In the presence of both space and time reversal symmetries, an $s$-wave $A_{1g}$ superconducting state is usually topologically trivial. Here, we demonstrate that an exception can take place in a type of nonsymmorphic lattice structure. We specify the demonstration in a time reversal invariant system with a centrosymmetric space group $P4/nmm$, the symmetry that governs iron-based superconductors, by showing the existence of a second-order topological state protected by a mirror symmetry. The topological superconductivity is featured by $2\mathcal{Z}$ degenerate Dirac cones on the (10) edge and $\mathcal{Z}$ pairs of Majorana modes at the intersection between the (11) and $(1\overline{1})$ edges. The topological invariance and Fermi surface criterion for the topological state are provided. Moreover, we point out that the previously proposed $s$-wave state in iron-based superconductors, which features a sign-changed superconducting order parameter between two electron pockets, is such a topological state. Thus, these results not only open a new route to pursue topological superconductivity, but also establish a measurable quantity to settle one long-lasting debate on the pairing nature of iron-based superconductors.

There is growing interest in using ultrafast light pulses to drive functional materials into nonequilibrium states with novel properties. The conventional wisdom is that above-gap photoexcitation behaves similarly to raising the electronic temperature and lacks the desired selectivity in the final state. Here, we report a novel nonthermal lattice instability induced by ultrafast above-gap excitation in SnSe, a representative of the $IV-VI$ class of semiconductors that provides a rich platform for tuning material functionality with ultrafast pulses due to their multiple lattice instabilities. The new lattice instability is accompanied by a drastic softening of the lowest-frequency $A_g$ phonon. This mode has previously been identified as the soft mode in the thermally driven phase transition to a $Cmcm$ structure. However, by a quantitative reconstruction of the atomic displacements from time-resolved x-ray diffraction for multiple Bragg peaks and excitation densities, we show that ultrafast photoexcitation with near-infrared (1.55 eV) light induces a distortion toward a different structure with $Immm$ symmetry. The $Immm$ structure of SnSe is an orthorhombic distortion of the rocksalt structure and does not occur in equilibrium. Density functional theory calculations reveal that the photoinduced $Immm$ lattice instability arises from electron excitation from the Se $4p$- and Sn $5s$-derived bands deep below the Fermi level that cannot be excited thermally. The results have implications for optical control of the thermoelectric, ferroelectric, and topological properties of the monochalcogenides and related materials. More generally, the results emphasize the need for ultrafast structural probes to reveal distinct atomic-scale dynamics that are otherwise too subtle or invisible in conventional spectroscopies.

The name active matter refers to any collection of entities that individually use free energy to generate their own motion and forces. Through interactions, active particles spontaneously organize in emergent large-scale structures with a rich range of materials properties. The active-matter paradigm is applied to living and nonliving systems over a vast dynamic range, from the organization of subnuclear structures in the cell to collective motion at the human scale. The diverse phenomena exhibited by these systems all stem from the defining property of active matter as an assembly of components that individually and dissipatively break time-reversal symmetry. This article outlines a selection of current and emerging directions in active matter research. It aims at providing a pedagogical and forward-looking introduction for researchers new to the field and a road map of open challenges and future directions that may appeal to those established in the area.

Giant and tunneling magnetoresistance are physical phenomena used for reading information in commercial spintronic devices. The effects rely on a conserved spin current passing between a reference and a sensing ferromagnetic electrode in a multilayer structure. Recently, we have proposed that these fundamental spintronic effects can be realized in unconventional collinear antiferromagnets with nonrelativistic alternating spin-momentum coupling. Here, we elaborate on the proposal by presenting archetype model mechanisms for the giant and tunneling magnetoresistance effects in multilayers composed of these unconventional collinear antiferromagnets. The models are based, respectively, on anisotropic and valley-dependent forms of the alternating spin-momentum coupling. Using first-principles calculations, we link these model mechanisms to real materials and predict an approximately 100% scale for the effects. We point out that, besides the giant or tunneling magnetoresistance detection, the alternating spin-momentum coupling can allow for magnetic excitation by the spin-transfer torque.

