The nature of superconductivity in heavy-fermion materials is a subject under intense debate, and controlling this many-body state is central for its eventual understanding. Here, we examine how proximity effects may change this phenomenon, by investigating the effects of an additional metallic layer on the top of a Kondo lattice and allowing for pairing in the former. We analyze a bilayer Kondo lattice model with an on-site Hubbard interaction, $-U$, on the additional layer, using a mean-field approach. For $U=0$, we notice a drastic change in the density of states due to multiple-orbital singlet resonating combinations. It destroys the well-known Kondo insulator at half filling, leading to a metallic ground state, which, in turn, enhances antiferromagnetism through the polarization of the conduction electrons. For $U\ne 0$, a superconducting Kondo state sets in at zero temperature, with the occurrence of unconventional pairing amplitudes involving $f$ electrons. We establish that this remarkable feature is only possible due to the proximity effects of the additional layer. At finite temperatures, we find that the critical superconducting temperature $T_c$ decreases with the interlayer hybridization. We have also established that a zero temperature superconducting amplitude tracks $T_c$, which reminisces the BCS proportionality between the superconducting gap and $T_c$.

The relative orientation (twist) of successive layers of stacked two-dimensional (2D) materials creates variations in the interlayer atomic registry. The variations often form a superlattice, called a moiré pattern, which can alter electronic properties. In this work we introduce a classification of the single-particle electronic structures that can occur in twisted stacks of 2D layers by characterizing them as “moiré molecules” or “moiré crystals.” The molecules generate localized electronic states and moiré flat bands, while the crystals are sometimes unconventional and produce electronic banding in the configuration basis. The underpinning of this classification is the duality between interlayer configuration and monolayer Bloch momentum in moiré Hamiltonians. We apply this understanding to diagrams of local electron density in untwisted geometries to produce intuitive and quantitative predictions of twistronic properties. We provide a conceptual introduction to this framework through a one-dimensional model, and then apply it to 2D twisted bilayers of the semimetal graphene, and of ${\mathrm{MoS}}_2$, a representative material of the transition metal dichalcogenide family of semiconductors. This level of thorough understanding of twistronic phenomena is vital in the search for new material platforms for localized moiré electrons.

We show that optical excitation of the Mott insulating phase of the one-dimensional Hubbard model can create a state possessing two of the hallmarks of superconductivity: a nonvanishing charge stiffness and long-ranged pairing correlation. By employing the exact diagonalization method, we find that the superposition of the $\eta$-pairing eigenstates induced by the optical pump exhibits a nonvanishing charge stiffness and a pairing correlation that decays very slowly with system size, in sharp contrast to the behavior of an ensemble of thermally excited eigenstates, which has a vanishing charge stiffness and no long-ranged pairing correlations. We show that the charge stiffness is indeed directly associated with the $\eta$-pairing correlation in the Hubbard model. Our finding demonstrates that optical pumping can actually lead to superconducting-like properties on the basis of the $\eta$-pairing states.

The macroscopic performance of ferroelectric and piezoelectric devices depends strongly on domain wall dynamics. It is clear that structural defects, such as vacancies, interstitials, and dopants codetermine the dynamics, but the microscopic understanding of the wall-defect interactions is still at an early stage. Hexagonal manganites are among of the most intensively studied systems with respect to static domain wall properties and thus are ideal model materials for studying domain wall mobility in the presence of defects. Here we study the mobility of domain walls in the hexagonal manganites and how it is affected by cation dopants using density functional theory calculations. The results are correlated with scanning probe microscopy measurements on single crystals, to confirm an increasing domain wall roughness for the dopants we predict to pin the walls. The pinning originates from elastic strain fields around the walls interacting with the local crystal perturbations surrounding a dopant. The pinning strength is correlated with the local change in order parameter amplitude caused by the dopant. As a computationally friendly alternative to large supercell calculations, we demonstrate that domain wall pinning can be predicted from the dopants’ effect on the free-energy landscape of polarization switching. This approach allows to directly probe the effect of defects on domain wall mobility using a fraction of the computational cost, opening the door to detailed modeling and understanding of the critical pinning process of domain walls.

