The identification of spin-nematic states is challenging due to the absence of Bragg peaks. However, the study of dynamical physical quantities provides a promising avenue for characterizing these states. In this study, we investigate the dynamical properties of spin-nematic states in three-dimensional quantum spin systems in a magnetic field, using a two-component boson theory that incorporates magnons and bimagnons. Our particular focus lies on the dynamical spin structure factor at zero temperature and the nuclear magnetic resonance (NMR) relaxation rate at finite temperatures. Our findings reveal that the dynamical structure factor does not exhibit any diverging singularity across momentum and frequency while providing valuable information about the form factor of bimagnon states and the underlying structure of spin-nematic order. Furthermore, we find a temperature dependence in the NMR relaxation rate proportional to $T^3$ at low temperatures, similar to canted antiferromagnets. A clear distinction arises as there is no critical divergence of the NMR relaxation rate at the spin-nematic transition temperature. Our theoretical framework provides a comprehensive understanding of the excitation spectrum and the dynamical properties of spin-nematic states, covering arbitrary spin values $S$ and encompassing site and bond nematic orders. Additionally, we apply the same methodology to analyze these dynamical quantities in a canted antiferromagnetic state and compare the results with those in the spin-nematic states.

Semiconductor qubit devices suffer from the drift of important device parameters as they are operated. The most important example is a shift in qubit operating frequencies. This effect appears to be directly related to the heating of the system as gate operations are applied. We show that the main features of this phenomenon can be explained by the two-level systems that can also produce charge noise if these systems are considered to form an interacting random-field glass. The most striking feature of the theory is that the frequency shift can be nonmonotonic in temperature. The success of the theory and the questions it raises considerably narrow the possible models for the two-level systems.

Theoretical and experimental studies have verified the existence of “magic angles” in twisted bilayer graphene, where the rotation angle between layers gives rise to flat bands and consequently exotic correlated phases. Recently, magic-angle phenomena have been predicted and reported in other graphene systems, for instance, multilayers with alternating twist angles and trilayers with identical twist angles between consecutive layers. In this paper, we present a comprehensive theoretical study on flat bands in general twisted graphene systems. Using the continuum model in the chiral limit, we demonstrate the existence of flat bands in a variety of multilayers where the ratios between twist angles are rational and develop a framework for predicting magic-angle sets in trilayer configurations with arbitrary ratio of rotation angles. Our results are corroborated by tight-binding calculations. Remarkably, the technique we developed can be extended to systems with many layers of graphene. Our results suggest that flat bands can exist in graphene multilayers with angle disorder, that is, narrow samples of turbostatic graphite, point to the existence of a continuous, connected magic surface in trilayer configuration space, and compare favourably with contemporary experiments on trilayer moiré quasicrystals.

Tantalum ditelluride (${\mathrm{TaTe}}_2$) belongs to the family of layered transition metal dichalcogenides but exhibits a unique structural phase transition at around 170 K that accompanies the rearrangement of the Ta atomic network from a “ribbon chain” to a “butterfly-like” pattern. While multiple mechanisms including Fermi surface nesting and chemical bonding instabilities have been intensively discussed, the origin of this transition remains elusive. Here we investigate the electronic structure of single-crystalline ${\mathrm{TaTe}}_2$ with a particular focus on its modifications through the phase transition, by employing core-level and angle-resolved photoemission spectroscopy combined with first-principles calculations. Temperature-dependent core-level spectroscopy demonstrates a splitting of the Ta $4f$ core-level spectra through the phase transition indicative of the Ta-dominated electronic state reconstruction. Low-energy electronic state measurements further reveal an unusual kink-like band reconstruction occurring at the Brillouin zone boundary, which cannot be explained by Fermi surface nesting or band-folding effects. On the basis of the orbital-projected band calculations, this band reconstruction is mainly attributed to the modifications of specific Ta $5d$ states, namely, the $d_{XY}$ orbitals (the ones elongating along the ribbon chains) at the center Ta sites of the ribbon chains. The present results highlight the strong orbital-dependent electronic state reconstruction through the phase transition in this system and provide fundamental insights towards understanding complex electron-lattice-bond coupled phenomena.

