The ferrimagnetic semiconductor ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$ keeps the records of colossal magnetoresistance (CMR) and angular magnetoresistance (AMR). Here we report tuning the electronic transport properties via doping and pressure in ${\left({\mathrm{Mn}}_{1\text{−}x}{\mathrm{Mg}}_x\right)}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$. As the substitution of nonmagnetic ${\mathrm{Mg}}^{2+}$ for magnetic ${\mathrm{Mn}}^{2+}$, ferrimagnetic transition temperature $T_C$ gradually decreases, while the resistivity increases significantly. At the same time, the CMR and AMR are both enhanced for the low-doping compositions (e.g., $x=0.1$ and 0.2), which can be attributed to the doping-induced broadening of the band gap and a larger variation range of the resistivity when undergoing a metal-insulator transition by applying a magnetic field along the $c$ axis. On the contrary, $T_C$ rises with increasing pressure due to the enhancement of the magnetic exchange interactions until a structural transition occurs at $\sim 13$ GPa. Meanwhile, the activation gap is lowered under pressure and the magnetoresistance is decreased dramatically above 6 GPa where the gap is closed. At 20 and 26 GPa, weak drops in resistance indicating a superconducting transition at $\sim 5$ K are observed. The results reveal that doping and pressure are effective methods to tune the activation gap, and correspondingly, the CMR and AMR in nodal-line semiconductors, providing an approach to investigate the magnetoresistance materials for novel spintronic devices.

In this work we introduce an anyon condensation web that interconnects a broad class of two-dimensional (2D) finite gauge theories with multipolar conservation laws at a microscopic level. We refer to such theories as spatially modulated since their generators act nonuniformly across the system and have a strong position dependence. We find that condensation of appropriate set of anyons triggers the emergence of additional spatially modulated symmetries, which has the general effect of increasing the number of super-selection anyon sectors. As explicit examples, we start with the rank-2 toric code model and implement various anyon condensation protocols, resulting in a range of 2D higher-rank theories, each with a distinct gauge structure. We also expand the scope of anyon condensation by introducing lattice defects into spatially modulated theories and demonstrate that these geometric defects can be viewed as effective anyon condensations along the branch cut. Furthermore, we introduce the Multipartite Entanglement Mutual Information measure as a diagnostic tool to differentiate among various distinct multipole conserving phases. A captivating observation is the ultraviolet sensitivity of the mutual information sourced from multipartite entanglement in such modulated gauge theories, which depends on the geometric cut and the system size, and exhibits periodic oscillations at large distances.

Exponential observables, formulated as $ln\langle e^{\widehat{X}}\rangle$ where $\widehat{X}$ is an extensive quantity, play a critical role in the study of quantum many-body systems, examples of which include the free energy and entanglement entropy. Given that $e^X$ becomes exponentially large (or small) in the thermodynamic limit, the accurate computation of the expectation value of this exponential quantity presents a significant challenge. In this paper, we propose a comprehensive algorithm to quantify these observables in interacting fermion systems, utilizing the determinant quantum Monte Carlo method. We have applied this algorithm to the two-dimensional square-lattice half-filled Hubbard model and $\pi$-flux t-V model. In the Hubbard model case at the strong-coupling limit, our method showcases a significant accuracy improvement on free energy compared to conventional methods that are derived from the internal energy, and in the t-V model, we indicate that the free energy offers a precise determination of the second-order phase transition. We also illustrate that this approach delivers highly efficient and precise measurements of the $n\mathrm{th}$ Rényi entanglement entropy. Even more noteworthy is that this improvement comes without incurring increases in computational complexity. This algorithm effectively suppresses exponential fluctuations and can be easily generalized to other models.

Layered transition metal chalcogenides have stimulated wide research interest due to their many exotic physical properties. In this paper, we study the magnetotransport properties of the exfoliated nodal-line semimetal $\mathrm{TaNiT}{\mathrm{e}}_5$. A giant positive magnetoresistance (MR) is observed when the current is parallel to the crystallographic $c$ axis, while it is strongly diminished when the current flows along the $a$ axis. The observed giant MR is gradually suppressed either on reducing the thickness of the nanoflake or on increasing temperature. By performing MR measurement in tilted magnetic fields, the interlayer coupling is found to be weakened both by reducing the thickness and by increasing temperature. We propose a mechanism of electron-electron interaction assisted interlayer transport as an origin of the giant MR. The mechanism is likely to provide an explanation for giant MR in other layered materials.