We investigate the Josephson radiation emitted by a junction made of a quantum dot coupled to two conventional superconductors. At strong coupling, such a junction accommodates two Andreev bound states, which are slightly detached from the continuum outside the superconducting gap. Furthermore, when the dot is tuned close to resonance, the residual backscattering opens a small gap near the Fermi energy between the discrete states. Under voltage bias, we formulate a stochastic model that accounts for nonadiabatic processes, which change the occupations of the Andreev bound states. This model allows one to calculate the current noise spectrum and determine the Fano factor. Analyzing the finite-frequency noise, we find that the model may exhibit either an integer or a fractional AC Josephson effect, depending on the bias voltage and the size of the gaps in the Andreev spectrum. Our results assess the limitations in using the fractional Josephson radiation as a probe of topological superconductivity.

Double perovskites extend the design space for new materials and they often host phenomena that do not exist in their parent perovskite compounds. Here we present a detailed first-principles study of the correlated double perovskite ${\mathrm{Sr}}_2{\mathrm{VNbO}}_6$, where the intercationic charge transfer and strength of electronic correlations depend strongly on the cation order. By using density functional theory + embedded dynamical mean field theory, we show that this compound has a completely different electronic structure than either of its parent compounds despite V and Nb being from the same group in the Periodic Table. We explain how the electronic correlations' effect on the crystal structural parameters determines on which side of the Hund's metal–Mott insulator transition the material is. Our results demonstrate the emergence of Hund's metallic behavior in a double perovskite that has $d^1$ parents and underline the importance of electronic correlation effects on the crystal structure.

Many-body localization is a unique physical phenomenon driven by interactions and disorder for which a quantum system can evade thermalization. Although the existence of a many-body localized phase is now well established in one-dimensional systems, its fate in higher dimensions is an open question. We present evidence for the occurrence of a transition to a many-body localized regime in a two-dimensional quantum dimer model with interactions and disorder. Our analysis is based on the results of large-scale simulations for static and dynamical properties of a consequent number of observables. Our results pave the way for a generic understanding of occurrence of a many-body localization transition in a dimension larger than one and highlight the unusual quantum dynamics that can be present in constrained systems.

A theoretical framework for the study of electronic stopping of particle projectiles in crystalline solids is proposed. It neither relies on perturbative or linear-response approximations nor on an ideal metal host. Instead, it exploits the discrete translational invariance in a space-time diagonal for a constant velocity projectile following a trajectory with crystalline periodicity. By means of Floquet theory, (stroboscopically) stationary solutions are characterized, and previous perturbative and jellium models are recovered. The threshold-velocity effect in insulators is analyzed based on quasienergy conservation.

Monolayer graphene placed with a twist on top of AB-stacked bilayer graphene hosts topological flat bands in a wide range of twist angles. The dispersion of these bands and gaps between them can be efficiently controlled by a perpendicular electric field, which induces topological transitions accompanied by changes of the Chern numbers. In the regime where the applied electric field induces gaps between the flat bands, we find a relatively uniform distribution of the Berry curvature. Consequently, interaction-induced valley- and/or spin-polarized states at integer filling factors are energetically favorable. In particular, we predict a quantum anomalous Hall state at filling factors $\nu =1,3$ for a range of twist angles $1^{\circ }<\theta <1.4^{\circ }$. Furthermore, to characterize the response of the system to magnetic field, we computed the Hofstadter butterfly and the Wannier plot, which can be used to probe the dispersion and topology of the flat bands in this material.

Most currently used approximations for the one-particle Green's function $G$ in the framework of many-body perturbation theory, such as Hedin's GW approximation or the cumulant $\mathrm{GW}+\mathrm{C}$ approach, are based on a linear-response approximation for the screened interaction $W$. The extent to which such a hypothesis is valid and ways to go beyond have been explored only very little. Here we show how to derive a cumulant Green's function beyond linear response from the equation of motion of the Green's function in a functional derivative formulation. The results can be written in a compact form, which opens the possibility to calculate the corrections in a first-principles framework using time-dependent density functional theory. In order to illustrate the potential importance of the corrections, numerical results are presented for a model system with a core level and two valence orbitals.

