Motivated by twisted transition metal dichalcogenides (TMDs), we study an extended Hubbard model with both onsite and off-site repulsive interactions, in which Mott insulating states with concomitant charge order occur at fractional fillings. To resolve the charge ordering as well as the fate of the local moments formed thereby, we perform large-scale density matrix renormalization group calculations on cylindrical geometries for several filling fractions and ranges of interaction strength. Depending on the precise parameter regime, both antiferromagnetically ordered as well as quantum-disordered states are found, with a particularly prominent example being a quantum spinliquid type ground state on top of charge ordering with effective kagomé geometry. We discuss the different mechanisms at play in stabilizing various electronic and magnetic states. The results suggest that moiré TMDs are a promising venue for emergent quantum magnetism of strongly interacting electrons.

The interaction between surface-lying nitrogen-substituted fullerenes (radical azafullerene, ${\mathrm{C}}_{59}{\mathrm{N}}^{•}$) with subsurface negative nitrogen-vacancy complexes $\left({\mathrm{NV}}^{-}\right)$ in diamond is investigated using first-principles calculations. We consider $(2\times 1)$ reconstructed (001) oriented diamond surfaces with both H- and F-surface termination. The charge stability of ${\mathrm{NV}}^{-}$ when in close proximity to both the nearby surface and the spin active azafullerene is discussed, in the context of diamond band bending arising from surface-induced changes in electron affinity (EA). In the case of the hydrogenated surface, the system spin is quenched, yielding a negatively charged azafullerene $\left({\mathrm{C}}_{59}{\mathrm{N}}^{-}\right)$ and neutrally charged ${\mathrm{NV}}^0$ as the most stable electronic configuration. In contrast, fluorinating the surface favors the negatively charged ${\mathrm{NV}}^{-}$ and conserves the ${\mathrm{C}}_{59}{\mathrm{N}}^{•}$ neutrality and stabilizes uncompensated free spins. This opposing behaviour is attributed to surface charge doping emerging from different band bending effects associated with the surface EA. This study is consistent with experimentally observed photoluminescence quenching, and shows that surface passivation by fluorination could efficiently tackle the problem of charge transfer between adsorbed molecules and shallow NV centers.

The transient Pauli blocking effect offers a promising route for achieving ultrafast optical switching in semiconductors, enabling a rapid switching from an initially opaque state to a relatively transparent state upon photoexcitation. Herein, we demonstrate broadband ultrafast optical switching in degenerate InN thin films, spanning the visible to near-infrared spectral range, using pump-probe transient transmittance measurements. To elucidate the underlying physical mechanism, we perform probe-energy-resolved analysis for ultrafast dynamics, and we develop a theoretical model based on a quasiequilibrium Fermi-Dirac distribution. The model successfully captures the experimental transients and yields an electron-phonon coupling constant of $1.0\times {10}^{17}{\mathrm{W}/\mathrm{m}}^3\mathrm{K}$, along with an electronic specific-heat coefficient ranging from 1.52 to $2.02\mathrm{mJ}/\mathrm{mol}{\mathrm{K}}^2$, which allows direct prediction of the spectral switching window. Notably, we demonstrate that the Pauli blocking effect can be induced solely by a laser-excitation-driven rise in electronic temperature, without requiring significant carrier injection into the conduction band in degenerate semiconductors. These findings offer new insights for designing ultrafast optical modulators, shutters, and photonic devices for next-generation communication and computing technologies.

Monolayer ${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$ has been considered as the first large-gap Kane-Mele quantum spin Hall insulator and is therefore expected to exhibit distinctive topological phases and physical properties. Here, we use monolayer ${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$ as the work substance to demonstrate a quantum Stirling cycle driven by an external electric potential and an off-resonant light. Through theoretical analyses and numerical calculations, we find that monolayer ${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$ is an excellent material to perform a quantum Stirling cycle, while on the other hand, the quantum Stirling cycle can be used to characterize the topological phase transitions (TPTs) in monolayer ${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$. The results show that all four types of quantum thermal machine modes can be realized, including heat engine, refrigerator, Joule pump, and cold pump in the quantum sense. Moreover, it is easy to perform the transform among these operating modes by adjusting the electric potential, off-resonant light, and reservoir temperatures, illustrating the machine's high versatility. Furthermore, when operated as a quantum heat engine, the device achieves an efficiency that significantly exceeds that of Xene-based quantum Stirling engines. Most importantly, we demonstrate that across all four operational modes, the extrema in the work output provide clear signatures for detecting the TPTs in monolayer ${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$. Additionally, in the heat engine mode, the TPTs can be detected at both low and high temperatures, in sharp contrast to graphenelike Xene monolayers such as stanene, where quantum Stirling engines fail to detect the TPTs at high temperatures.