The observation of zero-bias conductance peaks in vortex cores of certain Fe-based superconductors has sparked renewed interest in vortex-bound Majorana states. These materials are believed to be intrinsically topological in their bulk phase, thus avoiding potentially problematic interface physics encountered in superconductor-semiconductor heterostructures. However, progress towards demonstration of non-Abelian statistics of vortex Majorana modes is hindered by our inability to measure the topological quantum state of a nonlocal vortex Majorana state, i.e., the charge of a vortex pair. In this paper, we theoretically propose a microwave-based charge parity readout of the Majorana vortex pair charge. A microwave resonator above the vortices can couple to the charge allowing for a dispersive readout of the Majorana parity. Our technique may also be used in vortices in conventional superconductors and allows one to probe the lifetime of vortex-bound quasiparticles, which is currently beyond existing scanning tunneling microscopy capabilities.

Lead-free, stable photovoltaic materials with promising optoelectronic features have recently been discovered in chalcogenide perovskites. However, a thorough theoretical analysis of excitonic and polaronic properties has never been done because it would require enormous computing. Here, we elucidate the role of excitonic and polaronic effects in a sequence of chalcogenide perovskites ${AB\text{Se}}_3$ ($A$ = Ca, Sr, Ba; $B$ = Zr, Hf), including their needlelike ($\alpha$-phase) and distorted ($\beta$-phase) structures, along with their relative stability and optoelectronic properties by employing state-of-the-art density functional theory, density functional perturbation theory (DFPT), and many-body perturbation theory [$GW$ and Bethe-Salpeter equation (BSE)]. We find all the investigated perovskites to be dynamically as well as mechanically stable and possess direct electronic ($G_0{W}_0$) band gap in the range of 1.02–1.97 eV. We investigate the interplay of the ionic and electronic components of the dielectric screening through the DFPT and BSE methods and find that the electronic component predominates. Interestingly, they exhibit smaller exciton binding energy (0.02–0.10 eV) than conventional halide and sulfur-based chalcogenide perovskites. The polaronic mobility for electrons (holes) is in the range of $8.26–77.59{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ (19.05–100.49 ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$), which also found to be much higher than that of sulfur-based chalcogenide perovskites, owing to less carrier-phonon interactions in the former. All the examined properties suggest $\beta \text{−}{AB\text{Se}}_3$ to be promising environmentally friendly stable materials for photovoltaic applications. This has been further confirmed by estimating spectroscopic limited maximum efficiency, which is calculated as $\sim 17.5$%–23% for these materials.

Magnetic excitons and spin-orbit transitions are investigated in the pseudospin-$\frac{1}{2}{\mathrm{CoTiO}}_3$ Dirac magnon system using inelastic neutron scattering. The magnon density of states consists of two magnon peaks with a gapless Dirac node nestled between them. Above 38 K, the Néel temperature ($T_{\mathrm{N}}$), the magnon modes are renormalized to lower energies. Magnetic excitations attributed to spin-orbit exciton multiplet transitions follow the same temperature dependence as the magnons, with the intensity dissipating quickly above $T_N$. On the other hand, energy level transitions arising from the crystal field and spin-orbit coupling (with an estimated $\lambda \approx 14\mathrm{meV}$) are gradually thermally populated, continuing through $T_N$ and reaching a maximum at $\sim 100\mathrm{K}$. The excitations significantly broaden but persist through room temperature.

This study investigates the fascinating reality of noncollinear antiferromagnet ${\mathrm{Mn}}_3\mathrm{Pt}$ thin film and its potential applications in spintronic devices, particularly in the context of all-antiferromagnetic tunnel junctions. By employing atomic-scale Landau-Lifshitz-Gilbert (LLG) simulations, we explored the spin dynamics driven by spin transfer torque (STT) in a ${\mathrm{Mn}}_3\mathrm{Pt}$-based all-antiferromagnetic tunnel junction. The investigation revealed an intriguing phenomenon: a spin-flop transition occurring within the ${\mathrm{Mn}}_3\mathrm{Pt}$ sublattices under the influence of STT. This transition involves a transformation from a triangular antiparallel alignment to a quasiparallel configuration, demonstrating the complex dynamics achievable through STT manipulation. Importantly, the study identifies magnetic anisotropy as a key determinant of the stability of the spin-flop process, highlighting that larger magnetic anisotropy leads to a more robust transition. Furthermore, through frequency analyses conducted via simulations and theoretical considerations, the study elucidates the presence of terahertz oscillations. Notably, the frequency of these oscillations exhibits a linear increase with the increase of current density. These findings not only deepen our understanding of the oscillation properties of noncollinear antiferromagnets but also pave the way for potential applications in high-frequency spin oscillators.

