Two-dimensional (2D) ferromagnetic materials with high-temperature intrinsic magnetic ordering, electrically tunable spin polarity, and significant magnetocaloric effect (MCE) are highly desirable for designing nanoscale spintronic devices and developing multifunctional materials. In this work, we employ first-principles calculations in conjunction with atomic spin dynamics (ASD) simulations to identify a 2D monolayer phase of ${\mathrm{CrS}}_2$ with space group $P\overline{4}{2}_{1}m$, exhibiting high-Curie-temperature ($T_{\mathrm{C}}$) Ising ferromagnetism and bipolar magnetic semiconducting characteristics. The intrinsic ferromagnetism originates from the strong superexchange interactions between Cr cations mediated by S anions. Moreover, we explore its layer-dependent electronic structure and find that even-layered ${\mathrm{CrS}}_2$ systems exhibit altermagnetism with the momentum-dependent spin splitting. ASD simulations demonstrate that the ${\mathrm{CrS}}_2$ monolayer possesses an excellent MCE with the maximum magnetic entropy change $-\mathrm{\Delta }{S}_{\mathrm{M}}^{max}$ of 8.03 $\mu \mathrm{J}/{\mathrm{m}}^2$/K, the maximum adiabatic temperature change $\mathrm{\Delta }{T}_{\mathrm{ad}}^{max}$ of 2.17 K, and a relative cooling power (RCP) of 722.70 $\mu \mathrm{J}/{\mathrm{m}}^2$ under an out-of-plane magnetic field of 5 T. Additionally, we systematically investigate the effects of biaxial strain on its magnetic phase and MCE. Our results indicate that a moderate compressive strain induces a magnetic phase transition from ferromagnetism to antiferromagnetism and significantly enhances the MCE by increasing $-\mathrm{\Delta }{S}_{\mathrm{M}}^{max}, \mathrm{\Delta }{T}_{\mathrm{ad}}^{max}$, and RCP. Furthermore, utilizing the carrier-doping controllable spin polarity of the bipolar magnetic semiconductor, we propose a conceptual model for a dual-gated magnetic tunnel junction, where the “on” and “off” states are achieved by switching the voltage signs of the dual gates. These findings not only deepen our understanding of the electrically tunable spin polarity, but also emphasize the profound impact of strain effects on magnetic ground states and MCE enhancement, thereby paving the way for future innovations in spintronic devices and magnetic refrigeration materials.

We theoretically investigate the second harmonic generation (SHG) of topological insulator surface states in a perpendicular magnetic field. Our theory is based on the microscopic expression of the second-order magneto-optical conductivity developed from the density matrix formalism, taking into account hexagonal warping effects on the surface states' band structure. Using numerically exact Landau-level energies and wave functions including hexagonal warping, we calculate the spectrum of SHG conductivities under normal incidence for different values of magnetic field and chemical potential. The imaginary parts of the SHG conductivities show prominent resonant peaks corresponding to one-photon and two-photon inter-Landau-level transitions. Treating the hexagonal warping term perturbatively, these transitions are clarified analytically within a perturbation theory from which approximate selection rules for the allowable optical transitions for SHG are determined. Our results show extremely high SHG susceptibility that is easily tunable by magnetic field and doping level for topological surface states in the far-infrared regime, exceeding that of many conventional nonlinear materials. This work highlights the key role of hexagonal warping effects in generating second-order optical responses and provides insights into the nonlinear magneto-optical properties of the topological insulators.

Holes in Si quantum dots are being investigated for the implementation of electrically addressable spin qubits. In this perspective, the attention has been focused on the electric-field induced transitions between the eigenstates belonging to the ground doublet. Here we theoretically extend the analysis to the first excited doublet. We show that—in a prototypical quantum dot structure—transitions involving the lowest excited states display Rabi frequencies that are several orders of magnitude larger than that of the transition between states of the ground doublet. A clear relation with the symmetries of the eigenstates emerges, as well as a wide tunability of the Rabi frequencies by means of the applied bias. A preliminary discussion on the possible implications of the present results for multilevel manipulation schemes and for multihole qubit encodings is provided.

We investigate the nonequilibrium populations of Andreev bound states (ABSs) in ballistic graphene three-terminal Josephson junctions (JJs). Our measurements reveal periodic resistance oscillations as a function of the superconducting phase, which is modulated by the applied magnetic flux. Additionally, we observe several voltage-induced crossovers, where resistance oscillations alternate between minima and maxima at integer flux quanta, corresponding to 0 and $\pi$ phase shifts, respectively. We utilize a microscopic model for long ballistic Andreev interferometers, accounting for both perturbative and nonperturbative scattering processes. This model attributes the $0\text{−}\pi$ crossovers to phase-sensitive Andreev reflections (ARs), which convert normal current into ABSs with nonequilibrium populations. Our combined theoretical and experimental approach suggests that unconventional energy-phase relations, such as $\pi$-shifted ABSs, can be created in multiterminal JJs without relying on complex Cooper quartet or Floquet physics.

