The inchworm expansion is a promising approach to solving strongly correlated quantum impurity models due to its reduction of the sign problem in real and imaginary time. However, inchworm Monte Carlo is computationally expensive, converging as $1/\sqrt{N}$ where $N$ is the number of samples. We show that the imaginary-time integration is amenable to quasi Monte Carlo, with parametrically better $1/N$ convergence, by mapping the Sobol low-discrepancy sequence from the hypercube to the simplex with the so-called Root transform. This extends the applicability of the inchworm method to, e.g., multiorbital Anderson impurity models with off-diagonal hybridization, relevant for materials simulation, where continuous-time hybridization expansion Monte Carlo has a severe sign problem.

Lorentzian-type media support optical phonons that oscillate with longitudinal polarization parallel to the wave direction, at a wave-vector-independent frequency at which the permittivity becomes zero. Here, we study the interactions between the longitudinal optical phonons and Lorentzian medium-based dispersive photonic time crystals (PTCs). We demonstrate that a stationary charge embedded in the PTCs can excite these longitudinal modes through the conversion of the static polarization field induced by the charge. Furthermore, the PTCs can develop a momentum band gap across the entire wave vector space to amplify the longitudinal modes. Remarkably, this infinite momentum band gap can be established with minimal temporal modulation of the refractive index when creating the PTCs. Our approach expands the range of waves that can be manipulated in PTCs and shows potential for observing the momentum band gap phenomenon in realistic optical experiments, where the modulation depth of the refractive index is severely constrained.

Two-dimensional terahertz spectroscopy (2DTS), a terahertz analog of nuclear magnetic resonance, is a new technique poised to address many open questions in complex condensed matter systems. The conventional theoretical framework used ubiquitously for interpreting multidimensional spectra of discrete quantum level systems is, however, insufficient for the continua of collective excitations in strongly correlated materials. Here, we develop a theory for 2DTS of a model collective excitation, the Josephson plasma resonance in layered superconductors. Starting from a mean-field approach at temperatures well below the superconducting phase transition, we obtain expressions for the multidimensional nonlinear responses that are amenable to intuition derived from the conventional single-mode scenario. We then consider temperatures near the superconducting critical temperature $T_c$, where dynamics beyond mean-field become important and conventional intuition fails. As fluctuations proliferate near $T_c$, the dominant contribution to nonlinear response comes from an optical parametric drive of counterpropagating Josephson plasmons, which gives rise to 2D spectra that are qualitatively different from the mean-field predictions. As such, and in contrast to one-dimensional spectroscopy techniques, such as third harmonic generation, 2DTS can be used to directly probe thermally excited finite-momentum plasmons and their interactions. Our theory can readily be tested in cuprates, and we discuss implications beyond the present context of Josephson plasmons.

We study electron-electron Coulomb interactions in electronic systems whose Fermi surfaces possess a finite electric dipole density. Although there is no net dipole moment, we show that electric monopole-dipole interactions can become sufficiently strong in quasi-2D Dirac metals with spin-orbit coupling to induce unconventional odd-parity superconductivity, similar to the Balian-Werthamer state of ${}^3\mathrm{He}-\mathrm{B}$. Hence, materials with spin-orbit-induced band inversion, such as the doped topological insulators ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, and $\mathrm{SnTe}$, are natural candidate materials where our theory could be relevant. We discuss the conditions for an electric dipole density to appear on the Fermi surface and develop the formalism to describe its coupling to the plasmon field, which mediates the Coulomb interaction. A mechanism for the enhancement of dipolar coupling is then provided for quasi-2D Dirac systems. Within a large-$N$ renormalization group treatment, we show that the out-of-plane ($z$-axis) dipole coupling is marginally relevant, in contrast to the monopole coupling, which is marginally irrelevant. For physically realistic parameters, we find that dipole fluctuations can get sufficiently enhanced to result in Cooper pairing. In addition, we establish that the proposed pairing glue is directly measurable in the $z$-axis optical conductivity.