Strain tunability of magnetocrystalline anisotropy (MCA) in ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ is demonstrated for monolayer and bilayer structures. In the framework of density functional calculations, upon in-plane strain $\eta , -5\%\le \eta \le +5\%$, MCA exhibits contrasting behaviors for compressive and tensile strains. For tensile strain ($\eta >0$), perpendicular MCA is well retained with little change in the magnetic moments. For compressive strain ($\eta <0$), on the other hand, an abrupt decrease in $E_{\text{MCA}}$ and change in the magnetic moments are prominent. It is noteworthy that bilayer ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ even exhibits a sign change of $E_{\text{MCA}}$, indicating a transformation from perpendicular to in-plane magnetization. The feature of MCA is analyzed from three perspectives: (1) atomic decomposition, (2) the spin-orbit coupling (SOC) matrix in the $d$ and $p$ manifolds, and (3) occupation changes in the band structure. In atomic decomposition, the Te contribution predominates and accompanies a sign change in the bilayer structure. The SOC matrix in Te $p$ manifolds under the compressive strain $\langle m=\pm 1,\uparrow \left|L_z\right|m=\pm 1,\uparrow \rangle$ contribution to $E_{\text{MCA}}>0$ decreases. In the bilayer structure, $\langle m=\pm 1,\downarrow \left|L_x\right|m=0,\downarrow \rangle$ is responsible for the sign change of $E_{\text{MCA}}$ when $\eta =-5\%$.

We report the observation of the negative differential resistance (NDR) in the current-driven regime of superconducting MoN/Cu constrictions. Our calculations in framework of time-dependent Ginzburg-Landau equation reveal that in the constriction at large current there is a channel with dynamically suppressed superconducting order parameter along which vortices and antivortices move quasiperiodically over time. As the current increases the constriction switches to the less resistive state with much faster vortices moving periodically over time (quasiphase slip-line state). Microwave radiation experiments demonstrate the presence of Shapiro steps at low voltage, where a slow, time-periodic vortex motion is expected. Conversely, Shapiro steps are absent at high voltage, where the NDR appears, indirectly confirming the quasiperiodic regime.

The reduced phonon specularity $p$ from boundary roughness scattering plays a major role in the lower thermal conductivity in semiconducting and insulating nanowires and films. Although the well-known Ziman formula $p=exp(-4{\sigma }^2{q}_x^2)$, where $\sigma$ and $q_x$ denote the root-mean-square boundary roughness and the normal component of the incident phonon wave vector, respectively, and its variants are commonly used in the literature to estimate how roughness attenuates $p$, their validity and accuracy remain poorly understood, especially when the effects of mode conversion cannot be ignored. In this paper, we investigate the accuracy and validity of the more general Ogilvy formula, from which the Ziman formula is derived, by comparing its predictions to the $p$ values computed from atomistic Green's function simulations for an ensemble of rough boundaries in single-layer graphene. The effects of phonon dispersion, incident angle, polarization, mode conversion, and correlation length are analyzed. Our results suggest that the Ogilvy formula is remarkably accurate for $0<q_x<\frac{\pi }{4\sigma }$ when the lateral correlation length $L$ is large or the phonon is at normal incidence. At large $q_x$ in the short-wavelength limit, the $q_x$-dependence of $p$ becomes significantly weaker. In the large-$L$ limit, the numerical results suggest the existence of a minimum $p$ for short-wavelength phonons, given by $p\sim p_{0}exp(-{\pi }^2/4)$, where $p_0$ is the baseline specularity for the ideal boundary.