Dihedral (“$k$-atic”) liquid crystals (DLCs) are assemblies of microscopic constituent particles that exhibit $k$-fold discrete rotational and reflection symmetries. Generalizing the half-integer defects in nematic liquid crystals, two-dimensional $k$-atic DLCs can host point defects of fractional topological charge $\pm m/k$. Starting from a generic microscopic model, we derive a unified hydrodynamic description of DLCs with aligning or antialigning short-range interactions in terms of Ginzburg-Landau and Landau-Brazovskii-Swift-Hohenberg theories for a universal complex order-parameter field. Building on this framework, we demonstrate in particle simulations how adiabatic braiding protocols, implemented through suitable boundary conditions, can emulate anyonic exchange behavior in a classical system. Analytic solutions and simulations of the mean-field theory further predict a novel spontaneous chiral symmetry-breaking transition in antialigning DLCs, in quantitative agreement with the patterns observed in particle simulations.

We describe a qubit linearly coupled to a heat bath, either directly or via a cavity. The main focus of the paper is on calorimetric detection in a realistic circuit, specifically a solid-state qubit coupled to a resistor as an absorber. The bath in the model is formed of oscillators initially in the ground state with a distribution of energies and coupling strengths. A direct numerical solution of the Schrödinger equation for the full system including up to ${10}^6$ oscillators in the bath verifies the expected decay process. We address quantitatively the question of separation of the qubit and bath by adding a cavity in between which by detuning allows one to adjust the decay rate into a convenient regime for detection purposes. Most importantly, we propose splitting a quantum to two uncoupled baths and performing a cross-correlation measurement of their temperatures. This technique enhances significantly the signal-to-noise ratio of the calorimeter.

Resistive switching is of the key phenomena for applications such as nonvolatile memories or neuromorphic computing. ${\mathrm{V}}_3{\mathrm{O}}_5$, a compound of the vanadium oxide Magnéli series, is one of the rare materials to exhibit an insulator-metal transition above room temperature ($T_c\sim 415\mathrm{K}$). The switching mechanisms in this material are still not clear. Here, we demonstrate both static dc volatile resistive switching and fast oscillatory spiking regimes in ${\mathrm{V}}_3{\mathrm{O}}_5$ devices at room temperature (120 K below the phase transition temperature) by applying an electric field. We couple electrical measurements, operando optical imaging, and infrared measurements to track the reflectivity change and the temperature of a device during the resistive switching. We find that the resistive switching starts via thermal runaway deep in the insulating state and triggers the phase transition with the formation of a filament of the high-temperature phase. Furthermore, we capture optically and thermally the spiking oscillations that we link to the negative differential resistance regime and find the filament forms and dissolves via a periodic spatiotemporal instability that we describe by numerical simulations. Our studies play an important role in understanding the volatile resistive switching mechanisms and demonstrate that ${\mathrm{V}}_3{\mathrm{O}}_5$ can be a key vanadium oxide for neuromorphic computing.

Higgs and Goldstone modes, well known in high-energy physics, have been realized in a number of condensed matter physics contexts, including superconductivity and magnetism. The Goldstone-Higgs concept is also applicable to and gives rise to new insight on structural phase transitions. Here, we show that the Leggett mode, a collective mode observed in multiband superconductors, also has an analog in crystallographic phase transitions. Such structural Leggett modes can occur in the phase channel as in the original work of Leggett [Prog. Theor. Phys. 36, 901 (1966)]. That is, they are antiphase Goldstone modes (antiphasons). In addition, a new collective mode can also occur in the amplitude channel, an out-of phase (antiphase) Higgs mode, that should be observable in multiband superconductors as well. We illustrate the existence and properties of these structural Leggett modes using the example of the pyrochlore relaxor ferroelectric ${\mathrm{Cd}}_2{\mathrm{Nb}}_2{\mathrm{O}}_7$.