We study twisted bilayer ${\mathrm{MoTe}}_2$ systems at fractional fillings of the lowest hole band under an applied out-of-plane displacement field. By employing exact diagonalization in finite-size systems, we systematically map out the ground state quantum phase diagram for two filling fractions, $\nu =1/3$ and $2/3$, and provide a detailed characterization of each phase. We identify the phase transition between a fractional Chern insulator (FCI) and a layer-polarized charge density wave (CDW) at a filling fraction of $\nu =1/3$, denoted as CDW-1. Additionally, we demonstrate that the competition between the displacement field and twist angle leads to another phase transition from a layer-polarized CDW-1 to a layer-hybridized CDW-2, identified as a first-order phase transition. Furthermore, at $\nu =2/3$ filling of the lowest hole band, we observe that the FCI remains stable against the displacement field until it approaches proximity to a transition in single-particle band topology at a smaller twist angle.

Piezomagnetism, the linear response between strain and magnetic field, is a relatively unexplored cross correlation but has promising potential as a novel probe of time-reversal symmetry breaking in various classes of materials. Interestingly, there has been no report of piezomagnetism in ferromagnets, most archetypal time-reversal symmetry-broken materials. This half-century absence of piezomagnetic ferromagnets is attributable to complications originating from multiple-domain states, as well as from changes in the magnetic point group by rotation of the magnetic moment. Here, we report characteristic V-shaped magnetostriction in the Ising itinerant ferromagnet URhGe, observed by simultaneous multiaxis strain measurements utilizing optical fiber Bragg grating sensors. This magnetostriction occurs only under fields along the $c$ axis and does not scale with the square of magnetization. Such an unconventional feature indicates piezomagnetism as its origin. Our observation is owing to monodomain switching and Ising magnetization. The obtained piezomagnetic coefficients are fairly large, implying that Ising ferromagnets are promising frontiers when seeking for materials with large piezomagnetic responses.

Adding superconducting (SC) electron pairing via the proximity effect to pristinely nonsuperconducting materials can lead to a variety of interesting physical phenomena. Particular interest has recently focused on inducing SC into two-dimensional surface states (SSs), potentially also combined with nontrivial topology. We study the mechanism of proximity-induced SC into the Shockley-type SSs of the noble metals Ag(111) and Cu(111) grown on the elemental SC Nb(110) using scanning tunneling spectroscopy. The tunneling spectra exhibit an intriguing multitude of sharp states at low energies. Their appearance can be explained by Andreev bound states formed by the weakly proximitized SSs subject to lateral finite-size confinement. We study systematically how the proximity gap in the bulk states of both Ag(111) and Cu(111) persists up to island thicknesses of several times the bulk coherence length of Nb. We find that even for thick islands, the SSs acquire a gap, with the gap size for Cu being consistently larger than for Ag. Based on this, we argue that the SC in the SS is not provided through direct overlap of the SS wave function with the SC host but can be understood to be mediated by step edges inducing electronic coupling to the bulk. Our work provides important input for the microscopic understanding of induced superconductivity in heterostructures and its spectral manifestation. Moreover, it lays the foundation for more complex SC heterostructures based on noble metals.

Strong correlations occur in magic-angle twisted bilayer graphene (MATBG) when the octet of flat moiré minibands centered on charge neutrality (CN) is partially occupied. The octet consists of a single valence band and a single conduction band for each of four degenerate spin-valley flavors. Motivated by the importance of Hartree electrostatic interactions in determining the filling-factor-dependent band structure, we use a time-dependent Hartree approximation to gain insight into electronic correlations. We find that the electronic compressibility is dominated by Hartree interactions, that paramagnetic states are stable over a range of density near CN, and that the dependence of energy on flavor polarization is strongly overestimated by mean-field theory.

