While electron-phonon coupling (EPC) provides a well-established framework for understanding conventional superconductors, its role in Fe-based materials, where strong electronic correlations and competing orders are prevalent, remains an open question. Here, we integrate first-principles dynamical mean-field theory calculations with the epitaxial growth of single-layer $\text{Fe}X$ ($X$=Se, S, Te) films on ${\mathrm{SrTiO}}_3$(001) substrates, which allows us to control the $\mathrm{Fe}{X}_4$ tetrahedral angle and electron correlations. Using scanning tunneling spectroscopy, we demonstrate a superconducting dome where the pairing gap peaks at a magic angle of the ${\mathrm{FeSe}}_4$ tetrahedron. Crucially, our calculations show that the electron coupling to the $A_{1g}$ phonon mode is maximized at this magic angle in the FeSe film. Compared with superconductivity in Cs/K doped bilayer FeSe/${\mathrm{SrTiO}}_3$, our finding demonstrates that the primary boost to superconducting $T_c$ in monolayer FeSe/${\mathrm{SrTiO}}_3$, arises from intralayer correlations that reinforce the EPC within FeSe film.

ZnTe is arguably the most widely used nonlinear crystal for the generation and detection of terahertz radiation, used in conjunction with sub-bandgap optical excitation by femtosecond lasers operating near 800 nm. The terahertz dielectric function of ZnTe is the key parameter defining the efficiency and bandwidth of terahertz generation and detection. Here, we demonstrate that the terahertz dielectric function of ZnTe undergoes substantial transient modification at 800 nm sub-bandgap excitation under conditions typical for terahertz generation. These modifications arise from significant free-carrier generation via two-photon absorption of the 800-nm pump, accompanied by the pump-driven activation of the terahertz-active phonon modes. Using optical pump–terahertz probe spectroscopy, we characterize the terahertz dielectric function of ZnTe under 800-nm excitation as a function of pump fluence and pump-probe delay. Analysis of the experimental data within the Drude-Lorentz model provided the generated free-carrier density and momentum-scattering time, and oscillator strength of the pump-activated terahertz phonon modes, revealing their transient evolution in dependence on the excitation conditions.

Understanding the normal state is essential for uncovering the mechanism of high-$T_c$ superconductivity. We investigate magnetotransport in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ and ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Ca}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{10+\delta }$ single crystals over a wide doping range. While the in-plane resistivity and Hall coefficient show strong pseudogap-induced temperature dependence, the $T^2$ Hall-angle behavior and the modified Kohler's rule remain robust across all dopings. The onset temperatures of the pseudogap are clearly distinct from superconducting fluctuations, although they scale with the pseudogap magnitudes with a factor consistent with a $d$-wave superconductor. These results demonstrate that the pseudogap does not arise from superconducting fluctuations and instead suggest that it may originate from preformed Cooper pairing in the BCS-BEC crossover regime.

The transition-metal pnictide CrAs has recently attracted significant attention due to the emergence of superconductivity when the helimagnetic order is suppressed by hydrostatic pressure. However, the microscopic mechanism underlying the huge anisotropic spontaneous strain at the helimagnetic transition, which is incompatible with the spin structure, remains unresolved. Here, we have investigated this unconventional magnetoelastic coupling in CrAs through stress-strain relationship measurements and first-principles calculations. A superelasticlike nonlinear stress-strain curve emerges across the uniaxial stress-induced first-order helimagnetic-paramagnetic transition. Remarkably, uniaxial stress is found to have a substantially stronger influence on the helimagnetic transition than hydrostatic pressure. Theoretical calculations reveal that the observed drastic response against uniaxial stress originates from a strong entanglement between the anisotropic strain and orbital-dependent correlation. Our findings indicate that the spin-orbital coupling induced by local electron correlations plays a broadly important role in transition-metal pnictides, including iron pnictide superconductors.