${\mathrm{CeRh}}_2{\mathrm{As}}_2$ has recently been reported to be a rare case of a multiphase unconventional superconductor close to a quantum critical point (QCP). Here, we present a comprehensive study of its normal-state properties and of the phase (I) below $T_0\approx 0.4\mathrm{K}$ which preempts superconductivity at $T_c=0.26\mathrm{K}$. The second-order phase transition at $T_0$ presents signatures in specific heat and thermal expansion but none in magnetization and ac susceptibility, indicating a nonmagnetic origin of phase I. In addition, an upturn of the in-plane resistivity at $T_0$ points to a gap opening at the Fermi level in the basal plane. Thermal expansion indicates a strong-positive-pressure dependence of $T_0$, $d{T}_0/dp=1.5\mathrm{K}/\mathrm{GPa}$, in contrast to the strong-negative-pressure coefficient observed for magnetic order in Ce-based Kondo lattices close to a QCP. Similarly, an in-plane magnetic field shifts $T_0$ to higher temperatures and transforms phase I into another nonmagnetic phase (II) through a first-order phase transition at about 9 T. Using renormalized band-structure calculations, we find that the Kondo effect ($T_K\approx 30\mathrm{K}$) leads to substantial mixing of the excited crystalline-electric-field states into the ground state. This allows quadrupolar degrees of freedom in the resulting heavy bands at the Fermi level which are prone to nesting. The huge sensitivity of the quadrupole moment on hybridization together with nesting causes an unprecedented case of phase transition into a quadrupole-density-wave state at a temperature $T_0\ll T_K$, which explains the nature of phases I and II.

In the local or itinerant extreme, magnetic excitations can be described by the Heisenberg model which treats electron spins as localized moments, or by the itinerant-electron model where the exchange interaction between electrons leads to unequal numbers of electrons with up and down spins. However, the nature of the magnetic excitations has been elusive when both local moments and itinerant electrons are present in the intermediate range. Using inelastic neutron scattering, we provide direct spectroscopic evidence on the coexistence of and interplay between local moments and itinerant electrons in a van der Waals metallic ferromagnet ${\mathrm{Fe}}_{2.72}{\mathrm{GeTe}}_2$, which can sustain tunable room-temperature ferromagnetism down to the monolayer limit. We find that there exist ferromagnetic spin-wave excitations dispersing from the zone center at low energies resulting from local moments and a columnlike broad continuum at the zone boundary at high energies up to over 100 meV resulting from itinerant electrons. Unlike the two-dimensional crystal structure, the low-energy mode exhibits a three-dimensional nature, and the high-energy mode also has an out-of-plane dependence. Both modes persist well above the Curie temperature of 160 K. Our neutron spectroscopic data reveal that the low-energy spin waves at 100 K are more coherent than those at 4 K, which is evidence of the weakening of the Kondo screening at high temperatures. These results unambiguously demonstrate the coexistence of local moments and itinerant electrons and the Kondo effect between these two components in ${\mathrm{Fe}}_{2.72}{\mathrm{GeTe}}_2$. Such behaviors are generally expected in heavy-fermion systems with heavy $f$ electrons but are rarely clearly observed in materials with light $d$ electrons. These findings shed light on the understanding of magnetism in transition-metal compounds.

Determination of the symmetry property of superconducting gaps has been a central issue in studies to understand the mechanisms of unconventional superconductivity. Although it is often difficult to completely achieve the aforementioned goal, the existence of superconducting nodes, one of the few important experimental signatures of unconventional superconductivity, plays a vital role in exploring the possibility of unconventional superconductivity. The interplay between superconducting nodes and topology has been actively investigated, and intensive research in the past decade has revealed various intriguing nodes out of the scope of the pioneering work to classify superconducting order parameters based on the point groups. However, a systematic and unified description of superconducting nodes for arbitrary symmetry settings is still elusive. In this paper, we develop a systematic framework to comprehensively classify superconducting nodes pinned to any line in momentum space. While most previous studies are based on the homotopy theory, our theory is on the basis of the symmetry-based analysis of band topology, which enables systematic diagnoses of nodes in all nonmagnetic and magnetic space groups. Furthermore, our framework can readily provide a highly effective scheme to detect nodes in a given superconductor by using density-functional theory and assuming symmetry properties of Cooper pairs (called pairing symmetries). There are two main advantages of our method. One is that, while the symmetry-indicator theory cannot be directly applied to systems violating compatibility conditions, our diagnostic scheme focuses on such systems, which results in a complement of the symmetry-indicator theory. The other is that, although our method does not predict any pairing symmetry itself, our framework can diagnose not only the positions but also the dimensions of nodes such as line or surface nodes, which can reduce candidates of pairing symmetries. We substantiate the power of our method through the time-reversal broken and noncentrosymmetric superconductor CaPtAs. Our work establishes a unified theory for understanding superconducting nodes and facilitates determining superconducting gaps in materials combined with experimental observations.

