Spin relaxation of localized charge carriers in semiconductors is primarily governed by hyperfine interaction with the surrounding nuclear spin bath. While this mechanism is well-established in III-V bulk materials and quantum dots, its critical role in metal-halide perovskites (MHPs) has only recently emerged. Their inverted band structure induces an unusual hierarchy of hyperfine couplings, with hole interactions dominating electron interactions, particurlaly in Pb-based perovskites. Here, we adapt a spin relaxation model—originally developed for muon spin spectroscopy—to provide an exact description of longitudinal spin relaxation for localized carriers across arbitrary hyperfine correlation times. This approach is motivated by recent experimental evidence in ${\mathrm{MAPbI}}_3$, which places carrier spins in an intermediate correlation regime, where conventional monoexponential approximations fail. Our analysis reveals distinct hyperfine relaxation channels: electrons couple primarily to halogen nuclei, whereas hole interactions are governed by metal nuclei. This leads to a key prediction—perovskites with lighter halogens and metal cations exhibit significantly extended spin relaxation times. Applied to time-resolved Faraday rotation data obtained from two perovskite samples, our model extracts key microscopic parameters—including the carrier localization volume and hyperfine correlation time—demonstrating the necessity for the exact dynamical solution over a single-exponential approximation. These findings provide microscopic insight into hyperfine-driven spin relaxation in MHPs and establish a robust framework for characterizing carrier localization and spin dynamics.

Understanding how stacking controls the nonlinear optical response of two-dimensional materials is key to designing van der Waals heterostructures with tailored functionalities. Here, we establish a comprehensive symmetry-based framework mapping the structural configuration of ${\mathrm{MoS}}_2$ bilayers across four point groups ($D_{3h}, D_{3d}, C_{3v}, C_3$) to their second-order susceptibility tensor ${\chi }^{\left(2\right)}$. Using group-theory arguments benchmarked against first-principles response-function calculations, we demonstrate how symmetry breaking controls the activation and suppression of individual tensor elements in these systems. We show that the emergence of the in-plane component ${\chi }_{xxx}$ in twisted configurations ($C_3$ group) induces a rigid azimuthal rotation of the second-harmonic generation polar lobes, which remains frequency-independent across the entire optical spectrum, locking to half of the structural twist angle. Our findings establish a direct, wavelength-independent optical route for twist-angle determination and provide a clear roadmap for engineering nonlinear optical responses in two-dimensional materials.

The azimuthal angular dependence of the in-plane magnetoresistance provides a powerful probe for detecting rotational symmetry breaking in superconductors, which has significant implications for understanding the superconducting pairing symmetry and the intertwined orders. However, in two-dimensional or layered superconductors with strong anisotropy between in-plane and out-of-plane critical fields, measuring the in-plane magnetoresistance is particularly challenging as it is extremely sensitive to the field misalignment. Here we perform systematic electrical transport measurements on ultrathin FeSe films grown on $\mathrm{SrTi}{\mathrm{O}}_3$ substrates. By remounting the same FeSe film to change the field misalignment, two qualitatively different angular dependences of the in-plane magnetoresistance are observed in the superconducting transition region, indicating the extreme sensitivity of the measurement to field misalignment. The observed anisotropic in-plane magnetoresistance can be well captured by our geometric model considering the field misalignment effect. Our work provides a practical framework for disentangling the misalignment effect and investigating the in-plane rotational symmetry breaking in two-dimensional or layered superconductors.

We study coherent electric manipulation of magnetic field induced localized states at a double kink of a helical edge in a HgTe/CdHgTe quantum well. An in-plane magnetic field opens a gap in the one-dimensional (1D) edge spectrum, while changes in the edge orientation generate localized in-gap states at the kinks. We show that, for suitable geometry and magnetic field direction, two such states form an effective lithographically defined two-level subsystem. Using an edge-state model that includes both the localized levels and the fully propagating 1D edge states outside the magnetic field induced gap, we calculate the electric-dipole matrix elements and solve the time-dependent problem under resonant driving. The resulting dynamics exhibits Rabi oscillations with linear frequencies of 20–40 GHz for realistic parameters. We find that the propagating 1D edge states provide a leakage channel whose strength is strongly controlled by the driving amplitude: reducing the electric field suppresses leakage below the percent level while preserving GHz-scale coherent oscillations. These results establish a route from magnetic field induced corner-state physics to electrically driven two-level dynamics in a realistic two-dimensional topological-insulator edge geometry.

