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.

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 $T_1$.

A recently formulated mean-field model of glasses features an equilibrium, zero-temperature Replica-Symmetry-Breaking (RSB) transition in some parameter range. In this range, the model's solution in the Replica-Symmetric phase is expressed in terms of an effective, self-consistent random potential for uncoupled degrees of freedom, where the transition to the RSB phase is characterized by the emergence of spectral-edge localized modes and a pseudogapped quartic vibrational spectrum, resulting in a finite spin-glass susceptibility. These properties are preserved in numerical solutions of the model under nonequilibrium conditions, i.e., upon an instantaneous quench. Inspired by recent advances in computer glasses, we define a micromechanical response function—the linear response to local force monopoles—in the framework of the mean-field model. We establish exact relations between the force monopole stiffness and global susceptibilities, which suggest a close correspondence between the nonequilibrium RSB phase of the model and the above-mentioned effective random potential description. As such, the obtained micromechanical observables constitute a concrete realization of the collective degrees of freedom of the model, offering a bridge between a glassy mean-field model and finite-dimensional glasses. We show that the model's vibrational spectrum can be computed solely from the monopole response statistics and, by employing a marginal stability criterion, we extract a characteristic frequency/stiffness scale of soft glassy modes, which is related to the boson peak in finite-dimensional, laboratory glasses.

Bond disorder in frustrated pyrochlore antiferromagnets can give rise to fundamentally different quantum ground states depending on the nature of the local magnetic moments. Here, we show that the bond-disordered $J_{\mathrm{eff}}=\frac{1}{2}$ pyrochlore antiferromagnet ${\mathrm{NaCdCo}}_2{\mathrm{F}}_7$ realizes an unconventional spin-glass-like state with continued dynamics, in stark contrast to its isostructural $S=\frac{1}{2} {\mathrm{NaCdCu}}_2{\mathrm{F}}_7$ counterpart. High-field magnetization and Co $L_{2,3}$-edge x-ray absorption spectroscopy/x-ray magnetic circular dichroism establish spin-orbit-entangled $J_{\mathrm{eff}}=\frac{1}{2} {\mathrm{Co}}^{2+}$ moments with a substantial unquenched orbital contribution, consistent with local $XY$ anisotropy seen in the isostructural $\mathrm{Na}{A}^{\prime \prime }{\mathrm{Co}}_2{\mathrm{F}}_7$ ($A^{\prime \prime }=\text{Ca}$, Sr) analogs. muon spin relaxation and ${}^{23}\mathrm{Na}$ nuclear magnetic resonsonance (NMR) measurements reveal progressive slowing of spin fluctuations below $\sim 10\mathrm{K}$, culminating in a partially frozen state with persistent low-temperature dynamics that deviates from a canonical spin glass. Comparison with the isostructural bond-disordered pyrochlore ${\mathrm{NaCdCu}}_2{\mathrm{F}}_7$, which realizes a random-singlet state, reveals a fundamentally different response of spin-orbit-entangled ${\mathrm{Co}}^{2+}$ moments to bond disorder. These results identify spin-orbit coupling as a key ingredient governing the fate of bond-disordered frustrated pyrochlore magnets.

A bandwidth-controlled antiferromagnetic Mott-insulating phase in ${\mathrm{Ca}}_3{\left({\mathrm{Ru}}_{1\text{−}x}{\mathrm{Ti}}_x\right)}_2{\mathrm{O}}_7$ is realized through isovalent substitution of the Ru site. For a dilute substitution with only 1% Ti, the Mott insulator ground state remains nearly degenerate with the ground state of pristine ${\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$, where the Ru moments are ferromagnetically aligned within the metallic ${\mathrm{RuO}}_2$ bilayers, which are stacked in an antiferromagnetic fashion. The exceptionally shallow free-energy landscape of this doped compound arises from intertwined electron–electron and electron–lattice interactions. This makes its magnetic and transport properties highly sensitive to external perturbations. We systematically investigated magnetic-field-induced phase switching in ${\mathrm{Ca}}_3{\left({\mathrm{Ru}}_{0.99}{\mathrm{Ti}}_{0.01}\right)}_2{\mathrm{O}}_7$ to explore its magnetic $H\text{−}T$ phase diagram. With the field applied along the easy $b$ axis, parallel to the antiferromagnetic moments, the magnetization exhibits a first-order spin-flop transition at $\approx 6\mathrm{T}$, indicating reorientation of the $\mathrm{Ru}$ moments perpendicular to the field. The transition is accompanied by a decrease in the electrical resistance, but the spin-flop phase remains insulating. Above $10.5\mathrm{T}$, all Ru moments align with the $b$ axis, resulting in a forced ferromagnetic metallic phase. In contrast, neither spin-flop nor forced ferromagnetic phases are observed up to $14\mathrm{T}$ when the field is applied along the $a$ axis. While the electronic kinetic energy and the electron–lattice coupling contribute to the free-energy balance of this system, the resulting $H\text{−}T$ phase diagram is remarkably simple and closely resembles that of a canonical anisotropic antiferromagnet, albeit with substantially renormalized critical fields.

We studied acoustoelectric (AE) currents in materials that undergo charge density wave (CDW) transitions, induced by a surface acoustic wave (SAW) on a piezoelectric substrate. The polarity and magnitude of the AE current in ${\mathrm{NbSe}}_3$ and $2H\text{−}{\mathrm{TaSe}}_2$ were modulated due to their CDW transitions. We also found that the sign of the AE current depends on the SAW propagation direction with respect to the crystalline axis of the substrate. A phenomenological model assuming strain-modified conductivity can qualitatively account for the significant modulation of the AE current associated with the CDW transition, as well as the SAW propagation orientation dependence. The present results offer a powerful probe for exploring SAW-electron interactions in van der Waals materials, thereby highlighting their potential for advancing the emerging field of straintronics.