Recently, the Hofstadter spectrum of a twisted ${\mathrm{WSe}}_2/{\mathrm{MoSe}}_2$ heterobilayer was observed in experiment [C. R. Kometter et al. Nat. Phys. 19, 1861 (2023)], but the origin of Hofstadter states remains unclear. Here, we present a comprehensive theoretical interpretation of the observed Hofstadter states by calculating their accurate Hofstadter spectrum. We point out that the valley Zeeman effect, a unique feature of the transition metal dichalcogenide (TMD) materials, plays a crucial role in determining the shape of the Hofstadter spectrum, due to the narrow bandwidth of the moiré bands. This is distinct from the graphene-based moiré systems. We further predict that the Hofstadter spectrum of the moiré flat band, which was not observed in the experiment, can be observed in the same system with a larger twist angle $2^{\circ }\lesssim \theta \lesssim 3^{\circ }$. Our theory paves the way for further studies of the interplay between the Hofstadter states and correlated insulating states in such moiré lattice systems.

The layered transition metal dichalcogenide $1\text{T-TiS}{\mathrm{e}}_2$ is of great research interest, having intriguing properties of charge density waves (CDWs) and superconductivity under doping or pressure. The monolayer form of $1\text{T-TiS}{\mathrm{e}}_2$ also shows a CDW with a higher transition temperature than the bulk, indicating a stronger CDW interaction. By using the meta-generalized gradient approximation (metaGGA)-based model Bethe-Salpeter equation (mBSE) and many-body perturbation GW+BSE methods, we calculate the exciton binding energies and electron energy loss spectrum (EELS) for the $1\text{T-TiS}{\mathrm{e}}_2$ monolayer under different in-plane biaxial strains. We find that even without strain the $1\text{T-TiS}{\mathrm{e}}_2$ monolayer can have negative exciton energies at the Brillouin zone boundary point M, with a binding energy larger than the gap. The calculated EELS reinforces this picture, indicating exciton insulator (EI) states in $1\text{T-TiS}{\mathrm{e}}_2$ monolayer even without strain. The Wannier-Mott formula calculations of exciton binding energy corroborate results from GW+BSE. Small compressive strain enhances the EI state, and for tensile strains slightly less than 3%, the EI state in this monolayer persists. At large tensile strains, the material makes a transition to a normal semiconductor. Our results provide important information for understanding the quantum nature of this two-dimensional material. Our results from the standard ${\mathrm{G}}_0{\mathrm{W}}_0$@PBE+SOC+U+BSE approach are not qualitatively different from those of a more computationally efficient metaGGA-based $\mathrm{SCAN}+\mathrm{SOC}+\mathrm{U}+\mathrm{mBSE}+f_{xc}^{\mathrm{loc}}$ approach that employs a model BSE.

We investigate the experimentally observed achiral-to-chiral displacive phase transition of ${\mathrm{K}}_3{\mathrm{NiO}}_2$ using first-principles calculations. Phonon dispersion analysis of the achiral phase reveals the presence of an unstable phonon branch, with the low-symmetry chiral phases arising from a doubly degenerate zone-boundary phonon mode. The resulting energy landscape exhibits four energetically equivalent wells, described by left- and right-handed chiral space groups. Additionally, we demonstrate that substituting K with larger cations (e.g., Na, Rb, Cs) and replacing Ni with Ag or Au hardens this soft mode, consistent with experimental observations. Our calculations further show that the resulting instability may be externally tuned with the help of hydrostatic pressure and epitaxial strain, and we quantify the effect of these external stimuli by calculating the energy difference between the chiral and achiral structures and the behavior of the O-K-O bonding angle. In particular, we show that the chirality should disappear above $\sim 1\mathrm{GPa}$ or 3% tensile epitaxial strain. On the other hand, the chiral phases are additionally softened if compressive epitaxial strain is present. We discuss the effect of pressure and strain on other compounds undergoing an achiral-to-chiral transition, unveiling a possible general strategy to control chiral instabilities in materials.

