We theoretically study the energy and optical absorption spectra of alternating-twist multilayer graphene (ATMG) under a perpendicular electric field. We obtain analytically the low-energy effective Hamiltonian of ATMG up to pentalayer in the presence of the interlayer bias by means of first-order degenerate-state perturbation theory, and present general rules for constructing the effective Hamiltonian for an arbitrary number of layers. Our analytical results agree to an excellent degree of accuracy with the numerical calculations for twist angles $\theta \gtrsim 2.2^{\circ }$ that are larger than the typical range of magic angles. We also calculate the optical conductivity of ATMG and determine its characteristic optical spectrum, which is tunable by the interlayer bias. When the interlayer potential difference is applied between consecutive layers of ATMG, the Dirac cones at the two moiré Brillouin zone corners $\overline{K}$ and ${\overline{K}}^{\prime }$ acquire different Fermi velocities, generally smaller than that of monolayer graphene, and the cones split proportionally in energy resulting in a steplike feature in the optical conductivity.

In this paper, we study the Berry curvature induced linear and nonlinear magnon transport in bilayer van der Waals antiferromagnets, where we deduce forms for the spin and energy currents within the semiclassical Boltzmann formalism under the relaxation time approximation. Even in the absence of the Dzyaloshinskii-Moriya interaction, if we turn on the layer dependent electrostatic doping (ED) potential and anisotropy in the Heisenberg interactions, the linear response remains zero, whereas we obtain a nonzero nonlinear thermal Hall response resulting from higher moments of the Berry curvature. We show that there is a sign reversal of nonlinear thermal Hall conductivity with varying strength of ED potential, which can be potentially useful in spin based technologies. We also comment on the momentum and temperature dependence of the relaxation time, which can influence the transport properties.

The relevance of the single-band two-dimensional Hubbard model to superconductivity in the doped cuprates has recently been questioned, based on density matrix renormalization group (DMRG) computations that found superconductivity over an unrealistically broad doping region upon electron-doping, yet a complete absence of superconductivity for hole-doping. We report very similar results from DMRG calculations on a ${\mathrm{Cu}}_2{\mathrm{O}}_3$ two-leg ladder within the parent three-band correlated-electron Hamiltonian. The strong asymmetry found in our calculations are in contradiction to the deep and profound symmetry in the experimental phase diagrams of electron- and hole-doped cuprate superconductors, as seen from the occurrence of quantum critical points within the superconducting domes in both cases that are characterized by Fermi surface reconstruction, large jumps in carrier density, and strange metal behavior.

We investigate nonlinear transport signatures stemming from linear and cubic spin-orbit interactions in one- and two-dimensional systems. The analytical zero-temperature response to external fields is complemented by a finite-temperature numerical analysis, establishing a way to distinguish between linear and cubic spin-orbit interactions. We also propose a protocol to determine the relevant material parameters from transport measurements attainable in realistic conditions, illustrated by values for Ge heterostructures. Our results establish a method for the fast benchmarking of spin-orbit properties in semiconductor nanostructures.

Color centers in diamonds have emerged as a promising candidate for quantum information and quantum computing applications. Compared to the well-known and widely studied nitrogen-vacancy ${\mathrm{N}}_{\mathrm{C}}\text{−}{\mathrm{V}}_{\mathrm{C}}$ (NV) color center with $C_{3v}$ symmetry, the group-IV vacancy color centers ${\mathrm{V}}_{\mathrm{C}}\text{−}X\text{−}{\mathrm{V}}_{\mathrm{C}}$ (XV, $X=\mathrm{Si}$, Ge, Sn, Pb), exhibit structures with the $D_{3d}$ symmetry, which give rise to more stable coherent optical transitions for the zero-phonon line (ZPL) due to its inversion symmetry. Moreover, it is experimentally found that the ZPL peak of XV centers increases from Si to Sn to Ge to Pb, i.e., it does not vary monotonically with the atomic numbers. So far, the physical origin of the unusual local structures and the abnormal trend of ZPL of the XV centers are not well understood. In this paper, based on density-functional theory calculations and symmetry analysis, we demonstrate that the large size of the X atoms plays a dominant role in moving the X atoms away from the substitution site to the bond-center site between the two carbon vacancies to form the $D_{3d}$ structure that can effectively reduce the local strain energy. Meanwhile, we find that the abnormal trends of ZPL of the XV centers derive from a competition of the $p-p$ coupling and $p-d$ coupling between X atoms and the divacancy based on the band-coupling mechanism. Our study, therefore, provides insights into the origin of the abnormal trends of ZPL and the local structure of XV centers in diamonds.

