnetism systems.

Thermodiffusion designates a diffusive motion of particles driven by a temperature gradient. In liquid alloys, this effect can influence the concentration distribution during directional solidification and change the homogeneity of grown crystals. Using x-ray radiography (XRR), thermodiffusion in liquid Al–Ag has been measured, with compositions ranging from ${\mathrm{Al}}_{80}{\mathrm{Ag}}_{20}$ to ${\mathrm{Al}}_{50}{\mathrm{Ag}}_{50}$. The Soret coefficient is determined to be $(0.9\pm 0.3)\times {10}^{-3}{\mathrm{K}}^{-1}$, where the silver atoms diffuse to the cold end of the sample. Thanks to the time-resolved information from in situ XRR, the interdiffusion coefficient could simultaneously be measured and is in good agreement with previous interdiffusion measurements in this concentration range. The measured Soret coefficient is compared with the values predicted by the current theoretical models, which are found to be off by at least a factor of two.

We report the nature of magnetic structure, and microscopic spin-spin correlations and their dependence on the underlying crystal structure of the geometrically frustrated layered spin-$\frac{3}{2}$ maple leaf lattice (MLL) antiferromagnet ${\mathrm{Na}}_2{\mathrm{Mn}}_3{\mathrm{O}}_7$ by a comprehensive neutron diffraction study. Crystal structural studies by x-ray and neutron diffraction reveal that the MLL layers (constituted by ${\mathrm{Mn}}_3{\mathrm{O}}_7{}^{2-}$ units) are well separated by nonmagnetic Na layers. The studies also conclude the presence of stacking faults (in-plane sliding of magnetic MLL layers) as well as a distortion in the MLL of ${\mathrm{Mn}}^{4+}$. Temperature dependent magnetic susceptibility, heat capacity, and neutron diffraction data yield a short-range antiferromagnetic (AFM) ordering below $\sim 100\mathrm{K}$ without a long-range magnetic ordering down to 1.5 K. The analysis of the diffuse magnetic neutron scattering patterns by the reverse Monte Carlo method reveals two-dimensional (2D) spin-spin correlations within the MLL layers. Additionally, we establish a relation between the correlation length of the short-range magnetic ordering with the stacking faults through a varying synthesis condition. The present study, therefore, explores a microscopic picture of the crystal and spin structures, as well as their correlation; hence it provides experimental insight of the magnetic ordering in a MLL AFM. Further, we have outlined the formation of several 2D frustrated lattice geometries having triangular plaquettes, including the MLL, by crystal engineering of the triangular lattice, and their role in the stabilization of multiple unique chiral spin states which opens up the door for study of unique chiral spin states.

A closed-form formalism for modeling high harmonic generation (HHG) in solids is derived; its validity is tested for one-dimensional two-band inversion symmetric model solids; excellent agreement with the exact von Neumann equation is found. From the closed-form expressions, a diagnostic method is developed that allows one to separate resonant from nonresonant processes in nonlinear optics. This opens a deeper view into the dominant laser- and material-dependent mechanisms of high harmonic generation in solids. Midinfrared-driven HHG in semiconductors is dominated by the resonant interband current. As a result of the dynamic Stark shift, virtual processes gain in importance in near-infrared-driven HHG in dielectrics. Finally, comparison to experiments indicates the potential importance of little-explored processes, such as dephasing of the strong-field dynamics from coupling to the many-body environment of solids.

The studies for the discovery of new nontrivial degrees of freedom of Bloch electrons are at the forefront of condensed matter physics. The focus remains on the spin-orbit coupling (SOC)-induced pseudospin valley state and its interplay with time inversion asymmetry. Here we propose a distinct magnetic order-protected, momentum-dependent spin-splitting valley (SSV) state in polar bulk ${\mathrm{BiFeO}}_3$. The octahedral distortion-induced spatial asymmetry of the superexchange interaction path breaks spin degeneracy and antiferromagnetic time-reversal symmetry. The SSV polarization is caused by crystal symmetry-related intrinsic SOC and leads to nonzero anomalous Hall conductivity (AHC) and magneto-optical effects. The anomalous Hall conductivity and magneto-optical effects are expected to be most prominent if magnetic and electric dipoles are orthogonal. Due to the ferroelectric nature of ${\mathrm{BiFeO}}_3$, such unconventional magneto-optical effect and AHC in fully compensated magnetic order offers straightforward tunability under an electric field.