Exploiting current-induced spin-orbit torques (SOTs) to manipulate the magnetic state of dipolar-coupled nanomagnet systems with in-plane magnetic anisotropy, such as artificial spin ices, provides a route to local, electrically programmable control of the magnetization, with relevance for applications including neuromorphic computing and reconfigurable magnonics. Here, we demonstrate how the orientation of a nanomagnet relative to the direction of an applied electrical current impacts the threshold current density needed for all-electrical field-free magnetization switching, and how dipolar coupling between the nanomagnets influences the switching of interacting pairs and ensembles of nanomagnets. Using a material system designed to generate SOTs in response to electrical currents, we find that the current required to switch the magnetization of isolated nanomagnets varies nonmonotonically as the angle between the nanomagnet long axis and the current increases. This trend in the switching current is also seen in pairs of nanomagnets. These nanomagnet pairs can switch either sequentially or simultaneously, depending on their relative geometry (side-by-side or end-to-end) and the applied current density. In small artificial spin ices, we observe a similar angular dependence of the switching current, which can be used to control the magnetization orientation of specific subsets of nanomagnets. The experimental results are supported by micromagnetic modeling, which illustrates how the various current-induced torques can be exploited to control the magnetization switching. This work thus establishes SOT switching as a practical mechanism for programmable manipulation of dipolar-coupled nanomagnetic systems.

Gyrotropy in twisted bilayer graphene can be used as a signature of interlayer electronic coupling. Gyrotropy can emerge in the absence of interlayer coupling in time-reversal symmetric bilayer systems. This gyrotropy originates from the extrinsic geometry associated with the physical geometry of the system and is independent of the structure of the electronic states. We first illustrate this effect for a purely classical bilayer array of one-dimensional wires. Next we study pristine twisted bilayer graphene and show that the gyrotropy is entirely due to interlayer coupling. Finally we consider twisted bilayer ${\mathrm{MoTe}}_2$, first as a pristine model where the gyrotropy exactly vanishes, and then with weak strain and displacement fields where we show that the geometric gyrotropy can dominate the gyrotropy due to interlayer coupling. Our results call attention to the necessity to separate the contribution of extrinsic physical geometry from the contribution of intrinsic electronic states to the properties of twisted materials.

The properties of He in diamond are of interest with respect to geological dating and the creation of color centers with possible quantum applications. We report on the lattice location of the short-lived ion-implanted nuclear probe ${}^6\mathrm{He}$ ($t_{1/2}=807\mathrm{ms}$) in diamond, which was performed using the beta emission channeling method at CERN's ISOLDE facility. ${}^6\mathrm{He}$ was implanted with 30 keV into a single-crystalline chemical-vapor deposited diamond sample kept at a temperature ranging from 30 °C up to 800 °C. By means of comparing the measured emission channeling patterns along different crystallographic directions with simulated yields for a variety of possible sites, we conclude that the implanted ${}^6\mathrm{He}$ occupies tetrahedral (T) interstitial sites, in agreement with theoretical predictions that T sites should be the preferred position of He in diamond. Implantation at 800 °C resulted in a drop in the tetrahedral interstitial fraction by ∼18%, which we interpret as the onset of diffusion, ${}^6\mathrm{He}$ thus being able to change to lattice sites of low crystallographic symmetry, or reach the surface of the sample or escape to the bulk during its lifetime. We estimate the activation energy for interstitial migration of He to be around 1.63–2.89 eV, which agrees with theoretical predictions of 1.41–2.36 eV from the literature. Activation energies around 2 eV would mean that simple interstitial He cannot be stable in diamond on geological time scales, thus to remain inside, it should be bound to some defect in the material or exist in another form such as within inclusions of other minerals or liquids, or possibly small He bubbles.

We develop theoretical methods to predict the effects of paramagnetic impurities on the microwave response of conventional spin-singlet superconductors. Our focus is on superconducting devices and resonators with low concentrations of impurities and exchange interactions with conduction electrons. We connect the subgap quasiparticle spectrum generated by pair-breaking to the frequency and temperature dependence of the conductivity for superconductors operating at microwave frequencies. We report theoretical results for superconducting device performance—dissipation, quality factor, and frequency-shift anomalies—based on self-consistent calculations of the current response and penetration of the electromagnetic field at the vacuum-superconducting interface. Key results include the prediction of a nonmonotonic anomaly in the low-frequency superfluid fraction and penetration depth at very low temperatures related to the subgap quasiparticle spectrum. Dissipation of microwave power is predicted from intra- and interimpurity band transitions at GHz frequencies at low temperatures, including a physical mechanism responsible for residual resistance. We predict anomalies in the resonant frequency, $f\left(T\right)$, and quality factor, $Q\left(T\right)$, of high-Q SRF cavities operating in the GHz range at low temperatures that are sensitive to nonmagnetic and paramagnetic impurity disorder.