Topological spin textures in the itinerant ferromagnet $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3$ are studied combining Hall transport measurements and numerical simulations. We observe characteristic signatures of the topological Hall effect associated with skyrmions. A relatively large thickness of our films and the absence of heavy-metal layers make the interfacial Dzyaloshinskii-Moriya interaction an unlikely source of these topological spin textures. In addition, the transport anomalies exhibit an unprecedented robustness to magnetic field tilting and temperature variations. Our numerical simulations suggest that this unconventional behavior results from compact magnetic bubbles with skyrmion topology stabilized by magnetodipolar interactions and higher-order magnetocrystalline ansiotropies in an unexpected region of parameter space. This work presents a comprehensive understanding of the origin of the topological Hall effect in $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3$, incorporating the complex angular dependence of the magnetic anisotropy, intrinsic to this material.

We study the in-plane electronic anisotropy in the parent compounds of several families of Fe-based superconductors (${\mathrm{BaFe}}_2{\mathrm{As}}_2, {\mathrm{EuFe}}_2{\mathrm{As}}_2$, NaFeAs, LiFeAs, FeSe, and LaFeAsO) by polarization-resolved Raman scattering. We measure intensity of the fully symmetric $c$-axis vibration of As atom mode in the $XY$ scattering geometry and notice that the mode's intensity is significantly enhanced below the magnetostructural transition only for compounds showing magnetic ordering. In particular, we find that the intensity ratio of this As phonon in the $XY$ vs. $XX$ scattering geometries is proportional to the square of the ordered magnetic moment. We relate this As phonon intensity enhancement below the Néel temperature in iron pnictides to in-plane electronic anisotropy induced by the collinear spin-density wave order.

We investigate the stability of vortices in two-dimensional Bose–Einstein condensates. In analogy with rotating space-times and with a careful account of boundary conditions, we show that the dynamical instability of multiply quantized vortices in trapped condensates persists in untrapped, spatially homogeneous geometries and has an ergoregion nature with some modification due to the peculiar dispersion of Bogoliubov sound. Our results open perspectives to the physics of vortices in trapped condensates, where multiply quantized vortices can be stabilized by interference effects and singly charged vortices can become unstable in suitably designed trap potentials. We show how superradiant scattering can be observed also in the short-time dynamics of dynamically unstable systems, providing an alternative point of view on dynamical (in)stability phenomena in spatially finite systems.

A large ensemble of quantum vortices in a superfluid may itself be treated as a novel kind of fluid that exhibits anomalous hydrodynamics. Here we consider the dynamics of vortex clusters under thermal friction and present an analytic solution that uncovers a new universality class in the out-of-equilibrium dynamics of dissipative superfluids. We find that the long-time dynamics of the vorticity distribution is universal in the form of an expanding Rankine vortex (i.e., top-hat distribution) independent of initial conditions. This highlights a fundamentally different decay process to classical fluids, where the Rankine vortex is forbidden by viscous diffusion. Numerical simulations of large ensembles of point vortices confirm the universal expansion dynamics and further reveal the emergence of a frustrated lattice structure marked by strong correlations. We present experimental results of expanding vortex clusters in a quasi-two-dimensional Bose-Einstein condensate that are in excellent agreement with the vortex fluid theory predictions, demonstrating that the signatures of vortex fluid theory can be observed with as few as $N\sim 11$ vortices. Our theoretical, numerical, and experimental results establish the validity of the vortex fluid theory for superfluid systems.

We combine the shoving model of $T$-dependent viscosity of supercooled liquids with the Zwanzig-Mountain formula for the high-frequency shear modulus using the $g\left(r\right)$ of molecular dynamics simulations of metal alloys as the input. This scheme leads to a semianalytical expression for the viscosity as a function of temperature, which provides a three-parameter model fitting of experimental data of viscosity for the same alloy for which $g\left(r\right)$ was calculated. The model provides direct access to the influence of atomic-scale physical quantities such as the interatomic potential $\varphi \left(r\right)$ on the viscosity and fragile-strong behavior. In particular, it is established that a steeper interatomic repulsion leads to fragile liquids, or, conversely, that “soft atoms make strong liquids.”