We analyze electronic excitations (excitations generated by adding or removing one electron) in the bulk of fractional quantum Hall states in Jain sequence states, using composite fermion Chern-Simons field theory. Starting from a mean field approximation in which gauge field fluctuations are neglected, we use symmetry to constrain the possible composite fermion states contributing to the electronic Green's function and expect discrete infinitely sharp peaks in the electronic spectral function. We further consider the electronic excitations in particle-hole conjugate fractional quantum Hall states. Gauge field fluctuations play an increasingly important role in the electron spectral function as the filling factor approaches $1/2$, and evolve the discrete coherent peaks into a broad continuum even in the absence of impurities. At that limit, we calculate the dressed electron spectral function via the linked cluster approximation from the low to intermediate energy range. Finally, we compare our results with recent experiments.

We introduce an $\mathrm{SU}\left(M\right)$-symmetric disordered bipartite spin model with unusual characteristics. Although superficially similar to the Sachdev-Ye (SY) model, it has several markedly different properties for $M\ge 3$. In particular, it has a large nontrivial nullspace whose dimension grows exponentially with system size. The states in this nullspace are frustration-free and are ground states when the interactions are ferromagnetic. The exponential growth of the nullspace leads to Hilbert-space fragmentation and a violation of the eigenstate thermalization hypothesis. We demonstrate that the commutant algebra responsible for this fragmentation is a nontrivial subalgebra of the Read-Saleur commutant algebra of certain nearest-neighbor models such as the spin-1 biquadratic spin chain. We also discuss the low-energy behavior of correlations for the disordered version of this model in the limit of a large number of spins and large $M$, using techniques similar to those applied to the SY model. We conclude by generalizing the Shiraishi-Mori embedding formalism to nonlocal models, and apply it to turn some of our nullspace states into quantum many-body scars.

The thermal quasiparticles in a clean type-II superconductor with line nodes give rise to a quadratic low-temperature change of the penetration depth, $\mathrm{\Delta }\lambda \sim T^2$, as first shown by Kosztin and Leggett [I. Kosztin and A. J. Leggett, Phys. Rev. Lett. 79, 135 (1997)]. Here, we generalize this result to multiple nodes and compare it to numerically exact evaluations of the temperature-dependent penetration depth in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ using a high-precision tight-binding model. We compare the calculations to recent penetration depth measurements in high purity single crystals of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ [J. F. Landaeta et al., Phys. Rev. B 110, L100503 (2024)]. When assuming the order parameter to have ${\mathrm{B}}_{1\mathrm{g}}$ symmetry, we find that both a simple $d_{x^2-y^2}$-wave and complicated gap structures with contributions from higher harmonics and accidental nodes can accommodate the experimental data.

The medium-range order (MRO) in amorphous systems has been linked to complex features such as the dynamic heterogeneities in supercooled liquids or the plastic deformation of glasses. However, the nature of the MRO in these materials has remained elusive, primarily due to the lack of methods capable of characterizing this order. Here, we leverage standard two-body structural correlators and advanced many-body correlation functions to probe numerically the MRO in prototypical network glass formers, i.e., silica and sodium silicates, systems that are of great importance in natural as well as industrial settings. With increasing Na concentration, one finds that the local environment of Na becomes more structured and the spatial distribution of Na on intermediate length scales changes from blob-like to channel-like, indicating a growing inhomogeneity in the spatial Na arrangement. In parallel, we find that the Si-O network becomes increasingly depolymerized, resulting in a ring size distribution that broadens. The radius of gyration of the rings is well described by a power law with an exponent around 0.75, indicating that the rings are progressively more crumpled with increasing size. Using a recently proposed four-point correlation function, we reveal that the relative orientation of the tetrahedra shows a surprising transition at a distance around 4 Å, a structural modification that is not seen in standard two-point correlation functions. The order induced by this transition propagates to larger distances, thus affecting the structure on intermediate length scales. Furthermore, we find that, for Na-rich samples the length scale characterizing the MRO is nonmonotonic as a function of temperature, caused by the competition between energetic and entropic terms which makes that the sample forms complex mesoscpic domains. Finally, we demonstrate that the structural correlation lengths as obtained from the correlation functions that quantify the MRO are correlated with macroscopic observables such as the kinetic fragility of the liquids and the elastic properties of the glasses. These findings allow to reach a deeper understanding of the nature of the MRO in network glass formers, insight that is crucial for establishing quantitative relations between their MRO and macroscopic properties.