Multipole expansion plays an important role in the description of electromagnetic scatterers, allowing them to be accurately characterized by a small set of expansion coefficients. However, to describe electromagnetic excitations inside a scatterer, the current density in it should be decomposed into current multipoles, which include nonradiating current configurations (anapoles) that are absent in the classical field-based expansion. Unfortunately, the use of current multipoles has so far been limited by the absence of an exact and general expression for the current multipole moments beyond their point-multipole approximation. Here, we derive such an expression and present the exact mapping relations between the classical and current multipole moments. We use our theory to calculate the scattering and extinction cross sections for large, wavelength-scale, optical scatterers supporting multipole excitations up to the sixth order, showing perfect agreement with the Mie theory. We also demonstrate the ability of current multipole expansion to describe anapole excitations beyond the small-scatterer approximation, which allows us to derive the exact anapole condition and reveal the actual current configurations and their contributions to scattering. Finally, we show that current multipoles enable a rigorous and intuitive description of chiroptical effects in chiral photonic structures. Our theoretical framework is valid for electromagnetic scatterers of arbitrary sizes and shapes without restrictions on the multipole orders, complementing the existing theory of electromagnetic multipole expansion. The minimalistic and universal character of current multipoles makes them a convenient tool for characterizing and designing diverse electromagnetic scattering systems of arbitrary complexity.

The Lieb-Schultz-Mattis (LSM) theorem and its higher-dimensional extensions forbid the existence of a unique, symmetric, and gapped ground state at fractional fillings in quantum many-body systems with a conserved particle number (or spin angular momentum) and the conventional translation symmetry of Euclidean lattices. In this work, we propose a generalization of the LSM theorem to quantum many-body systems on hyperbolic lattices, i.e., regular tessellations of two-dimensional negatively curved space. By leveraging concepts from hyperbolic band theory in a many-body setting, we adapt Oshikawa's flux-threading argument to periodic hyperbolic lattices with a non-Euclidean (Fuchsian) translation symmetry and compute a lower bound to the ground-state degeneracy as a function of filling and lattice geometry. We explore the consequences of LSM constraints for gapped phases of hyperbolic quantum matter and suggest frustrated spin models on hyperbolic analogs of the square and triangular lattices as promising platforms for realizing symmetric spin liquids in hyperbolic space.

The interaction between the electrons and the lattice vibrations in a solid is responsible for various important effects, such as formation of polarons, temperature-dependent band gaps, phonon-limited carrier transport, and conventional superconductivity. Most works assume a linear electron-phonon interaction, where the electron only interacts with one phonon at a time. However, the validity of this assumption has not been verified in polar anharmonic materials, where large ionic displacements may invalidate the assumption of linear interaction. Here, we show that nonlinear electron-phonon interactions contribute significantly to the finite-temperature electron mobility of the inorganic lead halide perovskite ${\mathrm{CsPbI}}_3$. The effect of nonlinear interaction is taken into account using the recently derived expression for the long-range part of the one-electron-two-phonon matrix element. We calculate the electron mobility from first principles within the self-energy relaxation-time approximation, treating the electron-phonon coupling in the long-range approximation. Despite these approximations, the calculated mobilities are in good agreement with the available experimental data, while enabling us to isolate and quantify the contribution of nonlinear electron-phonon interactions relative to the conventional linear coupling. We find that the one-electron-two-phonon interaction modifies the temperature dependence of the mobility in ${\mathrm{CsPbI}}_3$ and reduces its room-temperature value by about 10%. This sizable contribution results from the combined effects of strong lattice anharmonicity and large thermal phonon populations, the latter being enhanced by the low phonon frequencies associated with the heavy constituent atoms. These results identify a regime in which nonlinear electron-phonon interactions become relevant and indicate that they should be explicitly considered for finite-temperature properties of anharmonic halide perovskites.

Integer and fractional Chern insulators exhibit a nonzero quantized anomalous Hall conductivity due to a spontaneous breaking of time-reversal symmetry. To identify nontrivial topology in their time-reversal symmetric many-body spectra, we introduce many-body Euler numbers as a counterpart to many-body Chern numbers. Exemplarily, we perform calculations in a topological Hubbard model that can realize Chern and fractional Chern insulating phases. Furthermore, we lay out a classification scheme to realize different topological phases in interacting systems using symmetry indicators in analogy to topological band theory.