Quasicrystal approximants (QCAs) are a class of crystalline materials that share structural and electronic similarities with quasicrystals while maintaining periodicity. Here we report on the structural, thermodynamic, electronic, magnetic, and transport properties of the QCAs of ${\mathrm{GdCd}}_{6-x}{\mathrm{Zn}}_x$ ($x=0.0$, 0.5, and 1.0) prepared by the flux method. The substitution of Cd by Zn results in a $4.7\%$ reduction in volume for $x=1.0$, as revealed by x-ray diffraction measurements. The chemical pressure also reduces the Néel temperature ($T_N$), as observed in specific heat, resistivity, and magnetic susceptibility measurements, which is caused by the reduction of the total density of states at the Fermi level induced by Zn doping, as suggested from density functional theory calculations. Interestingly, for a specific concentration of Zn ($x$ = 1.0), the system behaves differently, exhibiting tunable resistance at low temperatures under different values of an external magnetic field. These observations suggest that the physical properties of the ${\mathrm{GdCd}}_6$ approximant under chemical pressure can be interpreted within an electronic multiband scenario.

The structural and physical properties of two single-grain Au-Al-Gd 1/1 approximant crystals (ACs) with analyzed compositions $\mathrm{A}{\mathrm{u}}_{73.67}\mathrm{A}{\mathrm{l}}_{12.24}\mathrm{G}{\mathrm{d}}_{14.09}$ and $\mathrm{A}{\mathrm{u}}_{74.67}\mathrm{A}{\mathrm{l}}_{11.35}\mathrm{G}{\mathrm{d}}_{13.98}$ were thoroughly investigated. The two variants are isostructural (cubic, space group: $Im\overline{3}$), undergoing sharp antiferromagnetic (AFM) transitions at $T_{\mathrm{N}}=9.6$ K and $T_{\mathrm{N}}=8.3$ K, respectively. Specific-heat measurements in both samples evidenced sharp jump at $T_{\mathrm{N}}$, associated with the AFM transition, accompanied by broad Schottky-type anomalies at lower temperatures. Strikingly, the two 1/1 ACs exhibited markedly different responses to applied magnetic fields: one displayed a pronounced metamagnetic anomaly while the other did not. In this regard, variational calculations with adiabatic transformation of the magnetic field in realistic structural models identified the existence of two possible AFM phases: the cuboc phase with a metamagnetic anomaly and the cluster Néel phase without it, thereby clearly revealing the possible magnetic structures of the present AFM variants. These findings have led to the establishment of a comprehensive magnetic-phase diagram for Heisenberg-type 1/1 ACs. This work not only advances our understanding of magnetic-phase transitions in these complex systems but also suggests the existence of a broader spectrum of unexplored magnetic states in Tsai-type materials.

We report on the stability of the magnetic dipole string (DS), a three-dimensional magnetic texture formed by two coupled Bloch points with opposite topological charges, separated by an equilibrium distance. Previous studies demonstrated the stability of such configurations through geometric confinement or coupling with local perturbations in the magnetization field, such as skyrmion strings or dislocations in helical modulations. Here, we show that, in uniaxial chiral magnets, an isolated DS remains stable in an unperturbed vacuum, thus representing a true three-dimensional soliton. The phase diagram illustrates the stability of the DS embedded in the conical or helical phases across a broad range of material parameters and external magnetic fields. Using the geodesic nudged elastic band method applied to a regularized micromagnetic model, we demonstrate that isolated DSs are protected from collapse by an energy barrier. Stochastic spin-lattice simulations demonstrate that DSs can spontaneously nucleate during in-field annealing. This work aims to stimulate the experimental observation of DSs and further exploration of uniaxial chiral magnets.

Understanding exciton thermalization is critical for optimizing optoelectronic and photocatalytic processes in many materials. However, it is hard to access the dynamics of such processes experimentally, especially on systems such as monolayer transition metal dichalcogenides, where various low-energy excitations pathways can compete for exciton thermalization. Here, we study exciton dynamics due to exciton-phonon scattering in monolayer ${\mathrm{MoS}}_2$ from a first-principles, interacting Green's function approach, to obtain the relaxation and thermalization of low-energy excitons following different initial excitations at different temperatures. We find that the thermalization occurs on a picosecond time scale at 300 K but can increase by an order of magnitude at 100 K. The long total thermalization time, owing to the nature of its excitonic band structure, is dominated by slow spin-flip scattering processes in monolayer ${\mathrm{MoS}}_2$. In contrast, thermalization of excitons in individual spin-aligned and spin-anti-aligned channels can be achieved within a few hundred fs when exciting higher-energy excitons. We further simulate the intensity spectrum of time-resolved angle-resolved photoemission spectroscopy experiments and anticipate that such calculations may serve as a map to correlate spectroscopic signatures with microscopic exciton dynamics.