Using the Boltzmann transport equation (BTE), we study the evolution of nonequilibrium carrier distributions in simple $\left(sp\right)$ metals, assumed to have been instantaneously excited by an ultrafast laser pulse with photon energy $h\nu$. The mathematical structure of the BTE scattering integrals reveals that $h\nu$ is a natural energy scale for describing the dynamics. Normalizing all energy quantities by $h\nu$ leads to a set of three unitless parameters $\beta /\delta$, $\gamma$, and $\alpha$ that control the relaxation dynamics: $\beta /\delta$ is the normalized ratio of electron-phonon to electron-electron scattering strengths, $\gamma$ is the normalized phonon (lattice) temperature, and $\alpha$ is the normalized absorbed energy density. Using this theory, we systematically investigate relaxation times for the high-energy part of the distribution $\left({\tau }_H\right)$, energy transfer to the phonon subsystem $\left({\tau }_E\right)$, and intracarrier thermalization $\left({\tau }_{\text{th}}\right)$. In the linear region of response (valid when $\alpha$ is sufficiently small), we offer heuristic descriptions of each of these relaxation times as functions of $\beta /\delta$ and $\gamma$. Our results as a function of excitation level $\alpha$ show that many ultrafast experimental investigations lie in a transition region between low excitation (where the relaxation times are independent of $\alpha$) and high excitation (where the two-temperature model of carrier dynamics is valid). Approximate boundaries that separate these three regions are described by simple expressions involving the normalized parameters of our model.

Using principles of asymptotic analysis, we derive the exact leading corrections to the Thomas-Fermi kinetic energy approximation for Kohn-Sham electrons for slabs. This asymptotic expansion approximation includes crucial quantum oscillations missed by standard semilocal density functionals. Because these account for the derivative discontinuity, chemical accuracy is achieved at fourth order. The implications for both orbital-free electronic structure and exchange-correlation approximations are discussed.

The effect of doping on the parameters of an electron-hole liquid (EHL) in heterostructures based on transition metal dichalcogenides is studied. The phase diagram of the EHL is constructed. It is shown that for the formation of a high-temperature tightly bound EHL, as well as for the transition from the semiconducting (exciton) state to the semimetallic one (electron-hole plasma/liquid), it is advantageous to dope the energy band with a larger number of valleys. The transition from trion gas to electron-hole plasma is investigated using the modified Mott criterion and variational calculation with screened potential. The effect of doping on the metal-insulator transition in the equilibrium case without laser excitation is studied.

The path integral formulation of quantum mechanical problems including fermions is often affected by a severe numerical sign problem. We show how such a sign problem can be alleviated by a judiciously chosen constant imaginary offset to the path integral. Such integration contour deformations introduce no additional computational cost to the Hamiltonian Monte Carlo algorithm, while its effective sample size is greatly increased. This makes otherwise unviable simulations efficient for a wide range of parameters. Applying our method to the Hubbard model, we find that the sign problem is significantly reduced. Furthermore, we prove that it vanishes completely for large chemical potentials, a regime where the sign problem is expected to be particularly severe without imaginary offsets. In addition to a numerical analysis of such optimized contour shifts, we analytically compute the shifts corresponding to the leading and next-to-leading order corrections to the action. We find that such simple approximations, free of significant computational cost, suffice in many cases. We present a simulation of ${\mathrm{C}}_{60}$ fullerenes (buckyballs) that are successful over a wide parameter range.

In one dimension density-density interactions of particles reduce their mobility and hence the Drude weight, which controls the divergence of the optical conductivity at zero frequency, decreases. We study effects of pair-hopping events on this result in a one-dimensional system of spinless fermions. The considered model consists of the usual single-particle hopping and pair-hopping terms. In the absence of the density-density interactions, we first show that a variation of the pair-hopping amplitude results in a monotonic change of the Drude weight. We next demonstrate that weak nearest-neighbor density-density interactions increase the Drude weight, whereas in the regime of strong interactions the Drude weight decreases as expected. Our numerical findings are supported by bosonization results.