Exceptional points (EPs), with their intriguing spectral topology, have attracted considerable attention in a broad range of physical systems, with potential sensing applications driving much of the present research in this field. Here, we investigate spectral topology and EPs in systems with significant nonlinearity, exemplified by a nonequilibrium exciton-polariton condensate. With the possibility to control loss and gain and nonlinearity by optical means, this system allows for a comprehensive analysis of the interplay of nonlinearities (Kerr type and saturable gain) and non-Hermiticity. Not only do we find that EPs can be intentionally shifted in parameter space by the saturable gain, but we also observe intriguing rotations and intersections of Riemann surfaces and find nonlinearity-enhanced sensing capabilities. With this, our results illustrate the potential of tailoring spectral topology and related phenomena in non-Hermitian systems by nonlinearity.

$\beta$-Mn-type family alloys ${\mathrm{Mn}}_{3}TX$ ($T=\text{Co}$, Rh, and Ir; $X=\text{Si}$ and Ge) have a three-dimensional antiferromagnetic (AF) corner-shared triangular network, i.e., the hyperkagome lattice. The antiferromagnet ${\mathrm{Mn}}_3\mathrm{RhSi}$ shows magnetic short-range order over a wide temperature range of approximately 500 K above the Néel temperature $T_{\mathrm{N}}$ of 190 K. In this family of compounds, as the lattice parameter decreases, the long-range magnetic ordering temperature decreases. ${\mathrm{Mn}}_3\mathrm{CoSi}$ has the smallest lattice parameter and the lowest $T_{\mathrm{N}}$ in the family. The quantum critical point (QCP) from AF to the quantum paramagnetic state is expected near a cubic lattice parameter of 6.15 $\AA$. Although the Néel temperature of ${\mathrm{Mn}}_3\mathrm{CoSi}$ is only 140 K, the emergence of the quantum critical behavior in ${\mathrm{Mn}}_3\mathrm{CoSi}$ is discussed. We study how the magnetic short-range order appears in ${\mathrm{Mn}}_3\mathrm{CoSi}$ by using neutron scattering, $\mu \mathrm{SR}$, and bulk characterization such as specific heat capacity. According to the results, the neutron scattering intensity of the magnetic short-range order in ${\mathrm{Mn}}_3\mathrm{CoSi}$ does not change much at low temperatures from that of ${\mathrm{Mn}}_3\mathrm{RhSi}$, although the $\mu \mathrm{SR}$ short-range order temperature of ${\mathrm{Mn}}_3\mathrm{CoSi}$ is largely suppressed to 240 K from that of ${\mathrm{Mn}}_3\mathrm{RhSi}$. Correspondingly, the volume fraction of the magnetic short-range order regions, as shown by the initial asymmetry drop ratio of $\mu \mathrm{SR}$ above $T_{\mathrm{N}}$, also becomes small. Instead, the electronic-specific heat coefficient $\gamma$ of ${\mathrm{Mn}}_3\mathrm{CoSi}$ is the largest in this ${\mathrm{Mn}}_{3}T\mathrm{Si}$ system, possibly due to the low-energy spin fluctuation near the quantum critical point.

Topological materials have been a main focus of studies in the past decade due to their protected properties that can be exploited for the fabrication of new devices. Among them, Weyl semimetals are a class of topological semimetals with nontrivial linear band crossings close to the Fermi level. The existence of such crossings requires the breaking of either time-reversal $\left(T\right)$ or inversion $\left(I\right)$ symmetry and is responsible for the exotic physical properties. In this work we identify the full-Heusler compound ${\mathrm{InMnTi}}_2$, as a promising, easy to synthesize, $T$- and $I$-breaking Weyl semimetal. To correctly capture the nature of the magnetic state, we employed a novel $\mathrm{DFT}+U$ computational setup where all the Hubbard parameters are evaluated from first principles; thus preserving a genuinely predictive ab initio character of the theory. We demonstrate that this material exhibits several features that are comparatively more intriguing with respect to other known Weyl semimetals: the distance between two neighboring nodes is large enough to observe a wide range of linear dispersions in the bands, and only one kind of such node's pairs is present in the Brillouin zone. We also show the presence of Fermi arcs stable across a wide range of chemical potentials. Finally, the lack of contributions from trivial points to the low-energy properties makes the materials a promising candidate for practical devices.

