Non-Hermitian physics is a rapidly growing field that hosts rich intriguing phenomena with broad applications in photonics, condensed matter, and quantum information science. Yet, it is still unclear how to properly deal with the nonadiabatic dynamics in a driven non-Hermitian quantum system. Here, we introduce a novel theoretical framework based on time-dependent biorthogonal quantum formalism by redefining the associated state to ensure gauge-independent transition probabilities. Applying this framework to a linearly driven non-Hermitian system, our approach reveals universal power-law scaling of defect production that is consistent with the Kibble-Zurek mechanism in the adiabatic regime at exceptional points. In the fast quench regime, universal scaling behaviors are also found, which can be explained by the adiabatic-impulse approximation. Moreover, upon trespassing the $\mathcal{PT}$-broken region, the phenomenon of defect freezing, i.e., violation of adiabaticity, is observed.

We theoretically predict in-plane magnetic field induced orbital Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting states in twisted ${\mathrm{WSe}}_2$ homobilayers (${\mathrm{tWSe}}_2$), focusing on their dependence on layer polarization and Fermi surface geometry. For unpolarized layers, finite-momentum pairing emerges only at low temperatures and above a critical field $B_{c1,\parallel }$. When layer symmetry is broken, finite-momentum pairing is stabilized at any nonzero field, with a critical temperature higher than that of the zero-momentum state. Notably, we identify a phase transition, which separates two distinct FFLO phases, when there are two separate Fermi pockets residing in the two moiré minivalleys associated with opposite layers. We further discuss the effects of twist angles and applied field directions. Our findings establish ${\mathrm{tWSe}}_2$ as a promising platform for realizing and manipulating FFLO states via twist angles, displacement fields, and filling factors.

The thermal Hall conductivity of the trilayer cuprate ${\mathrm{HgBa}}_2{\mathrm{Ca}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{8+\delta }$ (Hg1223)—the superconductor with the highest critical temperature $T_c$ at ambient pressure—was measured at temperatures down to 2 K for three dopings in the underdoped regime ($p=0.09,0.10,0.11$). By combining a previously introduced simple model and prior theoretical results, we derive a formula for the inverse mean free path, $1/\ell$, which allows us to estimate the mean free path of $d$-wave quasiparticles in Hg1223 below $T_c$. We find that $1/\ell$ grows as $T^3$, in agreement with the theoretical expectation for a clean $d$-wave superconductor. Measurements were also conducted on the single layer mercury-based cuprate ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{6+\delta }$ (Hg1201), revealing that the mean free path in this compound is roughly half that of its three-layered counterpart at the same doping $(p=0.10)$. This observation is attributed to the protective role of the outer planes in Hg1223, which results in a more pristine inner plane. We also report data in an ultraclean crystal of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_y$ (YBCO) with full oxygen content ($p=0.18$), believed to be the cleanest of any cuprate, and find that $\ell$ is not longer than in Hg1223.

Experimental studies on moiré materials have predominantly focused on twisted hexagonal lattice with low-energy states near the $\mathrm{\Gamma }$ or K points, where the electronic dispersion is typically isotropic. In contrast, we introduce a class of semiconducting transition metal carbides (MXenes) $M_2\mathrm{C}{T}_2$ $(M=\mathrm{Ti}, \mathrm{Zr}, \mathrm{Hf}, \mathrm{Sc}, \mathrm{Y}; T=\mathrm{O}, \mathrm{F}, \mathrm{Cl})$ as a new platform for M-valley moiré materials, which exhibit pronounced anisotropic properties. Using ${\mathrm{Ti}}_2{\mathrm{CO}}_2$ and ${\mathrm{Zr}}_2{\mathrm{CO}}_2$ as representative examples, we perform large-scale ab initio calculations and demonstrate that their AB-stacked twisted homobilayer hosts three threefold rotational-symmetry-related M valleys with time-reversal symmetry. These systems show striking anisotropic band flattening in the conduction-band minimum. To elucidate the underlying physics, we construct a simplified moiré Hamiltonian that captures the essential features of the band structure, revealing the origins of anisotropic flattening through the mechanisms of band folding and interlayer tunneling. Our findings expand the current landscape of moiré materials, establishing valley- and spin-degenerate, two-dimensional arrays of quasi-one-dimensional systems as promising platforms for exploring many interesting correlated electronic phases.

