We propose a mechanism for insulating ferromagnetism in the honeycomb Hubbard model of semiconductor moiré superlattices. The ferromagnetism emerges at a critical charge transfer regime, stabilizing the quantum anomalous Hall state without Hund's coupling. We further note the ferromagnetic exchange applies to general charge transfer systems when breaking particle-hole symmetry.

We demonstrate the feasibility of using optical transmission to determine concentration-dependent hydrogen diffusion coefficients and activation energies of thin metallic glass films over a wide range of temperatures and concentrations. The hydrogen concentration's temporal and spatial profiles are simultaneously extracted without requiring a metal-insulator transition or hydride formation. Different contributions to chemical hydrogen diffusion are quantified and compared. The concentration dependence of the activation energy is in stark contrast to the activation energy found in epitaxial vanadium films of similar thickness. The chemical diffusion is found to increase with hydrogen concentration in the glass, whereas it decreases in the crystalline case driven by differences in the activation energy at concentrations below $0.3\mathrm{H}/\mathrm{M}$ and the thermodynamic factor at concentrations above that. This technique provides detailed insight into the concentration dependence of hydrogen diffusion and constrains theories treating the correlated motion of hydrogen and short-range hydrogen-hydrogen interaction. The method can be applied to other materials that show an optical response to hydrogen uptake.

The ability to tune and switch magnetic anisotropy to a perpendicular orientation is a key challenge for implementing two-dimensional magnets in spintronic devices. H-phase vanadium dichalcogenides $\mathrm{V}{X}_2$ $(X=\mathrm{Te}, \mathrm{Se}, \mathrm{S})$ are promising ferromagnetic semiconductors with large magnetic anisotropy energy (MAE). Recent work has shown that hole doping can switch their easy axis to out-of-plane, although the microscopic origin of this perpendicular magnetic anisotropy (PMA) remains unclear. Using density-functional-theory calculations, we demonstrate that the PMA enhancement arises from first-order spin-orbit coupling (SOC) acting on topmost degenerate valence states with nonzero orbital angular momentum projection $(m_l\ne 0)$. In this case, the ${\widehat{L}}_z{\widehat{S}}_z$ term dominates for perpendicular magnetization orientation, while in-plane orientations involve only weaker, second-order SOC contributions. The increased valence bandwidth leads to depletion of higher-energy states upon hole doping, stabilizing PMA. From this mechanism, we identify two transferable design principles for enhancing magnetic anisotropy under weak hole doping: (i) orbital degeneracy at the valence-band edge protected by point-group symmetry and (ii) finite SOC in the degenerate manifold. Notably, we identify multiple magnetic semiconductors that meet these criteria and display enhanced MAE under hole doping. Furthermore, we show that band engineering can strategically place these degenerate orbitals at the valence band edge, significantly boosting PMA when hole-doped. We also examine trends in ${\mathrm{VTe}}_2$, ${\mathrm{VSe}}_2$, and ${\mathrm{VS}}_2$ to determine the influence of crystal-field splitting, exchange interaction, and orbital hybridization on the valence band edges. These results provide both a fundamental understanding of PMA switching upon hole doping and a transferable strategy for tuning magnetic anisotropy, essential for designing high-performance spintronic materials.

We theoretically explore how light-driven optical phonons can be used to drive magnetic exchange interactions into interesting physical regimes by developing a general theory of spin-phonon pumping in magnetic insulators with nonequilibrium optical phonon distributions, focusing on the diabatic regime where phonon frequencies are much larger than the magnetic interactions. We present several applications of spin-phonon pumping two-dimensional nearest-neighbor Heisenberg, XYZ, and Kitaev models to examine what kind of further neighbor interactions and chiral fields can be generated, and how anisotropic couplings can be enhanced, showing that experimentally accessible nonequilibrium phonon distributions can generically drive significant frustration and realize a variety of spin-liquid regimes. This effect is described for both direct and superexchange mechanisms, and we derive simple geometric rules for which phonon modes are “spin-phonon” active and for which magnetic interactions. Spin-phonon pumping provides an intriguing possibility for preferentially pumping specific magnetic interaction terms. In addition to generating further neighbor interactions, such pumping can lead to increased magnetic anisotropy for initially weakly anisotropic models, and selectively pumping the Kitaev-Heisenberg model can suppress undesirable Heisenberg terms while enhancing Kitaev interactions.