We develop a neutral vortex fluid theory on closed surfaces with zero genus. The theory describes collective dynamics of many well-separated quantum vortices in a superfluid confined on a closed surface. Comparing to the case on a plane, the covariant vortex fluid equation on a curved surface contains an additional term proportional to Gaussian curvature multiplying the circulation quantum. This term describes the coupling between topological defects and curvature in the macroscopic level. For a sphere, the simplest nontrivial stationary vortex flow is obtained analytically and this flow is analogous to the celebrated zonal Rossby-Haurwitz wave in classical fluids on a nonrotating sphere. For this flow the difference between the coarse-grained vortex velocity field and the fluid velocity field generated by vortices is solely driven by curvature and vanishes in the corresponding vortex flow on a plane when the radius of the sphere goes to infinity.

We demonstrate gate-tunable giant field-dependent nonreciprocal transport (magnetochiral anisotropy) in a noncentrosymmetric superconductor $T_{\mathrm{d}}\text{−}{\mathrm{MoTe}}_2$ in the thin limit. Giant magnetochiral anisotropy (MCA) with a rectification coefficient (or a figure of merit) $\gamma =3.1\times {10}^6{\mathrm{T}}^{-1}{\mathrm{A}}^{-1}$ is observed at 230 mK, below the superconducting transition temperature $\left(T_c\right)$. This is one of the largest values reported so far and may be attributed to the reduced symmetry of the crystal structure. The temperature dependence of $\gamma$ indicates that ratchetlike motion of magnetic vortices is the origin of the MCA, as supported by our theoretical model. For bilayer (2 L) $T_{\mathrm{d}}\text{−}{\mathrm{MoTe}}_2$, we can successfully modulate $\gamma$ by gating. Our experimental results provide a new route to realizing electrically controllable superconducting rectification devices in a single material.

Inspired by current research on measurement-induced quantum phase transitions, we analyze the nonunitary Floquet transverse-field Ising model with complex nearest-neighbor couplings and complex transverse fields. Unlike its unitary counterpart, the model shows a number of steady phases, stable to integrability-breaking perturbations. Some phases have robust edge modes and/or spatiotemporal long-range orders in the bulk. The transitions between the phases have extensive entanglement entropy, whose scaling with the system size depends on the number of the real quasiparticle modes in the spectrum at the transition. In particular, the volume-law scaling appears on some critical lines, protected by pseudo-Hermiticity. Both the scaling of entanglement entropy in steady states and the evolution after a quench are compatible with the non-Hermitian generalization of the quasiparticle picture of Calabrese and Cardy at least qualitatively.

By precisely tuning the ground state of black phosphorus with pressure from the semiconducting to semimetallic state, we track a systematic evolution of the Seebeck coefficient. Thanks to a manifest correlation between the Seebeck coefficient and resistivity, the Seebeck response in each conduction regime, i.e., intrinsic, saturation, extrinsic, and variable range hopping (VRH) regimes, is identified. In the former two regimes, the Seebeck coefficient behaves in accordance with the present theories, whereas in the latter two regimes available theories do not give a satisfactory account for its response. However, by eliminating the extrinsic sample dependence in the resistivity $\rho$ and Seebeck coefficient $S$, the Peltier conductivity $\alpha =S/\rho$ allows us to unveil the intrinsic thermoelectric response, revealing vanishing fate for $\alpha$ in the VRH regime. The emerged ionized impurity scattering on entry to the semimetallic state is easily surpassed by electron-electron scattering due to squeezing of screening length accompanied by an increase of carrier density with pressure. Each carrier scattering participates an enhancement of the phonon drag contribution to the Seebeck effect, but creates the phonon drag peak with opposite sign at distinct temperature. In the low-temperature limit, a small number of carriers enhances a prefactor of $T$-linear Seebeck coefficient as large as what is observed in prototypical semimetals. A crucial but largely ignored role of carrier scattering in determining the magnitude and sign of the Seebeck coefficient is indicated by the observation that a sign reversal of the $T$-linear prefactor is concomitant with a change in dominant scattering mechanism for carriers.