The nonlinear Hall effect, a new member of the Hall effect family, has attracted significant interest due to the rich quantum quantities it can manifest. Recent discoveries not only suggest novel nonlinear applications but also provide a powerful probe of the quantum geometry of the electron Bloch wave functions, including the Berry curvature dipole, quantum metric dipole, and beyond. These new developments, however, also bring a new challenge. The transverse transport consists of both Hall and non-Hall contributions. Unlike in the linear regime, separating the Hall and non-Hall transverse contributions in the nonlinear regime remains a significant challenge. Without the ability to discern Hall from non-Hall, the proposed novel quantum geometrical properties of the nonlinear Hall effect cannot be systematically studied and isolated. In this work, we present a unifying experimental method that can discern the Hall and non-Hall transverse currents in both the linear and nonlinear regimes. In particular, we show that this differentiation can be achieved by comparing the measured conductivities if one swaps the directions between the applied current and measured voltage, effectively probing the symmetry or antisymmetry of the conductivity tensor. We experimentally demonstrate this method in a number of novel quantum materials, including odd-layer ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$, black phosphorus, two-dimensional ${\mathrm{WTe}}_2$ and even-layer ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$, which allows us to verify this method in both the linear and nonlinear regimes, elucidating their distinct quantum geometrical origins. Our results offer a powerful and broadly applicable technique for disentangling transverse responses, contributing to active studies of the nonlinear electric transport of novel quantum materials.

When a primary order breaks multiple symmetries, partially ordered phases that only break a subset of those symmetries, known as vestigial phases, may onset at a higher temperature. This concept has been applied to a wide range of systems, including iron pnictides, cuprates, van der Waals antiferromagnets, doped topological insulators, and twisted bilayer graphene. In general, a multicomponent primary order parameter (OP) supports multiple vestigial channels, each described by a quadratic (or higher-order) composite OP. However, the standard large-$N$ approach to the Ginzburg-Landau action of the primary OP has an intrinsic ambiguity in how one decouples the composite OPs, leading to situations in which one can seemingly enhance or eliminate altogether any vestigial instability. Here, we show that this ambiguity is a direct consequence of redundancy relations, such as Fierz identities, that relate different composite OPs, reflecting the fact that different vestigial channels interfere with each other and thus cannot be treated separately. To resolve this ambiguity, we propose an unbiased large-$N$ approach that respects both the redundancy relations and the underlying symmetry-group structure, and that gives unique values for the effective interactions of all vestigial channels. Our analysis reveals the generic existence of regions in the parameter space of quartic Landau coefficients where no vestigial order is stable, in contrast with the standard large-$N$ approach but in agreement with weak-coupling and variational approaches. We illustrate our results by analyzing the vestigial orders of charge-density waves, spin-density waves, and multicomponent superconductors in tetragonal, hexagonal, and cubic systems, respectively, revealing the presence of exotic vestigial phases describing spin-quadrupolar, charge-$4e$ superconducting, and altermagnetic orders.

The insulating state of one-dimensional Josephson junction (JJ) arrays is governed by collective charge dynamics and disorder-induced pinning, resulting in a finite critical voltage under dc bias. Here, we investigate the influence of fabrication-induced structural defects on the critical-voltage scaling of small-capacitance Aluminium-Aluminium oxide-Aluminium ($\mathrm{Al}/{\mathrm{AlO}}_{\mathit{x}}/\mathrm{Al}$) JJ arrays. Controlled variation of the aluminium evaporation rate produces pronounced changes in grain morphology and room-temperature junction resistance. Despite these substantial structural modifications, the normalized critical-voltage scaling is preserved, demonstrating that the collective transport behavior is remarkably robust against this class of fabrication-induced defects. In contrast, the deliberate introduction of nanoscale gaps into the junctions introduces additional junction-to-junction structural variations that systematically modify the normalized scaling behavior. Likewise, in situ postfabrication oxidation alters the scaling coefficient while preserving the functional form of the scaling law, indicating that the collective transport is sensitive to specific classes of structural modifications. These results establish which fabrication-induced structural defects influence the collective transport in insulating $\mathrm{Al}/{\mathrm{AlO}}_{\mathit{x}}/\mathrm{Al}$ Josephson junction arrays, providing new insight into the role of fabrication-induced structural disorder and practical guidance for the design of future quantum phase slip (QPS) devices.

In the metallic magnet ${\mathrm{NbFe}}_2$, the low temperature threshold of ferromagnetism can be investigated by varying the Fe concentration within a narrow homogeneity range. ${\mathrm{NbFe}}_2$ is one of a number of compounds where modulated order is found to mask the ferromagnetic quantum critical point. However, here we report the rare case where the masking modulated magnetic order has been fully refined. Spherical neutron polarimetry and high-intensity single-crystal neutron diffraction reveal the first case of a longitudinal spin-density wave masking the ferromagnetic quantum critical point. The spin-density wave is characterized by a large-wavelength incommensurate modulation of its low average moment. It is formed from ferrimagnetic building blocks with antiparallel ferromagnetic sheets. The existence of ferromagnetic sheets and compensation of the magnetization only over mesoscopic length scales show local similarity between the spin-density wave and the ferromagnetic parent phase and indicate the spin-density wave's unconventional nature as emerging from underlying ferromagnetic quantum criticality.