Three-dimensional Dirac semimetal ${\mathrm{Cd}}_3{\mathrm{As}}_2$ exhibits excellent properties including high carrier mobility and a linear dispersion relation, with promising applications in ultrafast and broadband photodetection and optical communications. Here, we demonstrate that the transient photovoltage in ${\mathrm{Cd}}_3{\mathrm{As}}_2$ nanobelt field-effect transistors can be greatly enhanced in devices with Schottky junctions. In such devices, the photovoltage polarity reverses at high pump fluence, persisting up to room temperature. The photovoltage can be modulated over 300% within a few picoseconds, providing a unique material platform for potential ultrafast optical switching applications. In contrast, the modulation is only up to 30% in devices with Ohmic contacts. We attribute the photovoltage reversal to the competing photothermoelectric and lateral photo-Dember effects, supported by numerical modeling.

High-$Q$ optical nanocavities are fundamental to modern optics and photonics, enabling enhanced light-matter interactions. Previous studies have demonstrated that high-$Q$ supercavity modes can be constructed within a single dielectric resonator by leveraging quasibound states in the continuum. However, their $Q$ factors are limited to a few tens or hundreds when such a resonator is subwavelength scale. Here, we propose a general recipe for achieving high-$Q$ resonances with $Q>10000$ in triple subwavelength dielectric resonators. This is realized through destructive interference between two resonant modes, optimized by structural tuning. Multipole analysis confirms that destructive interference across radiation channels suppresses loss, forming the ultrahigh-$Q$ states. These resonances can be efficiently excited by azimuthally polarized light due to improved mode overlap. As a key application, we demonstrate efficient second harmonic generation under this excitation, achieving a conversion efficiency of 1.64% at an incident intensity of $100{\text{MW/cm}}^2$. Our results may find exciting applications in developing ultracompact photonic devices with superior performance.

${\mathrm{BaV}}_{10-x}{\mathrm{Ti}}_x{\mathrm{O}}_{15}$ with $x=0.15$ exhibits coexisting orbital-ordered (OO) and orbital-disordered (OD) phases as well as slow dynamics during the phase transition from the OD phase to the OO phase. Owing to these slow dynamics and the fact that these two phases exhibit antiferromagnetic ordering with different transition temperatures, two-phase coexisting states can be considered magnetic nanoparticles in a crystal, whose size can be controlled relatively easily. Measuring the magnetic susceptibility of these states reveals that the Neel temperature of the OO phase decreases as the size of the OO phase decreases, similarly to conventional magnetic nanoparticles.

Interference-induced quantum corrections to electronic transport provide a powerful route to probe localization/antilocalization phenomena in condensed-matter systems. Here, we report quantum-interference effects in a three-dimensional (3D) system, namely, the full Heusler alloy ${\mathrm{V}}_2\mathrm{NbAl}$, that are consistent with the presence of chiral spin correlations at low temperatures (T). We observe a crossover from weak localization (WL) to weak antilocalization (WAL) together with a pronounced low-field Hall anomaly, often discussed in terms of a topological Hall contribution, which is attributed to local symmetry breaking arising from intrinsic antisite disorder. The extracted phase coherence length ($l_{\varphi }\simeq 23$ nm at 2 K) is significantly larger than the mean free path, confirming the quantum diffusive nature of charge transport. The present study explores the interplay between disorder-driven noncollinear magnetic correlations and unconventional quantum transport in ${\mathrm{V}}_2\mathrm{NbAl}$. Ab initio calculations reveal a metallic state with fully compensated ferrimagnetic (CF) behavior for the B2-disordered structure (as confirmed by XRD data). Overall, our results highlight how intrinsic disorder in a 3D-compensated Heusler alloy can give rise to unconventional quantum-interference effects and Hall transport behavior.