The hexagonal antiferromagnet MnTe has attracted enormous interest as a prototypical example of a spin-compensated magnet in which the combination of crystal and spin symmetries lifts the spin degeneracy of the electron bands without the need for spin-orbit coupling, a phenomenon called nonrelativistic spin splitting (NRSS). Subgroups of NRSS are determined by the specific spin-interconverting symmetry that connects the two opposite-spin sublattices. In MnTe, this symmetry is rotation, leading to the subgroup with spin splitting away from the Brillouin zone center, often called altermagnetism. MnTe also has the largest spontaneous magnetovolume effect of any known antiferromagnet, implying strong coupling between the magnetic moment and volume. This magnetostructural coupling offers a potential knob for tuning the spin-splitting properties of MnTe. Here, we use neutron diffraction with in situ applied pressure to determine the effects of pressure on the magnetic properties of MnTe and further explore this magnetostructural coupling. We find that applying pressure significantly increases the Néel temperature, but decreases the ordered magnetic moment. We explain this as a consequence of strengthened magnetic exchange interactions under pressure, resulting in higher $T_{\mathrm{N}}$, with a simultaneous reduction of the local moment of individual Mn atoms, described here via density functional theory. This reflects the increased orbital hybridization and electron delocalization with pressure. These results shed light on the competition between magnetic exchange interactions and the strength of individual magnetic moments and show that the magnetic properties of MnTe can be controlled by pressure, opening the door to improved properties for spintronic applications through tuning via physical or chemical pressure.

Motivated by experiments carried out in the near infrared using zero-loop-area Sagnac interferometers, we explore electromagnetic signatures of time-reversal symmetry breaking (TRSB) at microwave frequencies, using as a prototypical example a semiclassical conductor in a magnetic field. TRSB is generically accompanied by a skew-symmetric term in the electrodynamic response tensors (permittivity, conductivity, surface impedance), imparting a nonreciprocal phase shift to left- and right-circularly polarized electromagnetic waves reflected from the surface of such a material. We show that TRSB manifests as a difference in the surface reactance experienced by circularly polarized waves, and can be detected using a doubly degenerate resonator mode, such as the ${\mathrm{TE}}_{111}$ mode of a cylindrical cavity. In addition to the frequency splitting induced by TRSB, we show that, when interrogated by circularly polarized microwaves, the forward and reverse transmission responses of such a resonator break reciprocity, providing a crucial signature that distinguishes true Faraday effects (i.e., circular birefringence) from non-TRSB effects such as linear birefringence. In the limit that the sample is larger than the spot size (i.e., larger than the diameter of the microwave cavity), we show that the TRSB resonator has sensitivity to polar Kerr angle comparable to that of the zero-loop-area Sagnac and should provide complementary insights into unconventional superconductors such as ${\mathrm{UPt}}_3$ and ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ that have been observed to spontaneously break time-reversal symmetry.

Motivated by the appearance of Majorana fermions in a broad range of correlated and topological electronic systems, we develop a general method to compute the dynamical response of interacting Majorana fermions in the random-phase approximation (RPA). This can be applied self-consistently on top of Majorana mean-field theory backgrounds, thereby in particular providing a powerful tool to analyze generic behavior in the vicinity of (various heavily studied) exactly soluble models. Prime examples are quantum spin liquids (QSL) with emergent Majorana excitations, with the celebrated exact solution of Kitaev. We employ the RPA to study in considerable detail phase structure and dynamics of the extended Kitaev honeycomb $KJ\mathrm{\Gamma }$ model, with and without an applied field. First, we benchmark our method with Kitaev's exactly soluble model, finding a remarkable agreement. The interactions between Majorana fermions even turn out to mimic the effect of local ${\mathbb{Z}}_2$ flux excitations, which we explain analytically. Second, we show how small non-Kitaev couplings $J$ and $\mathrm{\Gamma }$ induce Majorana bound states, resulting in sharp features in the dynamical structure factor in the presence of fractionalization: such “spinon excitons” naturally appear, and can coexist and interact with the broad Majorana continuum. Third, for increasing couplings or field, our theory predicts instabilities of the Kitaev QSL (KQSL) triggered by the condensation of the sharp modes. From the high-symmetry momenta of the condensation we can deduce which magnetically ordered phases surround the KQSL, in good agreement with previous finite-size numerics. We discuss implications for experiments and the broad range of applicability of our method to other QSL and Majorana systems.