In the context of orbitronics, the rise of the orbital angular momentum generated at light metal interfaces from orbital textures via orbital Rashba-Edelstein effects now represent extraordinary alternatives to the usual heavy-metal spin-based materials. In the wake of very recent experimental results [S. Krishnia et al., Nano Lett. 23, 6785 (2023)], starting from state-of-the-art density functional theory simulations, we provide theoretical insights into the emergence of very strong orbital torques at the Co/Al interface featuring a strong orbital Rashba texture. By using linear response theory, we calculate the exerted orbital torque amplitudes, mainly of intraband field-like character, acting on the ultrathin Co. Moreover, we show that inserting a single atomic plane of Pt between Co and Al is enough to suppress the effect, raising questions about the anatomy of the torque action clearly behaving differently than in the standard way. This work opens routes to the engineering of spintronic devices.

We consider measurement-induced phase transitions in monitored quantum circuits with a measurement rate that fluctuates in time, remaining spatially uniform at each time. The spatially correlated fluctuations in the measurement rate disrupt the volume-law phase for low measurement rates; at a critical measurement rate, they give rise to an entanglement phase transition with “ultrafast” dynamics, i.e., space time ($x,t$) scaling $logx\sim t^{{\psi }_{\tau }}$. The ultrafast dynamics at the critical point can be viewed as a space-time-rotated version of an infinite-randomness critical point; despite the spatial locality of the dynamics, ultrafast information propagation is possible because of measurement-induced quantum teleportation. We identify temporal Griffiths phases on either side of this critical point. We provide a physical interpretation of these phases, and support it with extensive numerical simulations of information propagation and entanglement dynamics in stabilizer circuits. The implications of our results on the general stability of phase transitions and ordered phases to such temporal randomness are discussed.

The multiferroic ferroborate $\mathrm{GdF}{\mathrm{e}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_4$ with huntite-type structure exhibits magnetic ordering below $T_{\mathrm{N}}=38$ K and contains two magnetic subsystems associated with Gd and Fe ions. Competing anisotropies of these subsystems drive a spin-reorientation transition at $T_{\mathrm{SR}}=10.7$ K, switching the ground state from easy-axis to easy-plane. Using antiferromagnetic resonance, we investigate the spin dynamics across this transition. The observed incomplete softening of a magnon mode during both field- and temperature-induced spin-reorientation transitions indicates the first-order nature of the phase transition, which is accompanied by a discontinuous jump in the effective anisotropy field. We reproduce this behavior using a simple model that attributes the jump in the anisotropy field to the presence of an effective fourth-order anisotropy constant, responsible for the discontinuous character of the transition. Remarkably, for in-plane magnetic fields, we identify a new AFMR mode that persists from 12 K up to $T_{\mathrm{N}}$. This mode likely corresponds to the dynamics of a long-period incommensurate state, previously detected by resonant elastic x-ray scattering.