An empirical force-field of Buckingham type is derived for crystalline and glassy ${\mathrm{Li}}_2\mathrm{S}\text{−}{\mathrm{SiS}}_2$ binary alloys using fractional charges, short-range repulsion, and long-range attractive dispersion interactions. Parameters are fitted on low-temperature equilibrium polymorphs ${\mathrm{Li}}_2\mathrm{Si}{\text{S}}_3$, and permit to recover typical bond distances, cell lengths, and to predict elastic properties. We then use classical molecular dynamics to study the structural properties of corresponding glasses and a very good agreement is found with corresponding data obtained from neutron and x-ray scattering. The resulting glasses are composed of fourfold tetrahedral Si and twofold sulfur, whereas Li atoms experience a more complex environment, partially in defective octahedral or tetrahedral geometry with a coordination number close to three. Unlike the base ${\mathrm{SiS}}_2$, the resulting glass network does not contain edge-sharing tetrahedra and displays a rather broad distribution of tetrahedral species that are quantified using the usual $Q^n$ formalism.

The recent discovery of long-range magnetic order in atomically thin films has triggered particular interest in two-dimensional (2D) van der Waals (vdW) magnetic materials. In this paper, we perform a systematic theoretical study of the magneto-optical Schäfer-Hubert effect (MOSHE) in 2D vdW antiferromagnetic $M{\mathrm{PS}}_3$ ($M=\mathrm{Mn}$, Fe, Ni) with multifold intralayer and interlayer magnetic orders. The formula for evaluating the MOSHE in 2D magnets is derived by considering the influence of a nonmagnetic substrate. The MOSHE of monolayer and bilayer $M{\mathrm{PS}}_3$ is very large ($>2^{\circ }$), originating from the strong anisotropy of in-plane optical conductivity. The Schäfer-Hubert rotation angles are surprisingly insensitive to the orientations of the Néel vector, while the Schäfer-Hubert ellipticities are identified to be a good criterion to distinguish different interlayer magnetic orders. Our work establishes a theoretical framework for exploring novel 2D vdW magnets and facilitates the promising applications of the 2D $M{\mathrm{PS}}_3$ family in antiferromagnetic nanophotonic devices.

Understanding the rich physics of magnetization dynamics in perpendicular synthetic antiferromagnets (p-SAFs) is crucial for developing next-generation spintronic devices. In this work, we systematically investigate the magnetization dynamics in p-SAFs combining time-resolved magneto-optical Kerr effect (TR-MOKE) measurements with theoretical modeling. These model analyses, based on a Landau-Lifshitz-Gilbert approach incorporating exchange coupling, provide details about the magnetization dynamic characteristics including the amplitudes, directions, and phases of the precession of p-SAFs under varying magnetic fields. These model-predicted characteristics are in excellent quantitative agreement with TR-MOKE measurements on an asymmetric p-SAF. We further reveal the damping mechanisms of two precession modes coexisting in the p-SAF and successfully identify individual contributions from different sources, including Gilbert damping of each ferromagnetic layer, spin pumping, and inhomogeneous broadening. Such a comprehensive understanding of magnetization dynamics in p-SAFs, obtained by integrating high-fidelity TR-MOKE measurements and theoretical modeling, can guide the design of p-SAF-based architectures for spintronic applications.

We use resonant inelastic x-ray scattering (RIXS) at the O $K$- and Cu $K$-edges to investigate the doping dependence and temperature dependence of low-energy plasmon excitations in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$. We observe a monotonic increase of the energy scale of the plasmons with increasing doping $x$ in the underdoped regime, whereas a saturation occurs above optimal doping $x\gtrsim 0.16$ and persists at least up to $x=0.4$. Furthermore, we find that the plasmon excitations show only a marginal temperature dependence, and possible effects due to the superconducting transition and the onset of strange metal behavior are either absent or below the detection limit of our experiment. Taking into account the strongly correlated character of the cuprates, we show that layered $t-J-V$ model calculations accurately capture the increase of the plasmon energy in the underdoped regime. However, the computed plasmon energy continues to increase even for doping levels above $x\gtrsim 0.16$, which is distinct from the experimentally observed saturation and reaches a broad maximum around $x=0.55$. We discuss whether possible lattice disorder in overdoped samples, a renormalization of the electronic correlation strength at high dopings, or an increasing relevance of nonplanar Cu and O orbitals could be responsible for the discrepancy between experiment and theory for doping levels above $x=0.16$.