We derive the $\mathcal{T}$-matrix formalism tailored for numerical analysis of second-harmonic (SH) generation from arbitrarily shaped particles made of centrosymmetric optical materials. First, the transfer matrix of a single particle is computed via the extended boundary condition method, in which the electromagnetic fields both at fundamental frequency and SH are expanded in vector spherical wave functions, and the integral formulation is satisfied away from the surface of the scatterer. We allow for the accurate physical description of the SH sources by taking into account both local surface and nonlocal bulk polarization contributions to the nonlinear polarization density source responsible for the generation of the SH signal by a particle. This single-particle formalism is then extended to arbitrary distributions of particles by incorporating into the formalism linear and nonlinear electromagnetic wave scattering from the particles in the cluster. Importantly from a practical point of view, our method can be applied to particles of arbitrary shape made of optical materials characterized by general frequency-dispersion relations, so that it can describe the linear and nonlinear optical response of clusters of metallic, semiconductor, or polaritonic particles, as well as mixtures of such particles. The approach proposed here is faster and more memory-efficient than well-established numerical techniques, especially in the analysis of spheroidal particles, due to the favorable symmetries of spherical wave basis functions used in the wave scattering analysis.

We investigate phase transitions in a non-Hermitian Su-Schrieffer-Heeger (SSH) model with an imaginary chemical potential via Krylov spread complexity and Krylov fidelity. The spread witnesses the $\mathcal{PT}$-transition for the non-Hermitian Bogoliubov vacuum of the SSH Hamiltonian, where the spectrum goes from purely real to complex (oscillatory dynamics to damped oscillations). In addition, it also witnesses the transition occurring in the $\mathcal{PT}$-broken phase, where the spectrum goes from complex to purely imaginary (damped oscillations to sheer decay). For a purely imaginary spectrum, the Krylov spread fidelity, which measures how the time-dependent spread reaches its stationary-state value, serves as a probe of previously undetected dynamical phase transitions.

In synthetic antiferromagnetic multilayers (SAFs), chiral magnetic structures such as spin spirals and skyrmions have been stabilized at room temperature by precisely tuning the effective perpendicular magnetic anisotropy, the Dzyaloshinskii-Moriya interaction, and the Ruderman-Kittel-Kasuya-Yoshida (RKKY) interlayer coupling. In this study, we investigate the dynamics of spin spirals on ultrashort timescales after femtosecond laser pumping in SAFs. The access to ultrafast magnetization dynamics, inaccessible by conventional optical techniques due to the zero net magnetization, has been enabled by the use of time-resolved circular dichroism in x-ray resonant magnetic scattering (CD-XRMS). A pair of two-dimensional x-ray scattering patterns for left and right elliptical polarization (EL and ER) have been recorded for each delay. In contrast to our previous findings in ferromagnetic multilayers, the magnetization ($\mathrm{EL}+\mathrm{ER}$) and dichroism ($\mathrm{EL}-\mathrm{ER}$) signals exhibit notably similar ultrafast dynamics, with demagnetization occurring on a timescale of $\sim 180$ fs, followed by rapid remagnetization within $\sim 500$ fs. This similarity in ultrafast dynamics can be attributed to the continuous rotation of magnetization in the spin spiral of SAFs, which evolves smoothly in space without forming abrupt domains or alternating domain walls. The ultrafast response and stability in its topological character highlight the potential of SAF-based chiral magnetic structures for future high-speed, energy-efficient data storage and processing applications.

Controlling the nonreciprocity of spin-wave propagation is crucial for various magnonics applications, however, it still poses a significant challenge. In a joint theoretical and experimental effort, we demonstrate such capability with pronounced tunability. Through modifying the exchange spring structures in SmCo/Fe bilayers, we show that the nonreciprocity can be continuously tailored in a wide range, similar to a system with a tunable Dzyaloshinskii-Moriya interaction from $-0.36$ to $-5.43\mathrm{mJ}/{\mathrm{m}}^2$, a value exceeding most reported ones in heavy metal/ferromagnet bilayers. Our findings create a tunable platform for magnonic devices, enhancing their potential in next-generation technologies.