Nonlinear spin excitations open up new options for microwave signals processing due to their ability to transfer data over large distances without distortions. Our work analyzes on the base of Whitham's theory possible types and stability of such nonlinear excitations in the form of the envelope solitary waves for different phases of the monoaxial chiral helimagnet, namely the conical phase, the forced ferromagnetic phase, and the soliton lattice. We found that only envelope solitons of the dark type can propagate in the monoaxial chiral helimagnet. It is shown that these excitations in the conical phase are stable at any wave numbers but only for the conical angles ${\theta }_0\lesssim {48}^{\circ }$ or ${\theta }_0\gtrsim {132}^{\circ }$. For the forced ferromagnetic state stable solitary wave of the given wave number $k$ can arise only within the narrow range centered around $q=D/J$, the width of which is determined by the strength of the easy-plane anisotropy $B$, i.e., $|k-q|\le \sqrt{q^2+2B}$. In the soliton lattice phase, the quasimomentum domain of the stable solitary waves lies symmetrically inside the magnetic Brillouin zone related to the soliton lattice period, which collapses at the critical field $H_C$ of the phase transition to the forced ferromagnetic state. This domain of stability rapidly shrinks in size as the field increases so that no stable solitary waves are possible above the threshold value $\sim 0.48H_C$.

It is widely accepted that the handedness of a resonant mode is an intrinsic property. We show that, by tailoring the polarization and handedness of alternating spin-orbit torques used as the driving force, the polarization state and handedness of inertial resonant modes in an antiferromagnet (AFM) can be actively controlled. In contrast with ferromagnets, whose resonant-mode polarization is essentially fixed, AFM inertial modes can continuously evolve from elliptic through circular to linear polarization as the driving polarization is varied. We further identify an inertia-dependent critical degree of driving polarization at which the mode becomes linearly polarized while its handedness reverses.

Within the framework of the Ginzburg-Landau model, it has been shown that in a composite system consisting of type-I and type-II superconductors with a large coherence length difference ${\xi }_{\mathrm{I}}\gg {\xi }_{\mathrm{II}}$, Abrikosov vortices may appear in the type-I superconductor as superconductivity is restored upon decreasing the magnetic field. The vortices enter the type-I superconductor through its boundary with the vacuum and reach the boundary with the type-II superconductor, which is in the Meissner state. We have found conditions under which the chain of vortices can exist at the I/II interface even at zero magnetic field.

The quantum states of an electron-hole pair in one-dimensional semiconductors under a static electric field are theoretically analyzed using a two-band model with on-site Coulomb interaction. In the absence of static field, the electron and hole are always bound, forming an exciton regardless of the Coulomb interaction strength, in contrast to what occurs in higher-dimensional semiconductors. The static field modifies the wave function of the electron-hole pair, turning bound states into continuum states. However, at low static fields, the linear optical spectra resemble those of the unbiased semiconductor, exhibiting a quadratic redshift of the main exciton absorption line as the field increases. When the static field exceeds a critical threshold, the exciton dissociates and the linear optical spectra exhibit signatures of the Wannier-Stark ladder with squally spaced peaks, making them a valuable tool for experimentally probing exciton dissociation.

The functional renormalization group (FRG) approach for spin models relying on a pseudofermionic description has proven to be a powerful technique in simulating ground-state properties of strongly frustrated magnetic lattices. A drawback of the FRG framework is that it is formulated in the imaginary-time Matsubara formalism and thus only able to access static correlations, a limitation shared with most other many-body approaches. A description of the dynamical properties of magnetic systems is the key to bridging the gap between theory and neutron-scattering spectra. We take the decisive step of expanding the scope of pseudofermion FRG to the Keldysh formalism, which, while originally developed to address nonequilibrium phenomena, enables a direct calculation of the equilibrium dynamical spin structure factors on generic lattices in arbitrary dimensions. We identify the principal features characterizing the low-energy spectra of exemplary zero-, one- and two-dimensional spin-$1/2$ Heisenberg models as well as the Kitaev honeycomb model while identifying current limitations of the method that have to be improved upon.