We have systematically investigated the electrical and structural properties of DySb, a potential magnetic topological semimetal, under applied pressure up to 26.8 GPa. At ambient conditions, DySb shows an antiferromagnetic phase transition at $T_N$ = 10.5 K. With increasing pressure, $T_N$ shows a nonmonotonic dependence of pressure and arrives the maximum at about 13 GPa. Just above that pressure, we see strong indications of superconductivity (SC) with an onset superconducting transition temperature $T_c^{\mathrm{onset}}$ of 2.2 K, concomitant with a structural phase transition from NaCl to CsCl structure. Extreme magnetoresistance as large as $1.4\times {10}^5\%$ is observed at $T=2$ K and $H=$ 9 T at ambient conditions with no sign of saturation. With increasing pressure, magnetoresistance (MR) is greatly suppressed at $P=13.5$ GPa, around which superconductivity emerges. Thus, DySb provides a good platform to study the relationship among MR, SC, and the structural phase transition.

Realization of diverse functionalities by tuning magnetic interactions in rare-earth perovskite oxide thin films opens up exciting technological prospects. Strain-induced tuning of magnetic interactions in rare-earth cobaltates and nickelates is of central importance due to their versatility in electronic transport properties. Here we report the spin reorientation induced switching of anisotropic magnetoresistance (AMR) and its tunability with strain in epitaxial ${\mathrm{LaCo}}_{0.5}{\mathrm{Ni}}_{0.5}{\mathrm{O}}_{3-\delta }$ thin films across the ferromagnetic transition. Moreover, with strain tuning, we observe a twofold to fourfold symmetry crossover in AMR across the magnetic transition temperature. The magnetization measurements reveal a ferromagnetic transition around 50 K. At temperatures below this transition, there is a subtle change in the magnetization dynamics, which reduces the ferromagnetic long-range ordering in the system. X-ray absorption and x-ray magnetic circular dichroism spectroscopy measurements at the Co and Ni $L$ edges reveal a Co spin state transition below 50 K, leading to the AMR switching and also the presence of ${\mathrm{Ni}}^{2+}$ and ${\mathrm{Co}}^{4+}$ ions evidencing the charge transfer from Ni to Co ions. Our work demonstrates the tunability of magnetic interactions mediated electronic transport in cobaltate-nickelate thin films, which is relevant in understanding Ni-Co interactions in oxides for their technological applications such as in AMR sensors.

We investigate the surface plasmons propagating on disordered bulk permalloys (Pys), whose magnetization breaks the time reversal symmetry, causing the surface plasmons to possess chirality in the absence of magnetic fields under the interplay of the anomalous Hall conductivity (AHC) and the boundary conditions at the surface. Based on the collisionless hydrodynamic model, we incorporate the effect of interband transitions into the dispersion of surface plasmons. As we show, the AHC will split the forward and backward propagating surface plasmons into two modes, while the $p\text{−}d$ transitions in bulk Py will work against the AHC, yielding slower propagation surface chiral Berry plasmons (CBPs), and even ceasing the presence of surface modes. The resulting surface CBP has a broad bandwidth ranging from visible light to infrared frequency. The free-field chirality may provide a new knob to control the chiral transport of light, as well as a new probe for detecting the intrinsic AHC in ferromagnetic metals.

Adding a Rashba term to the Hubbard Hamiltonian produces a model which can be used to learn how spin-orbit interactions impact correlated electrons on a lattice. Previous works have studied such a model using a variety of theoretical frameworks, mainly close to half filling. In this work, we determine the magnetic phase diagram for the Rashba-Hubbard model for arbitrary hole doping using a sine-square deformed lattice mean-field model with an unrestricted Ansatz, thus suppressing finite-size effects and allowing for inhomogeneous order. We find that the introduction of Rashba spin-orbit coupling significantly alters the ground-state properties of the Hubbard model and we observe an increasing complexity of the ground-state phase composition for increasing spin-orbit strength. We also introduce a gradual deformed envelope (GDE) technique building on the sine-square methodology to facilitate convergence towards ordered and defect-free ground-state configurations which is a challenge with the unrestricted ansatz at high interaction strengths. We observe that the use of the GDE technique significantly lowers the free energy of the obtained configurations. Moreover, we consider transient dynamics in the Rashba-Hubbard model by quenching the interaction strength. We find that the quench dynamics within a sine-square methodology allows for the simulation of quasiopen systems by using the zero-energy edge states as a particle reservoir. Interaction quenches at half filling show a tendency towards quench-induced spatial spin-magnitude inhomogeneity and a nonequilibrium system magnetization lower than equilibrium predictions, possibly related to a buildup of nonlocal correlations on the lattice.