We performed electrical resistivity and ${}^{13}\mathrm{C}$ nuclear magnetic resonance (NMR) measurements on $\lambda \text{−}{\left(\mathrm{BEDSe}\text{−}\mathrm{TTF}\right)}_2{\mathrm{FeCl}}_4$ under various pressures to obtain insights into the effect of $\pi \text{−}d$ interactions on the antiferromagnetic (AF) phase. The resistivity shows that the insulating state is maintained up to 2.0 GPa. ${}^{13}\mathrm{C}$ NMR measurements reveal that the AF transition occurs at 28, 31, and 23 K under 0.5, 1.0, and 2.0 GPa, respectively, indicating that a single AF phase in $\lambda \text{−}{\left(\mathrm{BEDSe}\text{−}\mathrm{TTF}\right)}_2{\mathrm{FeCl}}_4$ is observed under various pressures, whereas two AF phases were observed in $\lambda \text{−}{\left(\mathrm{BEDSe}\text{−}\mathrm{TTF}\right)}_2{\mathrm{GaCl}}_4$. The difference in phase diagrams between the magnetic anion salt, $\lambda \text{−}{\left(\mathrm{BEDSe}\text{−}\mathrm{TTF}\right)}_2{\mathrm{FeCl}}_4$, and the nonmagnetic anion salt, $\lambda \text{−}{\left(\mathrm{BEDSe}\text{−}\mathrm{TTF}\right)}_2{\mathrm{GaCl}}_4$, is attributed to $\pi \text{−}d$ interactions. These results provide insight into a possible microscopic role of $\pi \text{−}d$ interactions in the Fe salt. The magnetic properties of $\lambda \text{−}{\left(\mathrm{BEDSe}\text{−}\mathrm{TTF}\right)}_2{\mathrm{FeCl}}_4$ are described by the isolated cluster comprising two dimers and two anions with positive and negative $\pi \text{−}d$ interactions, suggesting antiferromagnetic coupling between two dimers in the cluster. We also discuss the possible AF structures of $\lambda \text{−}{\left(\mathrm{BEDSe}\text{−}\mathrm{TTF}\right)}_2{\mathrm{FeCl}}_4$.

We report on the longitudinal magnetoresistance (MR) in thin metal films on an Ising-type antiferromagnetic insulator, ${\mathrm{Na}}_5{\mathrm{Co}}_{15.5}{\mathrm{Te}}_6{\mathrm{O}}_{36}$ ($\mathrm{NCTO}$). Steep changes in the MR spectra with hysteresis were observed at spin-flip transitions driven by magnetic fields applied along the easy axis of the $\mathrm{NCTO}$ crystal. The MR jumps almost follow steplike changes in magnetization at the spin-flip transition. At very low temperatures where $\mathrm{Co}$ moments are partially frozen, the MR anomalies exhibit a tunnel-magnetoresistance-like shape. The observed MR anomalies at the spin-flip transition are attributed to strain effects via magnetostriction upon the magnetic-structure change of the $\mathrm{Co}$ nets in $\mathrm{NCTO}$ because similar MR jumps are observed in $\mathrm{Pt}/\mathrm{NCTO}, {\mathrm{Pt}/\mathrm{SiO}}_x/\mathrm{NCTO}$, and $\mathrm{Cu}/\mathrm{NCTO}$ heterostructures. Interestingly, we find that the high-field slopes of the MR spectra show opposite signs between $\mathrm{Pt}/\mathrm{NCTO}$ and $\mathrm{Cu}/\mathrm{NCTO}$ at low temperatures. Because similar MR spectra are observed for ${\mathrm{Pt}/\mathrm{SiO}}_x/\mathrm{NCTO}$, in which the interface magnetic interaction is negligible, the weak antilocalization due to the strong spin-orbit interaction of the Pt films is likely to contribute to the low-temperature MR.