The shift current effect, in materials lacking inversion symmetry, may potentially allow the performance of photovoltaics to surpass the Shockley-Queisser limit for traditional $p\text{−}n$ junction-based photovoltaics. Although the shift-current effect has been studied from first principles via second-order perturbation theory, an understanding of the dynamics of hot carriers is still lacking. We investigate the dynamics of the shift current in monolayer ${\mathrm{WS}}_2$ via real-time propagation time-dependent density functional theory (rt-TDDFT). We find that the shift current can be generated within 10–20 fs after turning on the lights, and dissipates within approximately a few tens of femtoseconds after turning off the lights. This property can be used for ultrafast photon detection. This work provides an important step toward understanding the dynamics of shift-current effects, which is crucial for device applications.

Iron-based superconductors, a cornerstone of low-temperature physics, have been the subject of numerous theoretical models aimed at deciphering their complex behavior. In this study, we present a comprehensive approach that amalgamates several existing models and incorporates experimental data to simulate the superconducting phase diagrams of the principal “122-type” iron-based compounds. Our model considers a multitude of factors including the momentum dependence of the superconducting gap, spin-orbital coupling, antiferromagnetism, spin-density wave, induced $\mathit{XY}$ potential on the tetrahedral structure, and electron-phonon coupling. We have refined the electron-phonon scattering matrix using experimental angle-resolved photoemission spectroscopy data, ensuring that relevant electrons pertinent to iron-based superconductivity are accounted for. This innovative approach allows us to calculate theoretical critical temperature $\left(T_{\mathrm{c}}\right)$ values for ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2, \mathrm{Ca}{\mathrm{Fe}}_2{\mathrm{As}}_2$, and $\mathrm{Sr}{\mathrm{Fe}}_2{\mathrm{As}}_2$ as functions of pressure. These calculated values exhibit remarkable agreement with experimental findings. Furthermore, our model predicts that $\mathrm{Mg}{\mathrm{Fe}}_2{\mathrm{As}}_2$ remains nonsuperconducting irrespective of the applied pressure. Given that 122-type superconductivity at low pressure or low doping concentration has been experimentally validated, our work serves as a powerful predictive tool for generating superconducting phase diagrams at high pressure empirically. This study underscores that the high transition temperatures and the precise doping and pressure dependence of iron-based superconductors are intrinsically linked to an intertwined mechanism involving a strong interplay between structural, magnetic, and electronic degrees of freedom.

Multiband superconductors are sources of rich physics arising from multiple order parameters, which show unique collective dynamics including Leggett mode as relative phase oscillations. Previously, it has been pointed out that the Leggett mode can be optically excited in the linear response regime, as demonstrated in a one-dimensional model for multiband superconductors [T. Kamatani et al., Phys. Rev. B 105, 094520 (2022)]. Here we identify the linear coupling term in the Ginzburg-Landau free energy to be the so-called Lifshitz invariant, which takes the form of $\mathit{d}·({\mathrm{\Psi }}_i^{*}\mathbf{\nabla }{\mathrm{\Psi }}_j-{\mathrm{\Psi }}_j^{*}\mathbf{\nabla }{\mathrm{\Psi }}_i)$, where $\mathit{d}$ is a constant vector and ${\mathrm{\Psi }}_i$ and ${\mathrm{\Psi }}_j(i\ne j)$ represent superconducting order parameters. We classify all pairs of irreducible representations of order parameters in the crystallographic point groups that allow for the existence of the Lifshitz invariant. We emphasize that the Lifshitz invariant can appear even in systems with inversion symmetry. The results are applied to a model of $s$-wave superconductors on a Kagome lattice with various bond orders, for which in some cases we confirm that the Leggett mode appears as a resonance peak in a linear optical conductivity spectrum based on microscopic calculations. We discuss a possible experimental observation of the Leggett mode by a linear optical response in multiband superconductors.