Entangled photon pairs are essential for many applications in quantum technologies. Recent theoretical studies demonstrated that different types of entangled Bell states can be created in a constantly driven four-level quantum emitter-cavity system. Unlike other candidates for the realization of the four-level emitter, semiconductor quantum dots unavoidably interact with their environment, resulting in carrier-phonon interactions. Surprisingly, phonons change the entanglement of emitted photon pairs in a qualitative way, already at low temperatures on the order of 4 K. While one type of Bell state can still be generated using small driving strengths, the other type is suppressed due to phonon interactions in strongly confined quantum dots. The degree of entanglement decreases with rising temperature and driving strength until it vanishes at a certain parameter value. Because it remains zero afterward, we encounter a phonon-induced transition between entangled and nonentangled photon emission that resembles a phase transition. The transition occurs at temperatures below 30 K and, independent of the driving strength, the concurrence as a function of the reduced temperature is found to obey a power law with exponent one near the transition point.

The one- and three-band Hubbard models which describe the electronic structure of cuprates indicate very different values of effective electronic parameters, such as the on-site Coulomb energy and the hybridization strength. In contrast, a comparison of electronic parameters of several cuprates with corresponding values from spectroscopy and scattering experiments indicates similar values in the three-band and cluster model calculations used to simulate experimental results. The Heisenberg exchange coupling $J$ obtained by a downfolding method in terms of the three-band parameters is used to carry out an optimization analysis consistent with $J$ from neutron scattering experiments for a series of cuprates. In addition, the effective one-band parameters $\tilde{U}$ and $\tilde{t}$ are described using the three-band parameters, thus revealing the hidden equivalence of the one- and three-band models. The ground-state singlet weights obtained from an exact diagonalization elucidates the role of Zhang-Rice singlets in the equivalence. The results provide a consistent method to connect electronic parameters obtained from spectroscopy and the three-band model with values of $J$ obtained from scattering experiments, band dispersion measurements, and the effective one-band Hubbard model.

Strongly correlated electron systems host a variety of poorly understood correlations in their high-temperature normal state. Unlike ordered phases defined by order parameters, regions of the normal state are often defined through unconventional properties such as strange metallic transport or spectroscopic pseudogaps. Characterizing the microscopic correlations in the normal state is necessary to elucidate mechanisms that lead to these properties and their connection to ground-state orders. Here we establish the presence of intertwined charge and spin stripes in the strange metal normal state of the Hubbard model using determinant quantum Monte Carlo calculations. The charge and spin density waves constituting the stripes are fluctuating and short ranged; yet they obey a mutual commensurability relation and remain microscopically interlocked, as evidenced through measurements of three-point spin-spin-hole correlation functions. Our findings demonstrate the ability of many-body numerical simulations to unravel the microscopic correlations that define quantum states of matter.

Diffusion currents are theoretically examined in two-dimensional Dirac materials, such as those of the transition metal dichalcogenides (TMD) family. The transversal effects are analogues of the valley Hall (VHE) and photogalvanic (PGE) transport phenomena in cases when the electron driving force is not an electric field but a gradient of electron density distribution in the sample. The latter can be created by a finite-sized laser spot or by the injection of electrons from other materials. We develop the theory of the diffusive VHE effect assuming the anisotropic electron-short-range-impurity skew scattering. The electron PGE-like transport caused by higher electron-density derivatives is analyzed assuming the trigonal warping anisotropy of electron valleys in a TMD monolayer. The nonlinear responses on an electron-density gradient are studied as well. The isotropic processes of electron scattering off the short-range and Coulomb centers are taken into account in the PGE-like transport theory.