We present an ab initio method to calculate the clamped Pockels tensor of ferroelectric materials from density-functional theory, the modern theory of polarization exploiting the electric-enthalpy functional, and automated first- and second-order finite-difference derivatives of the polarizations and the Hellmann-Feynman forces. Thanks to the functional-independent capabilities of our approach, we can determine the Pockels tensor of tetragonal barium titanate (BTO) beyond the local density approximation (LDA), with arbitrary exchange-correlation (XC) functionals, for example, PBEsol. The latter, together with RRKJ ultrasoft pseudopotentials (PP) and a supercell exhibiting local titanium off centering, enables us to stabilize the negative optical phonon modes encountered in tetragonal BTO when LDA and norm-conserving PP are combined. As a result, the correct value range of $r_{51}$, the largest experimental Pockels coefficient of BTO, is recovered. We also reveal that $r_{51}$ increases with decreasing titanium off centering for this material. The lessons learned from the structural, dielectric, and vibrational investigations of BTO will be essential to design next-generation electro-optical modulators based on the Pockels effect.

We investigate the relationship between Krylov complexity and operator quantum speed limits (OQSLs) of the complexity operator and level repulsion in random/integrable matrices and many-body systems. An enhanced level repulsion corresponds to increased OQSLs in random/integrable matrices. However, in many-body systems, the dynamics is more intricate due to the tensor product structure of the models. Initially, as the integrability-breaking parameter increases, the OQSL also increases, suggesting that breaking integrability allows for faster evolution of the complexity operator. At larger values of integrability breaking, the OQSL decreases, suggesting a slowdown in the operator's evolution speed. Information-theoretic properties, such as scrambling, coherence, and entanglement, of Krylov basis operators in many-body systems are also investigated. The scrambling behavior of these operators exhibits distinct patterns in integrable and chaotic cases. For systems exhibiting chaotic dynamics, the Krylov basis operators remain a reliable measure of these properties of the time-evolved operator at late times. However, in integrable systems, the Krylov operator's ability to capture the entanglement dynamics is less effective, especially during late times.

Zero-field magnetic noise, characterized by the magnetic autocorrelation function $S_s\left(t\right)$, has been observed, perhaps surprisingly, to depend on sample shape $s$. The reasons for this are identified and general expressions are derived that relate the autocorrelation functions for systems of different shape to an underlying “intrinsic” form. Assuming the fluctuation-dissipation theorem, it is shown that, for any noise that relaxes monotonically, the effect of sample shape is to reduce both the noise amplitude and mean relaxation time by a factor of $1+N{\chi }_i$, where $N$ is the demagnetizing factor and ${\chi }_i$ the intrinsic susceptibility, but that only the exponential decay retains the same functional form for finite $N$. In frequency space, where $S_s\left(t\right)$ Fourier transforms into the power spectrum $S_s\left(\omega \right)$, the above two factors combine to suppress the zero frequency amplitude of $S_s\left(\omega \right)$ by ${(1+N{\chi }_i)}^2$, while, at high frequency, sample shape dependence becomes negligible. These results are applied to various magnetic systems and experiments, including to tests of the fluctuation-dissipation and noise measurements in spin ice, to spin glasses, to surface magnetism, and to ferromagnetic critical behavior. They may be used to imply a general result that, for any near to equilibrium magnetic system with monotonic relaxation, the internal $B$ field will relax more slowly, and with larger amplitude, than the internal $H$ field, both by factors of $1+{\chi }_i$.

The formation of two-band nodal points in gapless topological phases, referred to as conventional Weyl nodes, relies solely on translational symmetry. However, when coupled with other spatial and spatiotemporal symmetries, unconventional Weyl nodes with pronounced chirality, high degeneracy, and complementary quaternion charges can manifest. In this paper, we identify ${\mathrm{ReO}}_3$ as an ideal unconventional Weyl semimetal in which rotation and screw symmetries as well as their combination with time-reversal symmetry play a crucial role. To show this, we first revisit in detail the algebraic determination of the chirality of doubly and triply degenerate Weyl nodes from the spinful irreducible representations of the occupied and unoccupied bands, and then combine it with the complementary $C_{2}T$-symmetry-protected patch Euler class and non-Abelian frame charges that indicate the pinning of the Weyl nodes on $C_{2}T$-invariant planes. Notably, we find one of the Weyl nodes in ${\mathrm{ReO}}_3$ as the first example of a node with multiple nontrivial quaternion charges. Supporting our findings with first-principles calculations, we furthermore reveal very clear Fermi arc signatures of the high-chirality Weyl nodes at the Fermi level for different surface orientations. We finally investigate the effect of strain upon which the robustness of Weyl nodes clearly demonstrates their Chern (i.e., chirality conservation) and quaternionic (i.e., symmetry-plane pinning) topological nature.