In this work, we investigate dielectric materials for thermoreflectance-based thermal sensing by extracting key optical parameters using temperature-dependent spectroscopic ellipsometry in the midinfrared regime. Leveraging optical phonon resonances, we demonstrate that the thermoreflectance coefficients in infrared-active polar dielectrics rival, and in some cases exceed by an order of magnitude, those observed in commonly used metals that are typically used as temperature transducers in thermoreflectance measurements. We introduce a transducer figure of merit (FOM) that combines pump absorption and probe reflectance modulation at different wavelengths, serving as a design-oriented screening metric for comparing thermoreflectance transducer performance across materials and spectral regions. Our results show that polar materials can exhibit material-level FOM values up to eight times those of metal transducers. To demonstrate practical capability, we perform transient thermoreflectance measurements on a 100-nm thermally grown ${\mathrm{SiO}}_2$ film on silicon. These results position dielectric materials as compelling candidates for next-generation thermal metrology, broadening the design space for optical thermometry, with strong implications for high-resolution thermal mapping and characterization of layered device structures based on phonon probing.

Thouless pumping is a transport phenomenon whereby an insulating, time-periodic Hamiltonian induces a robustly quantized transfer of charge, per driving cycle, in the quasiadiabatic limit. In this work, we apply the shortcuts to adiabaticity method of counterdiabatic (CD) driving to the Rice-Mele model, an archetypal model for Thouless pumping. We show that the charge pumped rapidly across each bond of our CD Rice-Mele model is robustly quantized and determined by a Chern number. The CD Hamiltonian generally involves long-range, hence nonlocal, hopping. Therefore we also derive an exact, local, nearest-neighbor CD Hamiltonian for our lattice system. We show that our nearest-neighbor nonadiabatic protocol possesses the same topological robustness to on-site disorder as quasiadiabatic Thouless pumps and is moreover surprisingly robust to on-site disorder and noise errors in implementing the CD protocol for sufficiently rapid driving. The protocol also reproduces finite-temperature Thouless pump results away from the adiabatic limit. In addition, we develop a general method to produce nontrivial, finite-time quantized pumping starting from any Rice-Mele initial ground state using only nearest-neighbor hopping.

Magic-angle twisted bilayer graphene (MATBG) hosts narrow moiré bands with meV-scale dispersion and energy splittings, making its correlated phases sensitive to both material parameters and modeling choices in low-energy downfolding. We develop an ab initio quantum-embedding workflow that derives interacting flat-band Hamiltonians from Kohn-Sham density functional theory (KS-DFT) of a relaxed, unstrained structure. Our model combines constrained random phase approximation screening, controlled double-counting subtraction, and an automated gauge-fixing procedure based on the selected columns of the density matrix that is compatible with symmetry-resolved many-body calculations. Solving the resulting models using Hartree-Fock and coupled cluster singles and doubles, we recover robust insulating Kramers intervalley coherent states at charge neutrality ($\nu =0$) and at electron doping ($\nu =+2$). The main unique physical effect appears on the hole-doped side: At $\nu =-2$, we observe a fragile semimetal with a weak $\sqrt{3}\times \sqrt{3}$ Kekulé modulation and enhanced intervalley-scattering peaks in the Fourier-transformed local density of states. Although the underlying KS-DFT band structure is nearly particle-hole symmetric, the effective interacting Hamiltonian exhibits a pronounced particle-hole asymmetry at $\nu =\pm 2$ that we trace to momentum-dependent single-particle renormalizations generated by subtraction terms constructed from reference densities consistent with the KS-DFT filling. Our work provides a first-principles route for connecting microscopic electronic structure, screened interactions, subtraction choices, and scanning tunneling microscopy signatures in MATBG.

Scanning superconducting quantum interference device imaging of ${\mathrm{CaSb}}_2$ reveals dense vortex clusters with enhanced boundary susceptibility and suppressed internal vortex motion, features that are inconsistent with both isolated-vortex and flux tube behaviors. These measurements provide a local visualization of magnetic dynamics within vortex clusters in a weakly pinned superconductor, offering an alternative route to probe nonmonotonic vortex-vortex interactions that are typically expected in single-band type-II/1 or multiband type-1.5 superconductors. Although the superfluid density follows a single-gap BCS model, vortex clustering and spatially inhomogeneous dynamics are clearly observed in a regime where reported Ginzburg-Landau parameters place ${\mathrm{CaSb}}_2$ near the type-I/type-II boundary.

We demonstrate that the recent observations of spatial oscillations in the energy position of the superconducting coherence peaks in the cuprate, transition metal dichalcogenide, iron-based, and heavy-fermion superconductors are consistent with the possible presence of a pair-, charge-, or spin-density-wave phase. We show that for all three cases, the spatial oscillations of the superconducting order parameter, $\mathrm{\Delta }\left(\mathbf{r}\right)$ can be imaged via the critical Josephson current, $I_c\left(\mathbf{r}\right)$ measured in Josephson scanning tunneling spectroscopy experiments. Finally, we show that the spatial oscillations of the density waves and of $\mathrm{\Delta }\left(\mathbf{r}\right)$ exhibit relative phase shifts of $\mathrm{\Delta }\varphi =0$ or $\pi$ for a charge-density wave, and $\mathrm{\Delta }\varphi =\pi /2$ for the spin-density wave.