We study the electronic structure of alternating-twist tetralayer graphene, especially near its magic angle $\theta =1.{75}^{\circ }$, for different AA, AB, and SP sliding geometries at their middle interface that divides two twisted bilayer graphenes. This sliding dependence is shown for the bandwidths, band gaps, and $K$-valley Chern numbers of the lowest-energy valence and conduction bands as a function of twist angle and interlayer potential difference. Our analysis reveals that the AA sliding is most favorable for narrow bands and gaps, and the AB sliding is most prone to developing finite valley Chern numbers. We further analyze the linear longitudinal optical absorptions as a function of photon energy and the absorption map in the moiré Brillouin zone for specific transition energies. A self-consistent Hartree calculation reveals that the AA system's electronic structure is the most sensitive to variations in carrier density.

A plethora of experiments in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ have reached conflicting conclusions about the symmetry of the superconducting gap. To probe the gap's structure in k space, we use strain to continuously tune the band structure through a Van Hove singularity (VHS) while imaging the superconductivity with scanning superconducting quantum interference device microscopy. We find that the superfluid density peaks at the VHS and that the temperature dependence of the penetration depth is $T^2$ quadratic over the entire measured range of strain $\varepsilon$. These results are consistent with a gap structure that has vertical line nodes, experimentally confirming that nonlocal effects in the Meissner screening can lead to $T^2$ behavior and clarifying the nature of the low-energy excitations in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$.

By determining the superconducting lower and upper critical fields $H_{\mathrm{c}1}\left(T\right)$ and $H_{\mathrm{c}2}\left(T\right)$, respectively, in a high-purity spherical ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ sample via ac-susceptibility measurements, we obtain the temperature dependence of the coherence length $\xi$ and the penetration depth $\lambda$ down to $0.04T_c$. Given the high sample quality, the observed $T^2$ dependence of $\lambda$ at low temperatures cannot be explained in terms of impurity effects. Instead, we argue that the weak type-II superconductor ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ has to be treated in the nonlocal limit. By comparing our data with existing theory in that limit, the penetration depth in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ agrees with a gap structure having vertical line nodes, while horizontal line nodes cannot account for the observation. The work highlights the potential benefits of purifying other unconventional superconductors in order to access the fascinating nonlocal regime in more materials and to determine their Cooper pair wave functions.

In this work, we derive a generalized constitutive relation describing the current response to external electromagnetic fields in electrically biased quantum materials. While our semiclassical Boltzmann approach reveals the existence of electro-optical effects induced by the Berry curvature dipole of Bloch electrons, we also find a wealth of alternative electro-optical effects originating from the interplay between Berry curvature and magnetic moment. In particular, our symmetry analysis indicates the existence of a magnetoelectric electro-optical effect, derived from the simultaneous presence of Berry curvature and magnetic moment, which requires either time-reversal or inversion symmetry breaking. The revealed conductivity coefficients are explicitly written and we derive the tensor shape describing such alternative electromagnetic responses for chiral materials pertaining to space groups 152 and 198.

Stoner magnetism in bands endowed with Berry curvature is shown to be influenced by the coupling between spin-chirality density $\mathit{s}·({\partial }_x\mathit{s}\times {\partial }_y\mathit{s})$ and Berry's orbital magnetization. The key effect is that carriers moving in the presence of a spin texture see it as a source of a geometric magnetic field coupled to the carrier's orbital motion through a spin-dependent Aharonov-Bohm effect. This interaction was recently predicted to enable chiral magnons propagating along system boundaries. Here we show that it also favors chiral spin textures such as skyrmions—the topologically protected objects with particle-like properties, stabilized in the ground state. Unlike previously studied systems, here skyrmion textures can arise in the absence of microscopic spin-dependent interactions such as spin-orbit coupling or Zeeman coupling. The threshold for Stoner instability is found to soften, rendering chiral spin-ordered phases accessible under realistic conditions. We present a detailed analysis of the chiral effect for Bernal bilayer graphene and discuss the unique properties of skyrmion textures in graphene multilayers.

