We construct exact eigenstates of multicomponent Hubbard models in arbitrary dimensions by generalizing the $\eta$-pairing mechanism. Our models include the $\mathrm{SU}\left(N\right)$ Hubbard model as a special case. Unlike the conventional two-component case, the generalized $\eta$-pairing mechanism permits the construction of eigenstates that feature off-diagonal long-range order and magnetic long-range order. These states form fragmented fermionic condensates due to a simultaneous condensation of multicomponent $\eta$ pairs. While the $\eta$-pairing states in the SU(2) Hubbard model are based on the $\eta$-pairing symmetry, the exact eigenstates in the $N$-component system with $N\ge 3$ arise not from symmetry of the Hamiltonian but from a spectrum generating algebra defined in a restricted Hilbert space. We exploit this fact to show that the generalized $\eta$-pairing eigenstates do not satisfy the eigenstate thermalization hypothesis and serve as quantum many-body scar states. This result indicates a weak breakdown of ergodicity in the $N$-component Hubbard models for $N\ge 3$. Furthermore, we show that these exact eigenstates constitute integrable subsectors in which the Hubbard Hamiltonian effectively reduces to a noninteracting model. This partial integrability causes various multicomponent Hubbard models to weakly break ergodicity. We propose a method of harnessing dissipation to distill the integrable part of the dynamics and elucidate a mechanism of nonthermalization caused by dissipation. This work establishes the coexistence of off-diagonal long-range order and $\mathrm{SU}(N$) magnetism in excited eigenstates of the multicomponent Hubbard models, which presents a possibility of novel out-of-equilibrium pairing states of multicomponent fermions. These models unveil a unique feature of quantum thermalization of multicomponent fermions, which can experimentally be tested with cold-atom quantum simulators.

Johnson noise thermometry enables direct measurement of the electron temperature, a valuable probe of many-body systems. Practical use of this technique calls for nonequilibrium generalizations of the Johnson-Nyquist theorem. For a hydrodynamic Corbino device, however, a naive use of the Shockley-Ramo theorem alongside the “Corbino paradox” leads to yet another paradox: current noise through the contacts would seem to be completely insensitive to bulk fluctuations. In this work, we resolve the unphysical “Corbino Shockley-Ramo paradox” by correctly formulating the hydrodynamic Shockley-Ramo problem. This allows us to properly formulate the problem of current noise in a hydrodynamic multiterminal device of arbitrary geometry, as well as validate a previously unjustified assumption for rectangular geometry results. As an example, we compute the Johnson noise in a hydrodynamic Corbino device, where we find a suppression of Johnson noise with magnetic field. This unusual characteristic serves as a qualitative signature of viscous hydrodynamic behavior.

Motivated by recent experiments on correlated van der Waals materials, including twisted and rhombohedral graphene and twisted ${\mathrm{WSe}}_2$, we perform an analytical and numerical study of the effects of strong on-site and short-range interactions in fractionally filled ideal Chern bands. We uncover an extensive nontrivial ground state manifold within the band filling range $0<\nu <1$ and introduce a general principle, the “three-rule,” for combining flat band wave functions, which governs their zero-energy property on the torus geometry. Based on the structure of these wave functions, we develop a variational approach that reveals distinct phases under different perturbations: metallic behavior emerges from a finite dispersion, and superconductivity is induced by attractive Cooper channel interactions. Our approach, not reliant on the commonly applied mean-field approximations, provides an analytical expression for the macroscopic wave function of the off-diagonal long-range order correlator, attributing pairing susceptibility to the set of nontrivial zero-energy ground state wave functions. Extending to finite screening lengths and beyond the ideal limit using exact diagonalization simulations, we demonstrate the peculiar structure in the many-body wave function's coefficients to be imprinted in the low-energy spectrum of the topologically ordered Halperin spin-singlet state. Our findings also make connections to frustration-free models of noncommuting projector Hamiltonians, potentially aiding the future construction of exact ground states for various fractional fillings.