In this work, we analyze the shift in current conductivity in helical twisted trilayer graphene. Without loss of generality, we show that the density of states and the twist angle set an upper bound for this response, which is inversely proportional to the square of the twist angle. For the case of ABA stacking and at the magic angle, the shift photoconductivity can reach values of order ${10}^4\mu \mathrm{A}\mathrm{nm}{\mathrm{V}}^{-2}$ for frequencies below 50 meV, which can be attributed to the interband transitions between the two flattened middle bands close to the Fermi level. By tuning the twist angle, we demonstrate that the photoconductivity is shifted in the frequency range and it is further influenced by two additional factors: the magnitude of the quantum geometric measure and the energy separation between the bands. Furthermore, we propose a scenario in the AAA stacked configuration, where the photoconductivity can be of order ${10}^5\mu \mathrm{A}\mathrm{nm}{\mathrm{V}}^{-2}$ in the THz regime, revealing a potential influence of the stacking in the optimization of the shift current conductivity. Therefore a large density of states, a small twist angle, and the layer stacking are ingredients that hold promising functionality for photovoltaic applications in moiré heterostructures.

We report on our study of the electron interaction effects in topological two-dimensional (2D) materials placed in a quantizing magnetic field. Taking our cue from a recent experimental report, we consider a particular case of bismuthene monolayer with a strong spin-orbit interaction which can be a Weyl semimetal when placed on a specially tuned substrate. Interestingly, we observe that in some Landau levels of this material, the interaction effects are enhanced compared to those for a conventional 2D system and graphene monolayer. Such an enhancement of electron-electron interactions in these materials is largely due to an anisotropy present in the materials. Additionally, the interaction effects can be tuned by changing the coupling to the substrate and the strongest inter-electron interactions are observed when the system is a Weyl semimetal. The observed enhancement of the interaction effects can therefore be an important signature of the 2D Weyl fermions.

The interaction of weak light with correlated Weyl semimetals (WSMs) provides a unique platform to explore nonequilibrium phases close to the ground state where a fragile balance of competing interactions, correlations, and chirality stabilizes the topological state. In this study, we probe ${\left(\mathrm{TaS}{\mathrm{e}}_4\right)}_2\mathrm{I}$, a prototypical correlated WSM, under very weak optical pumping using the circular photogalvanic effect and Raman spectroscopy. Surprisingly, we find that there is strong suppression of chiral response in ${\left(\mathrm{TaS}{\mathrm{e}}_4\right)}_2\mathrm{I}$ under very low optical excitation power. This loss of chirality is attributed to an optically driven reversible topological phase transition into an achiral state distinct from the ground state. The microscopic mechanism of this transformation is supported by fluence-dependent Raman spectra, which reveal a new peak at low pump fluences that disappears above the threshold fluence suggesting the formation of symmetric $\mathrm{TaS}{\mathrm{e}}_4$ structures. This all-optical manipulation of topology at very low powers showcases a way to control the properties of topological materials with strong electron-electron and electron-phonon coupling that are not driven far away from their ground states.

We investigate the electric-field driven power-law random banded matrix (PLRBM) model where a variation in the power-law exponent $\alpha$ yields a delocalization-to-localization phase transition. We examine the periodically driven PLRBM model with the help of the Floquet operator. The level spacing ratio and the generalized participation ratio of the Floquet Hamiltonian reveal a drive-induced weak multifractal (fractal) phase accompanied by diffusive (subdiffusive) transport on the delocalized side of the undriven PLRBM model. On the localized side, the time-periodic model remains localized—the average level-spacing ratio corresponds to Poisson statistics and logarithmic transport is observed in the dynamics. Extending our analysis to the aperiodic Thue-Morse driven system, we find that the aperiodically driven clean long-range hopping model (clean counterpart of the PLRBM model) exhibits the phenomenon of exact dynamical localization on tuning the drive parameters at special points. The disordered time-aperiodic system shows diffusive transport followed by relaxation to the infinite-temperature state on the delocalized side, and a prethermal plateau with subdiffusion on the localized side. Additionally, we compare this with a quasiperiodically driven Aubry-André-Harper model that also undergoes a localization-delocalization transition. Unlike the disordered long-range model, it features a prolonged prethermal plateau followed by subdiffusion to the infinite temperature state, even on the delocalized side.