We show that vortices with circulating current, related with odd-frequency triplet pairing, appear in Josephson junctions where the barrier is a weak ferromagnet with strong spin-orbit coupling. By symmetry analysis we show that there is an additional term—a rotary invariant—in the superconducting free energy which allows for magnetoelectric effects even when the previously considered Lifshitz invariant vanishes. Using a microscopic model based on a modified Usadel equation incorporating those effects, we show that the size, shape, and position of these vortices can be controlled by manipulating Rashba spin-orbit coupling in the weak link, via gates, and we suggest that these vortices could be detected via scanning magnetometry techniques. We also show that the transverse triplet components of the pairing amplitudes can form a texture.

We propose a theory for bilayer nickelate materials, where a large tetragonal field—intrinsic or induced by epitaxial strain—lifts the orbital degeneracy and localizes the $3z^2-r^2$ orbital states. These states host local spins $S=1/2$ bound into singlets by strong interlayer coupling, and their dynamics is described by weakly dispersive singlet-triplet excitations (“triplons”). The charge carriers occupy the wide bands of $x^2-y^2$ symmetry, and their Cooper pairing is mediated by the high-energy triplon excitations. As the $x^2-y^2$ band filling increases, i.e., moving further away from the ${\mathrm{Ni}}^{3+}$ valence state, the indirect Ruderman-Kittel-Kasuya-Yosida interactions between local spins induce spin-density-wave order via triplon condensation. Implications of the model for compressively strained ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ films and electron-doped oxychloride ${\mathrm{Sr}}_3{\mathrm{Ni}}_2{\mathrm{O}}_5{\mathrm{Cl}}_2$ are discussed.

An important result in the theory of quantum control is the “universality” of 2-local unitary gates, i.e., the fact that any global unitary evolution of a system of $L$ qudits can be implemented by composition of 2-local unitary gates. Surprisingly, recent results have shown that universality can break down in the presence of symmetries: in general, not all globally symmetric unitaries can be constructed using $k$-local symmetric unitary gates. This also restricts the dynamics that can be implemented by symmetric local Hamiltonians. In this paper, we show that obstructions to universality in such settings can in general be understood in terms of superoperator symmetries associated with unitary evolution by restricted sets of gates. These superoperator symmetries lead to block decompositions of the operator Hilbert space, which dictate the connectivity of operator space, and hence the structure of the dynamical Lie algebra. We demonstrate this explicitly in several examples by systematically deriving the superoperator symmetries from the gate structure using the framework of commutant algebras, which has been used to systematically derive symmetries in other quantum many-body systems. We clearly delineate two different types of nonuniversality, which stem from different structures of the superoperator symmetries, and discuss its signatures in physical observables. In all, our work establishes a comprehensive framework to explore the universality of unitary circuits and derive physical consequences of its absence.

Antiferromagnets have attracted extensive attention in spintronics and valleytronics due to their intrinsic advantages, including the absence of stray field and ultrafast dynamics. However, the lack of spin splitting in conventional antiferromagnets severely limits their spintronic applications, particularly the realization of the anomalous valley Hall effect (AVHE). To overcome this limitation, we propose a general strategy by constructing an interlayer $A$-type antiferromagnetic (AFM) bilayer from a ferromagnetic (FM) valley-polarized monolayer. An out-of-plane electric field is applied to break the combined spatial inversion and time reversal $\widehat{P}\widehat{T}$ symmetry, thereby inducing spin splitting and enabling the AVHE. To validate this paradigm and provide a concrete material platform, we perform first-principles calculations to systematically investigate the spin-valley physics of monolayer and bilayer $\mathrm{Gd}{\mathrm{I}}_2$. The monolayer is FM, while the AB1-stacked bilayer stabilizes in an interlayer $A$-type AFM state without spin splitting. Both systems display strong intrinsic valley polarization, with valley splittings of 145.9 and 140.2 meV for the monolayer and bilayer, respectively; both values are sufficient to support room-temperature valleytronic applications. For bilayer $\mathrm{Gd}{\mathrm{I}}_2$, even a tiny out-of-plane electric field breaks the $\widehat{P}\widehat{T}$ symmetry, inducing the full-Brillouin-zone spin splitting and enabling the AVHE, while the interlayer AFM coupling and valley splitting remain robust. As the electric field increases further (exceeding $0.04\mathrm{V}/\AA$), an interlayer AFM-to-FM transition occurs, accompanied by an enhancement of the valley splitting due to the strengthened spin-orbit coupling. All of the above phenomena have also been demonstrated in bilayer $\mathrm{Y}{\mathrm{I}}_2$. These results not only deepen the understanding of spin-valley coupling in layered AFM systems but also highlight a universal paradigm for designing AFM bilayer systems with controllable spin splitting, valley splitting, and the AVHE.