We experimentally study the statistics of the transition path time taken by a submicron bead to successfully traverse an energy barrier created by two optical tweezers in two prototypical viscoelastic fluids, namely, aqueous polymer and micellar solutions. We find a very good agreement between our experimental distributions and a theoretical expression derived from the generalized Langevin equation for the particle motion. Our results reveal that the mean transition path times measured in such viscoelastic fluids have a nontrivial dependence on the barrier curvature and they can be significantly reduced when compared with those determined in Newtonian fluids of the same zero-shear viscosity. We verify that the decrease of the mean transition path time can be described in terms of an effective viscosity that quantitatively coincides with that measured by linear microrheology at a frequency determined by the reactive mode that gives rise to the unstable motion over the barrier. Therefore our results uncover the linear response of the particle during its thermally activated escape from a metastable state even when taking place in a non-Markovian bath.

Multiband superconductivity plays an important role in many emergent novel superconductors and has attracted great interest over the years. Various related experimental aspects have been intensely researched, but a quantitative understanding on the Cooper-pair transport remains still elusive, despite its fundamental and technological importance. We study a Josephson junction with a scanning tunneling microscope (STM), where both tip and sample are Pb, a prototypical type I two-band superconductor. We map the properties of the junction across a wide range of normal state conductances revealing in-gap features originating from multiple Andreev reflections (MARs) and the Josephson effect. We present the theoretical framework to extract the transmission through the transport channels and describe the Cooper-pair tunneling with quantitative precision through two superconducting bands. This paves the way for the understanding of increasingly complicated superconductors.

The simultaneous occurrence of electric-field controlled superconductivity and spin-orbit interaction makes two-dimensional electron systems (2DES) constructed from perovskite transition metal oxides promising candidates for the next generation of spintronics and quantum computing. It is, however, essential to understand the electronic bands thoroughly and verify the predicted electronic states experimentally in these 2DES to advance technological applications. Here, we present insights into the electronic states of the 2DES at oxide interfaces through comprehensive investigations of Shubnikov–de Haas oscillations in three different systems: EuO/${\mathrm{KTaO}}_3$, ${\mathrm{LaAlO}}_3$/${\mathrm{SrTiO}}_3$, and amorphous-${\mathrm{LaAlO}}_3$/${\mathrm{KTaO}}_3$. To accurately resolve these oscillations, we conducted transport measurements in high magnetic fields up to 60 T and low temperatures down to 100 mK. For 2D confined electrons at these interfaces, we observed a progressive increase of oscillations frequency and cyclotron mass with the magnetic field. We interpret these universal and intriguing findings by considering the existence of nontrivial electronic bands, for which the $E-k$ dispersion incorporates both linear and parabolic relations. In addition to providing experimental evidence for nonparabolic electronic states in ${\mathrm{KTaO}}_3$ and ${\mathrm{SrTiO}}_3$ 2DES, the unconventional oscillations presented in this study establish a paradigm for quantum oscillations in 2DES based on perovskite transition metal oxides, where the oscillation frequencies in $1/B$ exhibit quadratic dependence on the magnetic field.

Two-particle response functions are a centerpiece of both experimental and theoretical quantum many-body physics. Yet, due to their size and discontinuity structure, they are challenging to handle numerically. Recently, two advances were made to tackle this problem: first, the overcomplete intermediate representation (OIR), which provides a highly efficient compression of Green's functions in imaginary frequency, and second, partial spectral functions (PSFs), which allow for an efficient evaluation in real frequency. We show that there is a two-to-one correspondence between PSFs and OIR coefficients and exploit this fact to construct the OIR for three-or-more-particle propagators. We then use OIR to fit and compress imaginary-frequency data obtained from the numerical renormalization group (NRG), reaching a compression ratio of more than 400. Finally, we attempt to match the OIR data to partial Green's functions from NRG. Due to the overcompleteness, we achieve only qualitative agreement.