We report on the observation and study of the magnetoratchet effect in a graphene-based two-dimensional metamaterial formed by a graphite gate that is placed below a graphene monolayer and patterned with an array of triangular antidots. We demonstrate that terahertz/gigahertz excitation of the metamaterial leads to sign-alternating magneto-oscillations with an amplitude that exceeds the ratchet current at zero magnetic field by orders of magnitude. The oscillations are shown to be related to the Shubnikov–de Haas effect. In addition to the giant ratchet current oscillations, we detect resonant ratchet currents caused by the cyclotron and electron spin resonances. The results are well described by the developed theory considering the magnetoratchet effect caused by the interplay of the near-field radiation and the nonuniform periodic electrostatic potential of the metamaterial controlled by the gate voltages.

Understanding interfacial thermal transport across interfaces is crucial for unraveling heat transfer mechanisms in materials and devices. Interestingly, an effective metric to quantify the contributions from elastic and inelastic vibration scatterings at interfaces to interfacial thermal transport is still lacking. In this paper, we demonstrate that the significance of elastic and inelastic vibration scatterings at the interfaces on the interfacial thermal transport is determined by the competition between the similarity of vibrational density of states (VDOS) between two contacted leads and the anharmonicity of the interface. The VDOS similarity between two contacted leads, measured using the Kullback-Leibler (K-L) divergence value, is found to correlate strongly to the signature of elastic thermal transport at interfaces. Our calculations show that elastic vibration scatterings dominate the interfacial thermal transport at interfaces when its K-L divergence value is <0.2. For instance, the elastic vibration scatterings contribute >75% to the interfacial thermal conductance (ITC) of Ar/heavy Ar (h-Ar) interfaces when the K-L divergence value is <0.2. When the K-L divergence value is >0.2, which indicates the VDOS similarity between two contacted leads is significant, we find that both the elastic and inelastic vibration scatterings at interfaces contribute primarily to the interfacial thermal transport. Furthermore, for the interfaces with K-L divergence values >1, the ratio of ITC contributed by inelastic vibration scatterings can be quantitatively characterized by the interfacial anharmonic ratio (IAR), which is a measure of interfacial anharmonicity. Our calculations on Ar/h-Ar, Si/Al, and Si/Ge interfaces at various temperatures with K-L divergence values >1 show that the IAR is generally linear with the inelastic contribution to ITC with an error of 12.5%. Our results here advance the fundamental understanding of interfacial thermal transport resulting from elastic or inelastic vibration scatterings, which may benefit the thermal management design in related applications.

Despite the remarkable progress in emerging solar cell technologies such as hybrid organic-inorganic perovskites, there are still significant limitations related to the stability of the devices and their nonideal electrical behavior under certain external stimuli. We present a conceptual framework for characterizing photovoltaic devices by integrating cyclic voltammetry (CV) and impedance spectroscopy (IS). This framework is constructed from a microscopic, multimode perspective that explicitly accounts for drift, diffusion, displacement, and memory contributions. We derive comprehensive analytical expressions for current-voltage relationships and complex admittance. Our model reveals the inseparable connection between hysteresis behaviors in current-voltage characteristics observed in CV and the apparent capacitive and inductive behaviors seen in IS spectral analysis. We demonstrate how CV and IS naturally complement each other, providing a deeper microscopic understanding of device performance and limitations. Additionally, we establish the relationship between intrinsic material parameters and experimentally accessible extrinsic parameters such as light intensity, temperature, DC bias, voltage amplitude, and frequency. This framework enables unprecedented optimization of solar cell performance, marking a significant advancement toward sustainability.