Thin film nanomagnets show promise for a wide range of applications, such as in computing, data storage, sensors, and biomedical devices. Elongated single-domain nanomagnets are bistable, with the magnetization of its two stable states pointing in opposite directions along its long axis. We can switch the magnetization between the bistable states of the nanomagnet by applying an external field. Here, we investigate the switching characteristics of single-domain nanomagnets of different shapes, sizes, and aspect ratios, both in simulation and in experiment. We use a phenomenological approach to describe the angle dependence of the field required to switch the nanomagnets from one bistable state to the other. Our simulations and experimental results correspond well, and show that precise tuning of the angular switching characteristics is possible in a number of different ways. Such tuning of switching characteristics on a single-magnet level unlocks intriguing possibilities in nanomagnet device design, and could further pave the way for a variety of new nanomagnet-based technologies.

The frustrated magnet ${\mathrm{Bi}}_2{\mathrm{Fe}}_4{\mathrm{O}}_9$ has been reported to exhibit complex spin dynamics coexisting with conventional spin wave excitations. The magnetic ${\mathrm{Fe}}^{3+}$ ($S=5/2$) ions are arranged into a distorted two-dimensional Cairo pentagonal lattice with weak couplings between the layers, developing long-ranged noncollinear antiferromagnetic order below 245 K. In order to enable studies and modeling of the complex dynamics close to $T_N$, we have reexamined the magnetic excitations across the complete energy scale ($0<\hslash \omega <90\mathrm{meV}$) at 10 K. We discover two distinct gaps, which can be explained by introducing, respectively, easy axis and easy plane anisotropy on the two unequivalent Fe sites. We develop a refined spin Hamiltonian that accurately accounts for the dispersion of essentially all spin-wave branches across the full spectral range, except around 40 meV, where a splitting and dispersion are observed. Polarization analysis shows that the system has magnetic anisotropic fluctuations, consistent with our model. A continuum of scattering is observed above the spin wave branches and is found to principally be explained by an instrumental resolution effect. The full experimental mapping of the excitation spectrum and the refined spin Hamiltonian provides a foundation for future quantitative studies of spin waves coexisting with unconventional magnetic fluctuations in this frustrated magnet found at higher temperatures.

Temperature dependent magnetic properties of superconductor-ferromagnet-superconductor (SC/FM/SC) trilayers are studied both experimentally and theoretically, with a focus on ferromagnetic resonance (FMR). The influence of the SC and FM layer thicknesses on the FMR field is examined. To differentiate the mechanisms involved, we additionally investigate structures containing nonmagnetic metallic (M) or insulating (I) spacers (SC/FM/M/SC or SC/FM/I/SC). All the studied multilayers show large reductions in the FMR field below the critical temperature of the SC, except the system containing an insulating spacer (SC/FM/I/SC). This SC-induced FMR-shift (resonance field/frequency) is larger for thicker SC as well as FM layers, reaching a saturation value for very large thicknesses. To explain the measured results, an analytical model is developed, in which the FM-magnetization precession modulates the magnetic flux in the system, thereby inducing an alternating supercurrent in the SC, which in turn produces a dynamic back-action magnetic field on the FM that shifts its resonance frequency. The model considers closed current loops, where the FM layer conductively links the supercurrents flowing in the opposite directions in the two outer SC layers. Our results provide a practical route for increasing the operating frequency of magnonic devices.