Coherent feedback stabilizes a system toward a target state without the need of a measurement, thus avoiding the quantum backaction inherent to measurements. Here, we employ optical coherent feedback to remotely cool a nanomechanical membrane using atomic spins as a controller. Direct manipulation of the atoms allows us to tune from strong coupling to an overdamped regime. Making use of the full coherent control offered by our system, we perform spin-membrane state swaps combined with stroboscopic spin pumping to cool the membrane in a room-temperature environment to $T=216\mathrm{mK}$ (${\overline{n}}_m=2.3\times {10}^3\mathrm{phonons}$) in $200\mu \mathrm{s}$. We furthermore observe and study the effects of delayed feedback on the cooling performance. Starting from a cryogenically precooled membrane, this method would enable cooling of the mechanical oscillator close to its quantum mechanical ground state and the preparation of nonclassical states.

Angle-resolved photoemission spectroscopy (ARPES) is the most powerful technique to investigate the electronic band structure of crystalline solids. To completely characterize the electronic structure of topological materials, one needs to go beyond band structure mapping and access information about the momentum-resolved Bloch wave function, namely, orbitals, Berry curvature, and topological invariants. However, because phase information is lost in the process of measuring photoemission intensities, retrieving the complex-valued Bloch wave function from photoemission data has yet remained elusive. We introduce a novel measurement methodology and associated observable in extreme ultraviolet angle-resolved photoemission spectroscopy, based on continuous modulation of the ionizing radiation polarization axis. Tracking the energy- and momentum-resolved amplitude and phase of the photoemission intensity modulation upon polarization axis rotation allows us to retrieve the circular dichroism in photoelectron angular distributions (CDAD) without using circular photons, providing direct insights into the phase of photoemission matrix elements. In the case of two relevant bands, it is possible to reconstruct the orbital pseudospin (and thus the Bloch wave function) with moderate theory input, as demonstrated for the prototypical, layered, semiconducting, transition metal dichalcogenide $2\mathrm{H}\text{−}{\mathrm{WSe}}_2$. This novel measurement methodology in ARPES, which is articulated around the manipulation of the photoionization transition dipole matrix element, in combination with a simple tight-binding theory, is general and adds a new dimension to obtaining insights into the orbital pseudospin, Berry curvature, and Bloch wave functions of many relevant crystalline solids.

The spectral form factor (SFF), characterizing statistics of energy eigenvalues, is a key diagnostic of many-body quantum chaos. In addition, partial spectral form factors (PSFFs) can be defined which refer to subsystems of the many-body system. They provide unique insights into energy eigenstate statistics of many-body systems, as we show in an analysis on the basis of random matrix theory and of the eigenstate thermalization hypothesis. We propose a protocol that allows the measurement of the SFF and PSFFs in quantum many-body spin models, within the framework of randomized measurements. Aimed to probe dynamical properties of quantum many-body systems, our scheme employs statistical correlations of local random operations which are applied at different times in a single experiment. Our protocol provides a unified test bed to probe many-body quantum chaotic behavior, thermalization, and many-body localization in closed quantum systems which we illustrate with numerical simulations for Hamiltonian and Floquet many-body spin systems.

We present a comprehensive study of ${\mathrm{CaCu}}_3{\mathrm{Ru}}_4{\mathrm{O}}_{12}$ using bulk sensitive hard and soft x-ray spectroscopy combined with local-density $\text{approximation}+\text{dynamical}$ mean-field theory (DMFT) calculations. Correlation effects on both the Cu and Ru ions can be observed. From the Cu $2p$ core-level spectra, we deduce the presence of magnetic ${\mathrm{Cu}}^{2+}$ ions hybridized with a reservoir of itinerant electrons. The strong photon energy dependence of the valence band allows us to disentangle the Ru, Cu, and O contributions and, thus, to optimize the DMFT calculations. The calculated spin and charge susceptibilities show that the transition metal oxide ${\mathrm{CaCu}}_3{\mathrm{Ru}}_4{\mathrm{O}}_{12}$ must be classified as a Kondo system and that the Kondo temperature is in the range of 500–1000 K.