Stacking polymorphism is a common characteristic of van der Waals layered materials which can substantially modify their physical properties. Here, based on first-principles calculations combined with electron and phonon transport theories, we systematically investigate the thermodynamic stability, electronic structure, lattice dynamics, and thermoelectric performance of ${\mathrm{Sc}}_2{\mathrm{Si}}_2{\mathrm{Te}}_6$ with three high-symmetry stacking sequences, namely, AA, AB, and ABC. We find that the AA- and AB-stacked structures are nearly degenerate in energy with the experimentally reported ABC phase and that the maximum sliding barrier among these stacking sequences is only about 10 meV/atom, thereby accounting for the stacking faults observed experimentally. These three stacking sequences exhibit distinct electronic structures, with the conduction band minimum being highly sensitive to the stacking sequence. As a consequence, the conduction band degeneracies are 12, 2, and 8 for the ABC, AA, and AB stackings, respectively, leading to markedly different electronic transport properties near the band edge. The lattice thermal conductivity is governed primarily by three-phonon (3ph) scattering, whereas four-phonon scattering provides an additional reduction, particularly in the ABC stacking. Among the three structures, the AB stacking exhibits the lowest lattice thermal conductivity owing to its stronger 3ph scattering and lower phonon group velocity. As a result, the maximum thermoelectric figure of merit $ZT$ is achieved in the AB structure, followed closely by the ABC structure, whereas the AA structure shows a substantially reduced value. These results demonstrate that the stacking sequence exerts a non-negligible influence on the thermoelectric performance of ${\mathrm{Sc}}_2{\mathrm{Si}}_2{\mathrm{Te}}_6$.

From the condensed matter physics perspective, the most natural single orbital tight-binding Hamiltonians, and hence the most widely studied, contain two fermionic species, corresponding to spin-up and spin-down electrons. In cold atom systems, however, SU(N) symmetry, in which $N>2$ fermionic species reside within a single band, also occurs. To understand such experiments, the SU(N) Hubbard model has been increasingly studied. Here we present determinant quantum Monte Carlo simulations of the SU(N) Holstein Hamiltonian in which $N$ fermionic species couple to a single local phonon mode. We show that at half filling it has an insulating charge density wave phase (CDW) at low temperatures in which empty sites alternate with sites with $N$ particles. We determine the $N=3$ CDW phase diagram in the temperature $T$ versus electron-phonon coupling $\alpha$ plane at fixed phonon frequency ${\omega }_0$ and half filling $\rho =1.5$. The critical temperature $T_c$ for $N=3$ can be as high as two times the maximum attainable for $N=2$. We also obtain the $N$ dependence of $T_c$ for a representative, fixed, ${\omega }_0$ and $\alpha$.

The rattling chain model has provided a useful phenomenological guideline for identifying $A{M}_6{X}_6$-type kagome charge density wave (CDW) materials. In this work, we performed a comprehensive experimental and theoretical investigation of ${\text{TiV}}_6{\text{Sn}}_6$, a kagome CDW material predicted by the rattling chain model. However, resistivity, magnetization, and specific heat measurements reveal no evidence of a CDW transition in ${\text{TiV}}_6{\text{Sn}}_6$. Furthermore, unlike its rare-earth-based sister compound $R{\text{V}}_6{\text{Sn}}_6$ ($R$ = Sc, Y, Sm, and Gd-Lu), phonon calculations show that ${\text{TiV}}_6{\text{Sn}}_6$ exhibits no imaginary phonon modes under either compressive or tensile strain. The band structure and crystal orbital Hamilton population (COHP) curves of ${\text{TiV}}_6{\text{Sn}}_6$ exhibit recognizable differences from those of $R{\text{V}}_6{\text{Sn}}_6$. Further analysis attributes these differences to the higher electronegativity of Ti relative to the rare-earth elements. The enhanced electronegativity promotes mixed ionic-covalent bonding, where the covalent character suppresses the rattling motion of Ti, ultimately precluding the formation of a CDW. Our findings suggest that the electronegativity of the filler atom, in addition to its ionic radius, should be incorporated as a supplementary factor into the rattling chain model.

We theoretically develop a machine-learning-based method for inferring the random onsite-energy fluctuations in disordered two-dimensional (2D) generalized Hubbard models underlying spin-qubit platforms, using vision-based neural networks trained on tensor-network-generated charge-stability data. We investigate theoretically the feasibility of estimating the unknown disorder-induced chemical-potential fluctuations using simulated data generated from Hubbard-model-based charge stability diagrams of small 2D qubit arrays in the presence of random disorder. When all the Hubbard-model parameters are treated as unknowns because of the presence of random disorder, the onsite energy and onsite interaction are predictable with $R^2>0.90$ and $R^2>0.97$, respectively, for both the $3\times 3$ and $5\times 5$ qubit arrays, from our machine-learning algorithm using only observables from a $3\times 3$ region centered on the target dot. Thus, the most relevant experimental parameter for tuning spin qubits, the onsite energy, can be inferred reliably even in the fully disordered setting for computationally challenging $5\times 5$ arrays, which are already beyond the size of current semiconductor-qubit experiments.