A measurement-only (quantum) circuit (MOC) gives the possibility to realize the states with rich entanglements, topological orders, and quantum memories. This work studies the MOC, in which the projective-measurement operators consist of stabilizers of the toric code and competitive local Pauli operators. The former correspond to terms of the toric code on a triangular lattice and the latter to external magnetic and electric fields. We employ an efficient numerical stabilizer algorithm to trace evolving states undergoing phase transitions. We elucidate the phase diagram of the MOC system with the observables such as topological entanglement entropy (TEE), disorder parameters of 1-form symmetries, and emergent logical operators. We clarify the locations of the phase transitions through the observation of the above quantities and obtain precise critical exponents to examine if the observables exhibit the critical behavior simultaneously under the MOC and transitions belong to the same universality class. In contrast to the toric code (TC) Hamiltonian system and toric code MOC on a square lattice, the system on the triangular lattice is not self-dual nor bipartite and then coincidence by symmetries, such as critical behaviors across the TC and Higgs/confined phase, does not takes place. Then, the toric code MOC on the triangular lattice provides us a suitable playground to clarify the mutual relationship between the TEE, spontaneous symmetry breaking of the 1-form symmetries, and emergence of logical operators. Obtained results indicate that toric code MOC on the triangular lattice exhibits a few distinct phase transitions with different location and critical exponents and some of them are closely related with the two-dimensional percolation transition.

The microscopic mechanisms of heat transport in ${\mathrm{YAgTe}}_2$ are explored through advanced first-principles calculations combined with self-consistent phonon theory. The computed normal mode resolved residuals display a characteristic W-shaped profile, indicating significant quartic anharmonicity. Both cubic and quartic anharmonicities are thoroughly considered, including their contributions to phonon frequency renormalization and linewidth broadening. Furthermore, both diagonal and off-diagonal components of the heat flux operator are explicitly evaluated in the calculation of lattice thermal conductivity. The results are in excellent agreement with experimental values near room temperature. Our study identifies the primary cause of the low lattice thermal conductivity in ${\mathrm{YAgTe}}_2$ as strong anharmonicity from the rattling effect of Ag atoms, which leads to substantial three-phonon and four-phonon scattering. Notably, the competition between phonon hardening (described by the loop diagram) and phonon softening (due to the bubble diagram) plays a significant role in influencing particlelike propagation, particularly affecting group velocity and energy conservation. We also confirm that ${\mathrm{YAgTe}}_2$ with space group $P\overline{4}{2}_{1}m$ remains dynamically stable at elevated temperatures. There are minor discrepancies between our high-temperature theoretical predictions and experimental results, which may be attributed to the presence of mixed phases in the experiments, whereas our calculations assume a perfect lattice. A reassessment of the thermoelectric performance shows that, for $n$-type doping, the maximum $ZT$ of 2.11 is achieved at a carrier concentration of $9\times {10}^{19}{\text{cm}}^3$ at 700 K, while for $p$-type doping, the maximum $ZT$ is 1.34 at a carrier concentration of $4\times {10}^{19}{\text{cm}}^3$ at 700 K. These findings provide valuable insights for guiding future experimental studies.

It is now well understood that non-Kitaev spin interactions can be added to the Kitaev quantum spin liquid by applying external fields. Recent years have seen intensive discussion on the possible phase transitions that these spin interactions induce. In this paper, we will show through the perturbation theory the possibility of accessing a gapped spin liquid phase with a higher Chern number through, in contrast to the cases studied in literature, a continuous phase transition. Such a transition may be induced by external tuning parameters such as electric field and hydrostatic pressure.

Realizing topological spin textures such as skyrmions in two-dimensional magnets has attracted significant attention in recent years. Most efforts focus on breaking inversion symmetry to induce Dzyaloshinskii-Moriya interaction (DMI), which is often difficult to implement experimentally. Here, using first-principles calculations and Monte Carlo simulations, we predict that strain can drive the spontaneous formation of anti-biskyrmions (A2SK) in the centrosymmetric $\mathrm{V}{\mathrm{I}}_2$. We identify the bond-dependent Kitaev interaction—together with exchange frustration—as the key ingredient that stabilizes a topologically nontrivial lattice without DMI. The same mechanism can explain the A2SK recently predicted in monolayer $\mathrm{Ni}{\mathrm{I}}_2$. Our work thus leads to a better understanding of the formation of topological spin textures in centrosymmetric metal dihalides, and establishes Kitaev interaction as a new route to engineer such exotic states.