Electron-phonon interactions underpin a variety of phenomena, ranging from transport and superconductivity to polarons and ultrafast carrier dynamics. Despite being one of the most intensely studied subjects in condensed matter physics, research on electron-phonon physics has mostly focused on linear, first-order couplings. Second- and higher-order nonlinear couplings are commonly ignored because their calculations are too demanding and we lack computational frameworks that can provide both diagonal and off-diagonal coupling matrix elements. In this work, we report a theory and computational method for computing nonlinear electron-phonon interactions of any order in real materials. Our approach combines the advantages of unit-cell calculations of electron wave functions and supercell calculations of phonon perturbations, is systematically improvable, can be used with either semilocal or nonlocal exchange-correlation functionals, and is amenable to Wannier–Fourier interpolation. As a first proof of concept, we illustrate this method by computing second-order electron-phonon coupling matrix elements in diamond, lithium fluoride, and graphite as representative nonpolar semiconductors, polar semiconductors, and metals, respectively. Furthermore, we generalize the ab initio polaron equations to second-order electron-phonon couplings, and we show that second-order couplings are essential to achieve quantitative accuracy in polaron formation energies and hopping barriers. The present methodology will find application in the study of all properties and phenomena that are currently being investigated within the linear electron-phonon coupling approximation, from phonon-mediated superconductivity to excited-state dynamics, both in harmonic and anharmonic systems.

Under high pressures and temperatures, hydrocarbons demix forming separate carbon- and hydrogen-rich phases. This gives rise to diamond formation within icy planets affecting their internal dynamics and has implications for inertial fusion energy, where demixing in plastic ablators can seed implosion instabilities. Despite the widespread importance of this process, it remains poorly understood, with existing studies finding huge discrepancies in diamond formation conditions and timescales from 15 to 140 GPa. Moreover, there is limited data on the effect of composition. Here we report time-resolved x-ray heating of statically compressed hydrocarbons of various compositions and map the effects of pressure and composition on demixing timescales. Carbon-rich hydrocarbons with C:H ratios of 1:1 or greater form diamond on timescales of order $10\mu \mathrm{s}$ from 20 to 80 GPa, while hydrogen-rich ones take longer. This likely arises from the relative reactivities of saturated and unsaturated hydrocarbons and may support the existence of a theorized plateau in the chemical potential of hydrogen-rich warm dense carbon-hydrogen mixtures.

We propose a microscopic scheme that allows to describe ac Josephson effect in superconducting junctions in terms of dissipative quantum dynamics of subgap Andreev bound states. This approach is particularly useful for highly transparent junctions at subgap bias voltages in which case a nontrivial combination of Landau-Zener tunneling between Andreev levels and their instability due to quasiparticle escape into continuum may play an important role. In the low-bias regime, we evaluate the nonequilibrium current-phase relation of superconducting junctions at arbitrary transmissions and identify a sub-Ohmic phase-dependent dissipative contribution to the current controlled by quasiparticle dynamics near the superconducting gap edge.

Perovskite nanocrystals are a convenient model system for optical spin orientation and manipulation. However, their real potential might be underestimated due to the incomplete knowledge on spin relaxation times, which are obscured by the limited sensitivity of measurement techniques as well as by the insufficient understanding of the spin relaxation mechanisms in perovskites. In this work, we study the spin relaxation of charge carriers in perovskite nanocrystals both experimentally and theoretically. We address the electron and hole spins in ${\mathrm{CsPbI}}_3$ nanocrystals embedded in a glass matrix by the resonant spin inertia technique based on optically detected magnetic resonance. It allows us to determine the longitudinal spin relaxation time $T_1$ separately for electrons and holes, the $g$ factors, and the effective Overhauser field of the nuclear spin bath. At a temperature of 1.6 K, the $T_1$ time for electrons can be as long as 0.9 ms. We reveal the effect of the time-varying nuclear field fluctuations, which enhance the electron spin relaxation at low magnetic fields, and measure a rather long nuclear spin correlation time of about 56 $\mu \mathrm{s}$. We develop a model of the spin relaxation in nanocrystals based on a two LO phonon Raman process, which explains the observed temperature dependence of the time $T_1$.