Periodically driven quantum systems can exhibit a plethora of intriguing nonequilibrium phenomena that can be analyzed using Floquet theory. Naturally, Floquet theory is employed to describe the dynamics of atoms interacting with intense laser fields. However, this semiclassical analysis cannot account for quantum-optical phenomena that rely on the quantized nature of light. In this paper, we take a significant step to go beyond the semiclassical description of atom-photon coupled systems by unifying Floquet theory with quantum optics using the framework of full-counting statistics. This is achieved by introducing counting fields that keep track of the photonic dynamics. This formalism, which we dub “photon-resolved Floquet theory” (PRFT), is based on two-point tomographic measurements, instead of the two-point projective measurements used in standard full-counting statistics. Strikingly, the PRFT predicts the generation of macroscopic light-matter entanglement when atoms interact with multimode electromagnetic fields, thereby leading to complete decoherence of the atomic subsystem in the basis of the Floquet states. This decoherence occurs rapidly in the optical-frequency regime, but is negligible in the radio-frequency regime. Our results thus pave the way for the design of efficient quantum memories and quantum operations. Finally, employing the PRFT, we propose a quantum-communication protocol that can significantly outperform the state-of-art few-photon protocols by two orders of magnitude or better. The PRFT potentially leads to insights in various Floquet settings including spectroscopy, thermodynamics, quantum metrology, and quantum simulations.

We study the Hall constant in a homogeneous two-dimensional fluid of correlated electrons immersed in a perpendicular magnetic field, with special focus on the regime of low carrier density. The model consists of a one-band tight-binding model and a momentum-independent causal self-energy, representing interaction-induced correlations effects that are restricted to be local in space. We write general gauge-invariant equations for the conductivity tensor at first order in the magnetic field and solve them numerically—–analytically when possible—in a minimal model of anisotropic square lattice with constant self-energy. Our results show that deviations from the universal behavior of the Hall constant, as observed in the semiclassical regime, appear upon entering the quantum regime, where the Fermi energy and interaction are comparable energy scales.

We examine the dynamical magnetic response in a two-component resonating-valence-bond (RVB) description of the doped Mott insulator. The half-filled antiferromagnetic phase described by the Schwinger-boson mean-field theory will evolve into a bosonic-RVB state in the superconducting phase upon doping, where the doped holes introduce another fermionic itinerant spinon which forms a BCS-like RVB order. The spin excitations are thus composed of a resonance-like mode from the former and a weak dispersive mode from the itinerant component at the mean-field level. These two-component spinons are shown to give rise to an hourglasslike spin excitation at the random-phase approximation level via an antiferromagnetic coupling between the two modes, which provides an unconventional explanation of the experimental observations in the cuprate. In particular, we also discuss an instability towards an incommensurate magnetic order in this theoretical framework.

The magneto-optical Kerr effect (MOKE) in the extreme ultraviolet (EUV) regime has helped to elucidate some of the key processes that lead to the manipulation of magnetism on ultrafast timescales. However, as we show in this paper, the recently introduced spectrally resolved analysis of such data can lead to surprising experimental observations, which might cause misinterpretations. Therefore, an extended analysis of the EUV magneto-optics is necessary. Via experimental determination of the dielectric tensor, we find here that the nonequilibrium excitation in an ultrafast magnetization experiment can cause a rotation of the off-diagonal element of the dielectric tensor in the complex plane. In direct consequence, the commonly analyzed magneto-optic asymmetry may show time-dependent behavior that is not directly connected to the magnetic properties of the sample. We showcase such critical observations for the case of ultrafast magnetization dynamics in Ni, and give guidelines for the future analysis of spectrally resolved magneto-optical data and its comparison with theory.

We propose a frustration-free model for the Moore-Read quantum Hall state on sufficiently thin cylinders with circumferences $\lesssim 7$ magnetic lengths. While the Moore-Read Hamiltonian involves complicated long-range interactions between triplets of electrons in a Landau level, our effective model is a simpler one-dimensional chain of qubits with deformed Fredkin gates. We show that the ground state of the Fredkin model has high overlap with the Moore-Read wave function and accurately reproduces the latter's entanglement properties. Moreover, we demonstrate that the model captures the dynamical response of the Moore-Read state to a geometric quench, induced by suddenly changing the anisotropy of the system. We elucidate the underlying mechanism of the quench dynamics and show that it coincides with the linearized bimetric field theory. The minimal model introduced here can be directly implemented as a first step towards quantum simulation of the Moore-Read state, as we demonstrate by deriving an efficient circuit approximation to the ground state and implementing it on an IBM quantum processor.