The interfacial charge transfer and band-edge alignment coupled with strong visible light absorption in the heterostructure is a fundamental tuning criterion for both photovoltaic and photocatalytic efficiencies. We have envisaged how both criteria evolve under the influence of electric field and temperature in a novel van der Waals (vdW) heterostructure composed of the all-inorganic perovskite ${\mathrm{CsSnI}}_3$ and the two-dimensional (2D) MXene ${\mathrm{Zr}}_2{\mathrm{CO}}_2$. Using density functional theory (DFT), we have systematically investigated the structural, electronic, and optical responses of the ${\mathrm{CsSnI}}_3\text{−}{\mathrm{Zr}}_2{\mathrm{CO}}_2$ heterostructure under these external perturbations. Our results reveal that controlled tuning of thermal and electric field conditions effectively modulates the band alignment, enabling enhanced charge separation and extraction, crucial for optimizing photovoltaic performance. Beyond photovoltaics, we demonstrate that the synergistic influence of electric field and temperature promotes favorable band-edge positions relative to water redox potentials, alongside strong visible-light absorption, leading to a theoretical solar-to-hydrogen conversion efficiency approaching 30%. These insights underscore the promise of 2D perovskite-MXene van der Waals heterostructures as multifunctional materials for efficient solar energy harvesting and conversion.

Superconductivity in nickelates apparently takes place in two different Ni oxidation regimes, namely either for infinite-layer-type compounds close to ${\mathrm{Ni}}^{+}$, or for Ruddlesden-Popper materials close to ${\mathrm{Ni}}^{2+}$. The reduced ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_6$ bilayer with a nominal ${\mathrm{Ni}}^{1.5+}$ oxidation state may therefore serve as a normal-state mediator between the two known families of $3d^8$-like and $3d^9$-like superconducting nickelates. Using first-principles many-body theory, we explain its experimental 55 meV charge gap as originating from a different type of correlated (quasi-)insulator. Flat-band electrons of $\mathrm{Ni}\text{−}{d}_{z^2}$ character become localized from scattering with orbital-selective Mott-localized $\mathrm{Ni}\text{−}{d}_{x^2-y^2}$ electrons, by trading in residual hopping energy for a gain in local exchange energy in a ferromagnetic Kondo-lattice scenario. Most importantly, the flat-band electrons offer another route to unconventional superconductivity in nickelates at ambient pressure.

Electron correlation is a main driver of exotic quantum phases and their interplay. The $4Hb\text{−}\text{Ta}{\mathrm{S}}_2$ system, possessing an intrinsic heterostructure of $1T$- and $1H\text{−}\text{Ta}{\mathrm{S}}_2$ monolayers, offers a unique opportunity to control electron correlation by distorting the atomic lattice or tuning interlayer coupling. Here, we investigated intrinsically deformed charge-density waves (CDWs) in the $1T$ layer of $4Hb\text{−}\text{Ta}{\mathrm{S}}_2$ to elucidate and control their effects on flat bands using scanning tunneling microscopy and spectroscopy (STM/S) combined with first-principles calculations. We identified two types of CDW defects: Type 1 has structural distortion and locally suppressed flat bands, while Type 2 features an increased flat band filling factor of intact CDW structure. Density functional theory calculations indicate that a sulfur vacancy in the $1T$ layer distorts the CDW structure and gives rise to a Type 1, whereas a sulfur vacancy in the $1H$ layer reduces the interlayer charge transfer and leads to a Type 2. Furthermore, we demonstrated creating and erasing individual CDW defects via STM manipulation. Our findings provide a pathway to not only tune flat bands but also selectively manipulate the interaction between CDW, the atomic lattice, and interlayer coupling in strongly correlated systems with atomic precision.

As an ideal platform for studying interplays between symmetry, topology, and magnetism, the magnetic topological insulator (MTI) $\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$ has attracted extensive attention. However, its strong $n$-type intrinsic defects hinder the realizations of exotic phenomena. Stimulated by recent discoveries that Ge doping can efficiently tune the position of Fermi level, here we systematically investigate the band evolution and topological phase diagram with doping concentration from MTI $\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$ to strong topological insulator $\mathrm{GeB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$. Different from magnetically doped $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$, the topology here is determined by the competition of two band inversions arising from band folding of two time-reversal invariant momenta between antiferromagnetic and nonmagnetic/ferromagnetic unit cells. By employing a band momentum mapping method based on the first-principles calculations, besides the known MTI phase, remarkably, we find two classes of magnetic Dirac semimetal phases at antiferromagnetic state, two classes of Weyl semimetal phases at the ferromagnetic state and an intermediate trivial state at different doping regions. Interestingly, the trivial state can be tuned into a Weyl phase with two coexisting band inversions and extraordinarily long Fermi arcs by a small strain. Our work reveals diverse topological states with intrinsic quantum phenomena can be achieved with great potential for designing future electronic devices.