Quantum diffusion of hydrogen (H) on palladium (Pd) (111) was studied using two types of path integral simulations: quantum transition state theory (QTST) and ring polymer molecular dynamics (RPMD). The use of an artificial neural network potential trained by density functional theory calculations has made it feasible to perform path integral simulations considering nuclear quantum effects (NQEs) of a many-body Pd-H system with ab initio accuracy. The QTST result has shown a clear non-Arrhenius behavior in the diffusion coefficient $\left(D\right)$ of H below the temperature of 150–200 K due to the NQEs. Comparing the $D$ on Pd surface and bulk Pd, it was found that surface and bulk diffusion are competitive at high temperature. Consistent with this, it was observed in the RPMD simulation at 800 K that a part of the quantum trajectories of H on the surface bifurcates to the Pd subsurface. As the temperature decreases, the surface diffusion becomes much faster than the bulk diffusion. Furthermore, distinct quantum behaviors of H were identified at the surface and within the bulk. The $D$ values obtained from this study using a “flexible” Pd surface model differed considerably from those previously reported using “frozen” Pd surface models, indicating an important contribution from Pd vibrations coupled with H diffusion.

Materials that exhibit ferroaxial order hold potential for novel multiferroic applications. However, in pure ferroaxials, domains are not directly coupled to stress or static electric field due to their symmetry, limiting the ability to pole and switch between domains—features required for real-world applications. Here we propose a general approach to selectively condense and switch between ferroaxial domains with light. We show that circularly polarized light pulses on resonance with infrared-active phonons manifest helicity-dependent control over ferroaxial domains. Nonlinear contributions to the lattice polarizability play an essential role in this phenomenon. We illustrate the feasibility of our approach using first-principles calculations and dynamical simulations for the archetypal ferroaxial material $\mathrm{RbFe}{\left({\mathrm{MoO}}_4\right)}_2$. Our results are discussed in the context of future pump-probe optical experiments, where polarization, carrier frequency, and fluence threshold are explored.

We propose a simplified probabilistic model to investigate the fundamental physics of particle transport in parallel channels having bypasses and defects, mimicking the distinctive pathways found in a class of solid-electrolyte materials. Our results reveal that increases in the bypass interval and the number of defects induce significant macroscopic density fluctuations, substantially reducing overall flow rates. Notably, we find that the particle flow rate decreases exponentially with the product of bypass interval and defect probability. The decay exponent was dependent on the particle density, which reflects the nonlinear effects of particle congestion. Our findings may suggest that the presence of more bypass sites and fewer defects in the pathways has a significant impact on the performance of electrolyte materials.

The domain structure of a fluid ferroelectric nematic is dramatically different from the domain structure of solid ferroelectrics since it is not restricted by rectilinear crystallographic axes and planar surface facets. We demonstrate that thin films of a ferroelectric nematic seeded by colloidal inclusions produce domain walls (DWs) in the shape of conics such as a parabola. These conics reduce the bound charge within the domains and at the DWs. An adequate description of the domain structures requires one to analyze the electrostatic energy, which is a challenging task. Instead, we demonstrate that a good approximation to the experimentally observed polydomain textures is obtained when the divergence of spontaneous polarization—which causes the bound charge—is heavily penalized by assuming that the elastic constant of splay in the Oseen-Frank energy is much larger than those for twist and bend. The model takes advantage of the fact that the polarization vector is essentially parallel to the nematic director throughout the sample.

For a Weyl semimetal (WSM) in a magnetic field, a semiclassical description of the Fermi-arc surface state dynamics is usually employed for explaining various unconventional magnetotransport phenomena, e.g., Weyl orbits, the three-dimensional quantum Hall effect, and the high transmission through twisted WSM interfaces. For a half-space geometry, we determine the low-energy quantum eigenstates for a four-band model of a WSM in a magnetic field perpendicular to the surface. The eigenstates correspond to in- and out-going chiral Landau level (LL) states, propagating (anti)parallel to the field direction near different Weyl nodes, which are coupled by evanescent surface-state contributions generated by all other LLs. These replace the Fermi arc in a magnetic field. Computing the phase shift accumulated between in- and out-going chiral LL states, we compare our quantum-mechanical results to semiclassical predictions. We find quantitative agreement between both approaches.