We use uniaxial strain in combination with ultra-high-resolution resonant inelastic x-ray scattering (RIXS) at the oxygen-$K$ and copper-$L_3$ edges to study the excitations stemming from the charge ordering wave vector in ${\mathrm{La}}_{1.875}{\mathrm{Sr}}_{0.125}{\mathrm{CuO}}_4$. By detwinning stripe ordering, we demonstrate that the optical phonon anomalies do not show any stripe anisotropy. The low-energy charge excitations also retain an in-plane fourfold symmetry. As such, we find that both phonon and charge excitations are decoupled entirely from the strength of static charge ordering. The almost isotropic character of charge excitations is indicative of a quantum critical behavior and remains a possible source for the strange metal properties found in the normal state of cuprate superconductors.

We develop a theory for the Hall, Nernst, and thermal Hall effects in magnetic materials that harbor topological spin textures such as skyrmions. In addition to the ordinary response to an external magnetic field, there are two other contributions that have been described by two distinct theories: an intrinsic anomalous response arising from momentum-space Berry curvature and a topological response from the real-space Berry curvature related to the topological charge density of the spin texture. We develop here a unified semiclassical theory that incorporates the effects of all phase-space Berry curvatures on an equal footing within a controlled calculation. We analyze the electrical and thermal currents carried by electrons with arbitrary dispersion and spin-orbit coupling (SOC) of strength $\lambda$ interacting with arbitrary three-dimensional spin textures via an exchange coupling $J$. For small $\lambda /J$, we show that all linear response conductivities—electrical, thermoelectric, and thermal Hall—can be written as the sum of three contributions: ordinary, anomalous, and topological. We derive various general relations including the Weidemann-Franz, Kelvin, and Mott relations. We show that there is a topological phase transition as a function of $\lambda /J$ at which the semiclassical approach breaks down and the large $\lambda /J$ regime exhibits qualitatively different behavior with a vanishing topological Hall response.

The multicaloric effect, which represents the reversible entropy change that occurs when both external magnetic and electric fields are applied, is an interesting phenomenon characteristic to multiferroics. Targeting the multicaloric effect in the typical multiferroic ${\left({\mathrm{Fe}}_{0.95}{\mathrm{Zn}}_{0.05}\right)}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$, we experimentally evaluate the isothermal entropy change due to a magnetoelectric cross correlation. A pronounced cross-correlation-derived isothermal entropy change is found at the magnetic ordering temperature. By exploring the literature, we suggest that a magnetic phase transition may tend to involve a considerably larger cross-correlation-derived entropy change than that derived from the linear magnetoelectric effect.

Nonreciprocal critical currents without applying an external magnetic field have been observed recently in several superconductors, in various forms of graphene, a kagome compound and in an underdoped cuprate. A necessary requirement for this is that the usual supercurrent be accompanied by an antisymmetric chiral supercurrent, i.e., with the symmetry of a Hall current; equivalently that the superfluid density tensor have an antisymmetric chiral component. It also requires inversion breaking. The conditions for this phenomena are derived to find that their normal states must break time reversal and chirality and that the superconducting states must in addition be nonunitary. Each of the superconductors where spontaneous nonreciprocal critical currents are observed have shown some evidence for such broken symmetries in the normal state. The superconducting state of such materials have topological edge currents, but their projected electromagnetic part is in general not an integer. The edge states are protected in the superconductor due to a gap. The normal state should show a Kerr effect and, under ideal conditions, an anomalous Hall effect.

We performed a random-phase approximation calculation for a spin-valley-polarized model of the rhombohedral tetralayer graphene to study the possibility of a chiral superconductor from the Kohn-Luttinger mechanism. We included the realistic band structure and form factor in our calculation and solved the self-consistent equation numerically by sampling 20 000 points in the momentum space at a given temperature. Around the van Hove singularity, we find $p\text{−}ip$ pairing with the Chern number switching from $C=-1$ to $C=0$ through a gap closing at $\mathbf{k}=(0,0)$ (defined relative to $\mathbf{K}$). Although the superconductor is generically fully gapped at low temperature, we find the Bogoliubov Fermi surface at a temperature just below mean-field $T_c$. Besides, through calculation of the free energy, we conclude that the optimal Cooper pair momentum $\mathbf{Q}$ is generically finite and can be as large as $0.1k_F$. We dub the $\mathbf{Q}\ne 0$ phase as an incommensurate Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconductor to distinguish it from the $\mathbf{Q}=0$ phase. Compared to the $\mathbf{Q}=0$ phase, our incommensurate $\mathbf{Q}$ phase is a nematic superconductor if it is in the Fulde-Ferrell phase or exhibits a charge density wave if it is in the Larkin-Ovchinnikov phase. Our work demonstrates the rhombohedral tetralayer graphene as a wonderful platform to explore the Majorana zero-mode, FFLO physics, and the Bogoliubov Fermi surface within one single platform.