The possibility of realizing pair density wave (PDW) phases, in which Cooper pairs have a finite momentum, presents an interesting challenge that has been studied in a wide variety of systems. In conventional superconductors, this is only possible when external fields lift the spin degeneracy of the Fermi surface, leading to pair formation at an incommensurate momentum. Here, we study a second possibility, potentially relevant to transition metal dichalcogenides, in which the Fermi surface consists of a pair of pockets centered at the $\pm K$ points of the Brillouin zone as well as a central pocket at the $\mathrm{\Gamma }$ point. In the limit where these three pockets are identical, the pairing susceptibility has a logarithmic divergence at the nonzero wave vectors $\pm \mathbf{K}$, allowing for a weak-coupling analysis of the PDW instability. We find that repulsive electronic interactions combine to yield effective attractive interactions in the singlet and triplet PDW channels, as long as the $\mathrm{\Gamma }$ pocket is present. Because these PDW channels decouple from the uniform superconducting channel, they can become the leading unconventional pairing instability of the system. Upon solving the linearized gap equations, we find that the PDW instability is robust against small trigonal warping of the $\pm K$ pockets and small detuning between the $\mathrm{\Gamma }$ and $\pm K$ pockets, which affect the PDW transition in a similar way as the Zeeman magnetic field affects the uniform superconducting transition. We also derive the Ginzburg-Landau free energy for the PDW gaps with momenta $\pm \mathbf{K}$, analyzing the conditions for and consequences of the emergence of FF-type and LO-type PDW ground states. Our classification of the induced orders in each ground state reveals unusual phases, including an odd-frequency charge-$2e$ superconductor in the LO-type PDW.

The quadratic chain is a 1D nonperiodic lattice with lattice sites at $z_j=j^{2}d$ with $j\in \mathbb{W}$ and $d$ the underlying lattice constant. We treat the electronic structure of the quadratic chain in a continuum model. The electronic extended-state spectrum is singular-continuous with bands at wavevectors $k$ rational multiples of $\frac{\pi }{d}$ with critical states occurring at the transitions between extended and localized states as the potential parameter is varied for fixed $k$. Our treatment results in the exact classification of extended states and provides closed-form expressions to compute them. We further identify states at well-defined energy that are extended for small values of the potential parameter, become localized at higher values, and at yet higher ranges of the parameter, are again extended, dubbing such behavior reëntrant extended states.

The band structure for a crystal generally consists of connected components in energy-momentum space, known as band complexes. Here, we explore a fundamental aspect regarding the maximal number of bands that can be accommodated in a single band complex. We show that, in principle, a band complex can have no finite upper bound for certain space groups. This means infinitely many bands can entangle together, forming a connected pattern stable against symmetry-preserving perturbations. This is demonstrated by our developed inductive construction procedure, through which a given band complex can always be grown into a larger one by gluing a basic building block to it. As a by-product, we demonstrate the existence of arbitrarily large accordion-type band structures containing $N_C=4n$ bands, with $n\in \mathbb{N}$.

Charge carrier mobility is at the core of semiconductor materials and devices optimization, and Hall measurement is one of the most important techniques for its characterization. The Hall factor, defined as the ratio between Hall and drift mobilities, is of particular importance. Here we study the effect of anisotropy by computing the drift and Hall mobility tensors of a technologically important wide-band-gap semiconductor, $4H$-silicon carbide ($4H$-SiC), from first principles. With $GW$ electronic structure and ab initio electron-phonon interactions, we solve the Boltzmann transport equation without fitting parameters. The calculated electron and hole mobilities agree with experimental data. The electron Hall factor strongly depends on the direction of external magnetic field $\mathbf{B}$, and the hole Hall factor exhibits different temperature dependency for $\mathbf{B}\parallel c$ and $\mathbf{B}\perp c$. We explain this by the different equienergy surface shape arising from the anisotropic and nonparabolic band structure, together with the energy-dependent electron-phonon scattering.

Most semiconductors, in particular III-V compounds, have a complex valence band structure near the band edge, due to degeneracy at the zone center. One peculiar feature is the warping of the electronic dispersion relations, which are not isotropic even in the vicinity of the band edge. When the exciton, all important for the semiconductor optical properties, is considered, this problem is usually handled by using some kind of angular averaging procedure, that would restore the isotropy of the hole effective dispersion relations. In the present paper, we consider the problem of the exciton ground-state energy for semiconductors with zinc-blende crystal structure, and we solve it exactly by a numerical treatment, taking fully into account the warping of the valence band. In the resulting four-dimensional problem, we first show exactly that the exciton ground state is fourfold degenerate. We then explore the ground-state energy across the full range of allowed Luttinger parameters. We find that the correction due to warping may in principle be quite large. However, for the considered semiconductors with available data on the band structure, the correction turns out to be in the $10\%–15\%$ range.