The pairing of interacting fermions leading to superfluidity has two limiting regimes: the Bardeen-Cooper-Schrieffer (BCS) scheme for weakly interacting degenerate fermions and the Bose-Einstein condensation (BEC) of bosonic pairs of strongly interacting fermions. While the superconductivity that emerges in most metallic systems is the BCS-like electron pairing, strongly correlated electrons with poor Fermi liquidity can condense into the unconventional BEC-like pairs. Quantum spin liquids harbor extraordinary spin correlation free from order, and the superconductivity that possibly emerges by carrier doping of the spin liquids is expected to have a peculiar pairing nature. The present study experimentally explores the nature of the pairing condensate in a doped spin liquid candidate material and under varying pressure, which changes the electron-electron Coulombic interactions across the Mott critical value in the system. The transport measurements reveal that the superconductivity at low pressures is a BEC-like condensate from a non-Fermi liquid and crosses over to a BCS-like condensate from a Fermi liquid at high pressures. The Nernst-effect measurements distinctively illustrate the two regimes of the pairing in terms of its robustness to the magnetic field. The present Mott tuning of the BEC-BCS crossover can be compared to the Feshbach tuning of the BEC-BCS crossover of fermionic cold atoms.

We revisit the dissipative approach to producing and stabilizing spin-squeezed states of an ensemble of $N$ two-level systems, providing a detailed analysis of two surprising yet generic features of such protocols. The first is a macroscopic sensitivity of the steady state to whether $N$ is even or odd. We discuss how this effect can be avoided (if the goal is parity-insensitive squeezing) or could be exploited as a new kind of sensing modality to detect the addition or removal of a single spin. The second effect is an anomalous emergent long timescale and a “prethermalized” regime that occurs for even weak single-spin dephasing. This effect allows one to have strong spin squeezing over a long transient time even though the level of spin squeezing in the steady state is very small. We also discuss a general hybrid-systems approach for implementing dissipative spin squeezing that does not require squeezed input light or complex multilevel atoms, but instead makes use of bosonic reservoir-engineering ideas. Our protocol is compatible with a variety of platforms, including trapped ions, nitrogen-vacancy defect spins coupled to diamond optomechanical crystals, and spin ensembles coupled to superconducting microwave circuits.

Measuring the specific heat of herbertsmithite single crystals in high magnetic fields (up to 34 T) allows us to isolate the low-temperature kagome contribution while shifting away extrinsic Schottky-like contributions. The kagome contribution follows an original power law $C_p(T\to 0)\propto T^{\alpha }$ with $\alpha \sim 1.5$ and is found to be field independent between 28 and 34 T for temperatures $1\le T\le 4\mathrm{K}$. These are serious constraints when it comes to replication using low-temperature extrapolations of high-temperature series expansions. We manage to reproduce the experimental observations if about 10% of the kagome sites do not contribute. Between 0 and 34 T, the computed specific heat then has a minute field dependence supporting an algebraic temperature dependence in zero field, typical of a critical spin-liquid ground state. The need for an effective dilution of the kagome planes is discussed and is likely linked to the presence of copper ions on the interplane zinc sites. At very low temperatures and moderate fields, we also report some small field-induced anomalies in the total specific heat and start to elaborate on a phase diagram.

Ultrafast lasers are an increasingly important tool to control and stabilize emergent phases in quantum materials. Among a variety of possible excitation protocols, a particularly intriguing route is the direct light engineering of microscopic electronic parameters, such as the electron hopping and the local Coulomb repulsion (Hubbard $U$). In this work, we use time-resolved x-ray absorption spectroscopy to demonstrate the light-induced renormalization of the Hubbard $U$ in a cuprate superconductor, ${\mathrm{La}}_{1.905}{\mathrm{Ba}}_{0.095}{\mathrm{CuO}}_4$. We show that intense femtosecond laser pulses induce a substantial redshift of the upper Hubbard band while leaving the Zhang-Rice singlet energy unaffected. By comparing the experimental data to time-dependent spectra of single- and three-band Hubbard models, we assign this effect to an approximately 140-meV reduction of the on-site Coulomb repulsion on the copper sites. Our demonstration of a dynamical Hubbard $U$ renormalization in a copper oxide paves the way to a novel strategy for the manipulation of superconductivity and magnetism as well as to the realization of other long-range-ordered phases in light-driven quantum materials.