In this paper, we systematically explore several key properties of electronic states in one-dimensional (1D) helical crystals, including the inheritance of orbital angular momentum from local atomic orbitals to the entire helical structure, the helical momentum, and the emergence of helical-induced spin-orbit coupling (hSOC). We then apply this comprehensive theoretical framework to elucidate the electronic structure of the 1D helical crystal InSeI. Our analysis reveals the influence of hSOC, evident in spin-mixing energy gaps within the electronic band structure, as calculated through density functional theory. Utilizing a combination of tight-binding modeling and first-principles calculations, we ascertain the spin-polarized electric response and the chiral-switchable second-order photocurrent response of InSeI, characterized as the Landauer-Buttiker ballistic transport and shift current response, respectively. The results highlight the potential of 1D InSeI for applications in spintronics and optoelectronics. The overarching theoretical framework established in this work will prove invaluable for the investigation of other helical electronic systems.

We theoretically and computationally investigate bulk photovoltaic effects, with a specific focus on shift-current and injection-current. Initially, we perform a numerical analysis of the direct current (dc) induced by a laser pulse with a one-dimensional model, utilizing mean-field theories such as time-dependent Hartree–Fock and time-dependent Hartree methods. Our numerical results, obtained with mean-field theories, reveal that the dc component of the current, as a second-order nonlinear effect, exists even after irradiation with linearly polarized light as a second-order nonlinear effect, indicating the generation of injection-current. Conversely, when we employ the independent-particle approximation, no injection-current is generated by linearly polarized light. To develop the microscopic understanding of injection-current within the mean-field approximation, we further analyze the dc component of the current with the perturbation theory, employing the mean-field approximations, the independent-particle approximation, and the exact solution of the many-body Schrödinger equation. The perturbation analysis clarifies that the injection-current induced by linearly polarized light under the mean-field approximations is an artifact caused by population imbalance, created through quantum interference from unphysical self-excitation pathways. Therefore, investigation of many-body effects on the bulk photovoltaic effects have to be carefully conducted in mean-field schemes due to potential contamination by unphysical dc current. Additionally, we perform the first-principles electron dynamics calculation for ${\mathrm{BaTiO}}_3$ based on the time-dependent density functional theory, and we confirm that the above findings from the one-dimensional model calculation and the perturbation analysis apply to realistic systems.

Carefully designed metasurfaces can serve as effective platforms for strong coupling phenomena by the excitation of specific modes and the enhancement of light-matter interactions. However, strong coupling simultaneously involving Fabry-Pérot (FP) cavity modes, anapole states, and excitons has not been thoroughly explored. In this work, we numerically present the strong coupling of a FP cavity-anapole-exciton supported by a nano-optical metasurface of bulk $\mathrm{W}{\mathrm{S}}_2$-Si heterogeneous nanodisks inserted into a FP optical cavity. The excited multiorder FP cavity modes enable giant Rabi splittings with tunable population due to all generated odd-order FP cavity modes strongly coupling with the anapole state and exciton of bulk $\mathrm{W}{\mathrm{S}}_2$-Si heterogeneous nanodisks. Giant Rabi splittings of 643 and 335 meV are, respectively, achieved by the first-order FP and third-order FP cavity-anapole-exciton interactions. Our designed hybrid system provides a robust platform for exploring ultrahigh Rabi splittings and strong coupling of multiple optical responses in light-matter interactions.

The lacking of two-dimensional intrinsic room-temperature ferromagnetic semiconductors severely restricts the development of future low-dimensional semiconductor spintronic devices. Is it possible to indirectly obtain ferromagnetic semiconductors that work at room temperature from the available antiferromagnetic metal through quantum state regulation? Here we employ the density functional theory to systematically investigate the electronic and magnetic properties of the 1T-${\mathrm{CrTe}}_2$ monolayer under Li atomic adsorption. The 1T-${\mathrm{CrTe}}_2$ monolayer is an antiferromagnetic metal, which is not suitable for semiconductor spintronic device applications. Interestingly, a desired room-temperature ferromagnetic semiconductor with a large out-of-plane piezoelectricity, i.e., the ${\mathrm{LiCrTe}}_2$ monolayer, is achieved successfully after one Li atomic layer is adsorbed at the 1T-${\mathrm{CrTe}}_2$ surface. The Curie temperature is above room temperature and reaches 392 K, which remains above room temperature and increases with the increase of in-plane biaxial tensile strain. A semiconductor-to-metal conversion and a change of the orientation of the easy magnetization axis can be realized with robust room-temperature ferromagnetism through electrostatic doping. Our results indicate that the atomic adsorption is an effective strategy to achieve van der Waals room-temperature ferromagnetic semiconductors.