The electronic stopping of an ion in a material is at the heart of many application-relevant phenomena. For those applications, a calculation of the average electronic stopping power of the ion in the material of interest is often necessary. Computing such average values with first-principles simulations is not an easy task, especially for materials having ionic and/or covalent bonding, for which simple descriptors like the impact parameter or the nearest-neighbor distance are not representative of the electronic stopping power along a projectile trajectory. In this article, we present a method which can be applied to all kinds of materials to calculate average electronic stopping powers with real-time time-dependent density functional theory simulations (rt-TDDFT). By selecting initial projectile directions inside the crystal based on the material's ground state electronic density the projectile samples along its path, we are able to identify a priori $80\AA$ long trajectories suitable for average stopping power simulations. The method is validated on Si with protons and neutral Si projectiles by comparison to SRIM and experiments. Results with protons are in very good agreement with SRIM/experiments on a wide energy range and show excellent precision. With Si projectiles, calculated stopping powers are also in very good agreement with experiments below a few MeV. At higher energies, the rt-TDDFT methodology we employ fails to reproduce experimental stopping powers of Si projectiles into Si. Finally, although the results with Si projectiles show more deviations than with proton projectiles, the precision of our results is again very satisfactory compared to the existing deviations between different experiments and/or SRIM.

In topological photonics, the bulk-boundary correspondence (BBC) predicts the existence of boundary states through bulk topological invariants, typical examples being quantum anomalous Hall (QAH) phases responsible for boundary optical robust transport and second-order topological phases capable of corner local field enhancement. However, achieving multifold BBC, i.e., the coexistence of first-order and higher-order topological phases in a single photonic system, remains a critical unresolved scientific question. Here, we report the first experimental observation in a gyromagnetic photonic crystal, in which the QAH phase is induced by breaking the time-reversal symmetry and the second-order topological insulator phase is protected by the fourfold rotational symmetry. The photonic chiral edge and magnetic corner modes are confirmed by measuring the field distribution. This work not only enriches the classification of topological matter, but also provides new possibilities for the application of photonic crystals in the field of low-energy information processing.

Flat-band systems, featuring dispersionless electronic bands that localize carriers and amplify interaction effects, provide an ideal platform for realizing exotic quantum states such as superconductivity and fractional quantum Hall phases. However, designing such systems remains a major challenge because of the limited dimensional scope, poor generalizability, and complex constraints of existing construction methods. Here, an efficient and general method for constructing flat-band lattices is proposed, which assembles of compact localized state (CLS) clusters under coherent cancellation regularization, free from dimensional restrictions. Various CLS clusters were collected and designed as initial building blocks with different arrangements for constructing flat-band lattices. The existence of flat-bands in the resulting lattices, including the widely studied zero-energy flat-bands and the so-called “Yin-Yang” flat-bands, was confirmed based on tight-binding models. Finally, a series of graphdiyne allotropes with flat-bands were designed, based on these lattice models, including a superconductor (${\mathrm{C}}_{44}$) and a ferromagnetic metal (${\mathrm{C}}_{56}$) with flat-bands that remain at the Fermi level despite the inclusion of multiple-neighbor interactions under first-principles calculations. These results demonstrate the feasibility of designing flat-band materials by starting from CLS cluster analysis and lattice model construction. Potential metal-free light-element superconductors and magnetic materials can be designed under the guidance of the CLS theory.