We use the eigenstate thermalization hypothesis to derive a quantum master equation for a system that is weakly coupled to a chaotic finite-sized bath prepared in a pure state. We show that the emergence of Markovianity is controlled by the spectral function of the ETH and that local detailed balance emerges in the Markovian regime for a broad class of pure bath states. We numerically verify this result by comparing the master equation to dynamics computed using exact diagonalization of a chaotic Hamiltonian. We also compare the master equation to exact dynamics for an integrable bath and find that at finite size they strongly disagree. Our work puts forward eigenstate thermalization as a foundation for open quantum systems theory, thus extending it beyond ensemble bath preparations to chaotic many-body environments in generic pure states.

Unconventional superconductivity is often accompanied by non-Fermi liquid (NFL) behavior, which emerges near a quantum critical point (QCP)— a point where an electronic ordered phase is terminated at absolute zero under nonthermal parameters. While nematic orders, characterized by broken rotational symmetry, are sometimes found in unconventional superconductors, the role of nematic fluctuations in driving NFL transport behavior remains unclear. Here, we investigated electrical and thermoelectric transport properties in ${\mathrm{FeSe}}_x{\mathrm{Te}}_{1-x}$ thin films and observed hallmark NFL behavior: temperature-linear resistivity and logarithmic divergence of thermoelectricity at low temperatures. Notably, the thermoelectricity peaks sharply at the nematic QCP ($x=0.45)$, highlighting the dominant role of nematic fluctuations in the NFL transport. Furthermore, we found that the pair-breaking mechanisms in the superconducting phase crosses over from orbital- to Pauli-limited effects, indicating the mass enhancement near the nematic critical regime. These findings reveal the profound impact of nematic fluctuations on both normal-state transport and superconducting properties.

Shifting the Fermi level of the celebrated $A{M}_3{X}_5$ (135) compounds into proximity of flat bands strongly enhances electronic correlations and severely affects the formation of density waves and superconductivity. Our broadband infrared spectroscopy measurements of ${\mathrm{RbTi}}_3{\mathrm{Bi}}_5$ and ${\mathrm{CsTi}}_3{\mathrm{Bi}}_5$ combined with density-functional band-structure calculations reveal that the correlated Ti $d$-states are intricately coupled with the Bi $p$-states that form a tilted Dirac crossing. Electron-phonon coupling manifests itself in the strong damping of itinerant carriers and in the anomalous shape of the phonon line in ${\mathrm{RbTi}}_3{\mathrm{Bi}}_5$. An anomaly in these spectral features around 150 K can be paralleled to the onset of nematicity detected by low-temperature probes. Our findings show that the materials with low band filling open unexplored directions in the physics of kagome metals and involve electronic states of different nature strongly coupled with lattice dynamics.

Sliding ferroelectricity in van der Waals materials exhibits ultrafast polarization switching dynamics and high endurance, offering an avenue for the design of memories and neuromorphic devices. The unique polarization switching behaviors originate from the distinct characteristics of a domain wall (DW), which possesses broader width and faster motion compared to that of conventional ferroelectrics. Herein, using machine-learning-assisted molecular dynamic simulations and field theory analysis, we predict a soliton-like DW motion in sliding ferroelectric $3R\text{−}\mathrm{Mo}{\mathrm{S}}_2$ with nearly zero damping. It is found that the DW in sliding ferroelectrics exhibits uniformly accelerated motion under an external field, with its velocity ultimately reaching the relativistic-like limit due to continuous acceleration. Remarkably, the DW velocity remains constant even after the external field removal, completely deviating from the velocity breakdown observed in conventional ferroelectrics. The inertial DW motion with ultralow damping provides opportunities for applications of sliding ferroelectrics in memory devices based on DW engineering.