The formation of germanene on Ag(111) has been extensively studied and confirmed. However, the nature of the transition stage between the initial $\mathrm{A}{\mathrm{g}}_2\mathrm{Ge}$ alloy phase at 1/3 monolayer (ML) and the final quasifreestanding phase germanene phase at coverage $\ge 1$ ML still remains elusive. Using angle-resolved photoemission spectroscopy, scanning-tunneling microscopy (STM), and low-energy electron diffraction, we confirm the existence of striped-phase (SP) germanene in the transition stage. Along the armchair direction of SP, the period of higher-order coincidence corresponds to $11a_{\mathrm{AC}}=31a_{\mathrm{Ag}\left(111\right)}$, which accommodates four stripes on the bridge sites of the underlying Ag(111) lattice, as manifested in STM topography. On the electronic-structure side, the splitlike surface band of $\mathrm{A}{\mathrm{g}}_2\mathrm{Ge}$ centered at the surface zone boundary ${\overline{M}}_{\mathrm{Ag}\left(111\right)}$ evidently evolves to a steep, $\mathsf{V}$-shaped interfacial-state band, characteristic of SP germanene. Moreover, an anomalous broad Ag bulk-state band was observed to fully develop upon formation of SP germanene. The bulk-state band originates from the direct transition to the final-state bands of originally $d$ character at $\sim 22–23\mathrm{eV}$ above the Fermi level. Because of the strong hybridization between germanene π and Ag $d$ orbitals, the unoccupied Ag bulk final-state $d$ bands near the interface take on $p_z$ character, inducing the direct transition from the Ag bulk occupied $p_z$-valence band, following the selection rule for the photoemission cross section. A germanene identity of SP is thus confirmed. Our result indicates a very important missing segment of the evolution from the initial surface alloy phase to the final Xene layer, which is the coexistence of both in a common stripe lattice during a continuous transition.

Distinct from familiar $s$-, $p$-, or $d$-wave pairings, the monopole superconducting order represents a novel class of pairing order arising from nontrivial monopole charge of the Cooper pair. In the weak-coupling regime, this order can emerge when pairing occurs between Fermi surfaces with different Chern numbers in, for example, doped Weyl semimetal systems. However, the phase of monopole pairing order is not well-defined over an entire Fermi surface, making it challenging to design experiments sensitive to both its symmetry and topology. To address this, we propose a scheme based on symmetry and topological principles to identify this elusive pairing order through a set of phase-sensitive Josephson experiments. By examining the discrepancy between global and local angular momentum of the pairing order, we can unveil the monopole charge of the pairing order, including for models with higher pair monopole charge $|q_p|=1,2$, and 3. We demonstrate the proposed probe of monopole pairing order through analytic and numerical studies of Josephson coupling in models of monopole superconductor junctions. This work opens a promising avenue to uncover the unique topological properties of monopole pairing orders and to distinguish them from known pairing orders based on spherical harmonic symmetry.

In this paper, the optical response to a terahertz pulse was investigated in the transition metal chalcogenide $\mathrm{T}{\mathrm{a}}_2\mathrm{NiS}{\mathrm{e}}_5$, a candidate excitonic insulator. First, by irradiating a terahertz pulse with a relatively weak electric field (0.3 MV/cm), the spectral changes in reflectivity near the absorption edge due to third-order optical nonlinearity were measured, and the absorption peak characteristic of the excitonic phase just below the interband transition was identified. Next, by irradiating a strong terahertz pulse with a strong electric field of 1.65 MV/cm, the absorption of the excitonic phase was found to be reduced, and a Drude-like response appeared in the mid-infrared region. These responses can be interpreted as carrier generation by exciton dissociation induced by the electric field, resulting in the partial melting of the excitonic phase and metallization. The presence of a distinct threshold electric field for carrier generation indicates exciton dissociation via quantum-tunneling processes. The spectral change due to metallization by the electric field is significantly different from that due to the strong optical excitation across the gap, which can be explained by the different melting mechanisms of the excitonic phase in the two types of excitations.