Two-dimensional magnetic materials with tunable electronic and magnetic properties are promising for designing spintronics devices. Here we study, using first-principles calculations, the electronic, magnetic, and topological properties of monolayer ${\mathrm{NiAl}}_2{\mathrm{S}}_4$. Monolayer ${\mathrm{NiAl}}_2{\mathrm{S}}_4$ is in the noncollinear ${120}^0$-antiferromagnetic $\left({120}^0\text{−}\mathrm{AFM}\right)$ state and is an indirect band gap semiconductor. Although the ${120}^0$-AFM state in monolayer ${\mathrm{NiAl}}_2{\mathrm{S}}_4$ is robust against strain, its magnetic structure can be tuned effectively by carrier doping. Remarkably, monolayer ${\mathrm{NiAl}}_2{\mathrm{S}}_4$ can be tuned into a high Curie temperature $\left(T_C\right)$ half-metallic state by hole doping. The $T_C$ of ferromagnetic monolayer ${\mathrm{NiAl}}_2{\mathrm{S}}_4$ increases with hole-doping concentration and can be increased up to room temperature. For the hole-doped monolayer ${\mathrm{NiAl}}_2{\mathrm{S}}_4$ with the half-metallic state, before the spin-orbit coupling (SOC) is included, the two spin-up bands around the Fermi level are simultaneously degenerate at the $\mathrm{\Gamma }$ point (dominated by S $p_x/p_y$ states) and $\mathrm{K}/{\mathrm{K}}^{\prime }$ points (dominated by S $p_z$ state) with quadratic non-Dirac and linear Dirac band dispersions, respectively. When the SOC is included, topologically nontrivial gaps with Chern number $C=1$ will be opened, respectively, around the non-Dirac $\mathrm{\Gamma }$ point and Dirac $\mathrm{K}/{\mathrm{K}}^{\prime }$ points. More interestingly, the quantum anomalous Hall (QAH) effect with high Chern number $C=2$ can be achieved in the tensile-strained monolayer ${\mathrm{NiAl}}_2{\mathrm{S}}_4$ with 1.0 hole-doped per unit cell. The obtained high Chern number of $C=2$ results from the constructive coupling effect between the topological nontrivial non-Dirac and Dirac states, which is analyzed through a schematic depiction. Our results show that monolayer ${\mathrm{NiAl}}_2{\mathrm{S}}_4$ is a promising candidate for the exploration of high $T_C$ spintronics devices and high Chern number QAH effect.

We employ a time-dependent real-space local density-of-states method to study the movement and fusion of Majorana zero modes in the one-dimensional interacting Kitaev model, based on the time evolution of many-body states. We analyze the dynamics and both fusion channels of Majoranas using time-dependent potentials, either creating walls or wells. For fast moving Majoranas, we unveil nonequilibrium signatures of the “strong-zero-mode” operator (quasiparity degeneracy in the full spectrum) and its breakdown in the presence of repulsive Coulomb interactions. Focusing on forming a full electron after fusion, we also discuss the upper and lower limits on the Majorana speed needed to reduce nonadiabatic effects and to avoid poisoning due to decoherence.

Describing nonequilibrium properties of quantum many-body systems is challenging due to high entanglement in the wave function. We describe the evolution of local observables via the influence matrix (IM), which encodes the effects of a many-body system as an environment for local subsystems. Recent works found that in many dynamical regimes the IM of an infinite system has low temporal entanglement and can be efficiently represented as a matrix-product state (MPS). Yet, direct iterative constructions of the IM encounter highly entangled intermediate states—a temporal entanglement barrier (TEB). We argue that TEB is ubiquitous, and elucidate its physical origin via a semiclassical quasiparticle picture that exactly captures the behavior of integrable spin chains. Further, we show that a TEB also arises in chaotic spin chains, which lack well-defined quasiparticles. Based on these insights, we formulate an alternative light-cone growth algorithm, which provably avoids TEB, thus providing an efficient construction of the thermodynamic-limit IM as a MPS. This work uncovers the origin of the efficiency of the IM approach for studying thermalization and transport.