Transition-metal chalcogenide monolayers, with their intrinsic out-of-plane asymmetry and strong spin-orbit coupling, provide an exceptional platform for emergent excitonic and valley-dependent optoelectronic phenomena. Here, we present a comprehensive many-body investigation of the electronic structure and excitonic properties of the CrSO Janus monolayer using density functional theory, quasiparticle $G_0{W}_0$ calculations, and the spinorial Bethe-Salpeter equation. The quasiparticle band structure reveals an indirect fundamental gap of 0.94 eV and a direct optical gap of 1.97 eV, highlighting pronounced many-body renormalization effects. Spinorial $G_0{W}_0\_\mathrm{BSE}$ calculations uncover a strongly correlated excitonic manifold composed of both dark and bright excitons with large binding energies. In particular, a spin-forbidden dark exciton ${\xi }_0^d$ emerges at $1.58\mathrm{eV}$, followed by a spin-allowed bright exciton ${\xi }_1^b$ at $1.70\mathrm{eV}$, yielding a sizable spin-orbit-induced dark-bright splitting of 119 meV. Analysis of the excitonic spin texture reveals a dominant triplet character for the dark state and a mixed singlet-triplet configuration for the bright exciton. The reciprocal-space exciton wave functions exhibit strong valley localization around the $K^{+}$ and $K^{-}$ valleys, demonstrating the valley-selective nature of the optical transitions. Furthermore, electron-hole interactions dramatically reshape the optical response, enhancing the absorption intensity by up to 7.4 times relative to the independent-particle picture and generating broadband optical activity spanning 1.77–3.10 eV across the visible and near-ultraviolet regions. These results establish the CrSO Janus monolayer as a promising excitonic material for next-generation valleytronic, optoelectronic, and visible-light-driven energy-conversion technologies.

The two-dimensional metallic quantum critical problem is a long-standing puzzle that is widely believed to hold the key to resolving ubiquitous non-Fermi-liquid behavior in strongly correlated electronic systems. In this study, we present a nonperturbative renormalization group analysis of the metallic Ising-nematic quantum critical point in two dimensions, formulated directly around an intrinsically nonlocal infrared boson propagator. Rather than treating the anomalous dynamical critical exponent $a$ as a fixed phenomenological parameter, we regard it as an intrinsic component of the fixed-point data to be determined from the internal consistency of the low-energy patch field theory under highly anisotropic scaling dimensions ($\left[k_0\right]=a+1$, $\left[k_x\right]=2$, $\left[k_y\right]=1$). While the leading two-loop diagrammatics vanish identically due to kinematic pole configurations, our three-loop evaluation reveals a profound structural asymmetry between the sectors: The fermion self-energy and Yukawa vertex receive nonvanishing logarithmic corrections, whereas the corresponding bosonic counterterm remains strictly zero. Consequently, we find that no self-consistent, intersecting fixed-point solution for the exponent $a$ exists within the three-loop truncation, failing to reproduce the physical value of $a\approx 1.85$ observed in quantum Monte Carlo simulations. We conjecture that the cross-linked topology of the four-loop boson self-energy diagrams is exactly marginal and yields the minimal, mandatory bosonic counterterm required to restore multisector self-consistency. Our framework establishes a rigid multiloop matching scheme necessary to uniquely pin down the critical exponent and uncovers a stable phase space for field anomalous dimensions.

Coherent control of two-level quantum systems is typically achieved using resonant driving fields, forming the basis of qubit operations. Here, we report a mechanism for inducing complete Rabi oscillations in monochromatically driven two-level systems even when the drive frequency is much larger than the qubit's Larmor frequency. This effect—Rabi oscillations induced by nonresonant gyrating drive (RING)—requires a gyrating control field, i.e., an elliptic drive trajectory that encloses a nonzero area per cycle. We illustrate the effect with numerical simulations and provide an analytical understanding via a simple effective Hamiltonian derived from Floquet theory and perturbation theory. We show that RING enables coherent oscillations without relying on resonant energy exchange, thereby allowing the integration of high-pass noise filtering. We detail a realization in electrically driven spin-orbit qubits, where the mechanism provides access to non-Abelian phases at nonzero magnetic field and enables amplification of the Rabi frequency using the same gate voltage amplitudes at higher drive frequencies. Our results broaden the landscape of quantum control by establishing a route to coherent manipulation under strongly off-resonant driving conditions.

Spin point groups classify magnetic phases in the weak spin-orbit coupling regime and characterize the static properties of altermagnetic phases, but their dynamical consequences remain largely unexplored. Here, we derive selection rules for high-harmonic generation of charge and spin currents by extending dynamical symmetry to include spin point-group operations. Since spin currents transform under both real- and spin-space operations, whereas charge currents transform only under real-space operations, spin-current selection rules can reveal magnetic information that is inaccessible to charge-current harmonics. In a minimal altermagnetic model, an axis-aligned linearly polarized drive is nondiagnostic for distinguishing ferromagnetic and altermagnetic phases, although the antiferromagnetic phase is distinguished by the absence of the corresponding spin-current harmonics. A diagonal linearly polarized drive distinguishes the three spin-point-group phases within the weak-coupling spin-group description, whereas a single-helicity circularly polarized drive provides a sharper spin-current-harmonic criterion for distinguishing them from magnetic-point-group mimics. These results establish spin-current harmonics as a dynamical probe of spin-group symmetry. Finite spin-orbit coupling lifts these zeros only perturbatively: the forbidden harmonics acquire amplitudes analytic in the coupling strength, so that the fingerprint becomes a small same-order helicity contrast, below $3\times {10}^{-2}$ at low orders for couplings up to ${10}^{-2}$ of the hopping in our model.

We present an analytically solvable minimal model for the relaxation of low-frequency magnons in magnetic insulators arising from magnon-phonon and magnon-magnon interactions. The model establishes a direct connection between microscopic relaxation processes and Gilbert damping, and reveals how magnon decay evolves from bulk systems to the monolayer limit. We find that magnon-phonon coupling produces Gilbert damping of comparable magnitude in three- and two-dimensional magnets, with qualitative differences between flexural phonons in freestanding monolayers and three-dimensional phonons in substrate-supported layers. By contrast, non-Gilbert damping due to four-magnon scattering is strongly enhanced in two dimensions, where it becomes independent of spin-orbit coupling. To benchmark the model against real materials, we introduce a numerical approach for computing magnon damping from ab initio–derived spin Hamiltonians. We demonstrate that the central conclusions of the model remain valid for magnons in bulk yttrium iron garnet and several van der Waals magnetic insulators: $\mathrm{CrSBr}, {\mathrm{CrI}}_3$, and ${\mathrm{CrPS}}_4$.

We study the formation of a trion liquid in doped low-dimensional semiconductors with strong electron-hole interactions and analyze its signatures in angle-resolved photoemission spectroscopy (ARPES). We show that this strongly correlated state of matter forms naturally in the vicinity of the phase boundary between a normal band insulator and an excitonic insulator upon doping. By studying the photoemission spectrum, we show that a partially occupied trion band gives rise to in-gap features in the ARPES spectrum with vanishing spectral weight at the Fermi energy. We demonstrate our findings using a 1D microscopic model employing exact, unbiased, matrix product state (MPS)-based calculations.

We study the interplay between a Coulomb impurity and quasiholes in a fractional quantum Hall (FQH) state at finite temperatures. While a repulsive impurity can pin a quasihole and stabilize the FQH state, an attractive impurity cannot bind quasiholes. We demonstrate that at finite temperatures, a quasihole can be thermally ionized from a repulsive impurity, resulting in a thermal ionization crossover. We propose an experimental setup using exciton sensing to detect such a thermal ionization of quasiholes.

Contact processes play an important role in classical nonequilibrium dynamics, describing the spreading of diseases, the dynamics of earthquakes and forest fires, and the distribution of information through the internet. Here we show that their quantum counterpart, where the spreading occurs through coherent couplings, displays even richer dynamics and offers new means of control. A quantum contact process on a topologically nontrivial one-dimensional lattice can be confined to a protected subspace corresponding to either a single site or a fully excited lattice. Furthermore, excitation spreading can be controlled to occur in quantized steps and on demand when employing topological pumps. We show that the many-body dynamics of excited domains can be mapped to an effective single-particle model, which also determines the topological properties. Throughout this work, we consider a specific type of contact process corresponding to coherent Rydberg facilitation in a tweezer array of trapped atoms in a one-dimensional lattice.