The current carrying capability of type II superconductors under magnetic fields is determined to a large extent by the interaction of superconducting vortices with pinning centers. Vortices are arranged in lattices with varying degrees of disorder depending on the balance between the intervortex interactions and the pinning strength. We analyze here vortex arrangements in disordered vortex lattices of different superconducting systems, single crystals (Co-doped ${\mathrm{NbSe}}_2$, LiFeAs, and ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$), and amorphous W-based thin films (with critical temperatures $T_c$ from 4 K to 35 K and critical fields from 3.4 T to more than 90 T). We calculate for each case the structure factor and number variance and compare to calculations on an interacting set of partially pinned particles. We find that random density fluctuations appear when pinning overcomes interactions and show that the suppression of density fluctuations is correlated to the presence of interactions. We discuss the results within the framework of hyperuniform distributions and find that all studied lattices follow a similar increase of the number variance with the defect density.

Studying the disappearance of superconductivity at the end of the overdoped region of the cuprate phase diagram offers a different approach for investigating the interaction which is responsible for pairing in these materials. In the underdoped region this question is complicated by the presence of charge and stripe ordered phases as well as the pseudogap. In the overdoped region the situation appears simpler with only a normal phase, a superconducting phase, and impurity scattering. Here, for the overdoped region, we report the results of a combined dynamic cluster approximation (DCA) and a weak Born impurity scattering calculation for a $t\text{−}{t}^{\prime }\text{−}U$ Hubbard model. We find that a decrease in the $d$-wave pairing strength of the two-particle scattering vertex is closely coupled to changes in the momentum and frequency structure of the magnetic spin fluctuations as the system is overdoped. Treating the impurity scattering within a disordered BCS $d$-wave approximation, we see how the combined effects of the decreasing $d$-wave pairing strength and weak impurity scattering lead to the end of the $T_c$ dome.

We propose a non-Hermitian rhombic lattice and investigate the Aharonov-Bohm caging, which forms at the exceptional point (EP) of the rhombic lattice at which all the lattice eigenstates are coalesced. The compact localized eigenstates and the localization dynamics are investigated. The excitation intensity can be constant or can polynomially increase with time as a consequence of the peculiar dynamics at the EP. The excitation can be completely localized in the two diamond plaquettes of the rhombic lattices and the localization area can be controlled by the gain and loss. Our findings shed light on the Aharonov-Bohm cage in the non-Hermitian physics and are beneficial for light engineering in the non-Hermitian metamaterials.

We extend a field theoretic approach for the investigation of the electronic charge-current density response of crystalline systems to arbitrary electromagnetic fields. The approach leads to the introduction of microscopic polarization and magnetization fields, as well as free charge and current densities, the dynamics of which are described by a lattice gauge theory. The spatial averages of such quantities constitute the fields of macroscopic electrodynamics. We implement this formalism to study the modifications of the orbital electronic properties of a class of insulators due to uniform dc electric and magnetic fields, at zero temperature. To first order in the electric and magnetic fields, the free charge and current densities vanish; thus the linear effect of such fields is captured by the first-order modifications of the microscopic polarization and magnetization fields. Associated with the dipole moment of the microscopic polarization (magnetization) field is a macroscopic polarization (magnetization), for which we extract various tensors relating it to the electric and magnetic fields. We focus on the orbital magnetoelectric polarizability (OMP) tensor, and find the accepted expression as derived from the “modern theory of polarization and magnetization.” Since our results are based on the spatial averages of microscopic polarization and magnetization fields, we can identify the distinct contributions to the OMP tensor from the perspective of this microscopic theory, and we establish the general framework in which extensions to finite frequency can be made.

We introduce new classes of hydrodynamic theories inspired by the recently discovered fracton phases of quantum matter. Fracton phases are characterized by elementary excitations (fractons) with restricted mobility. The hydrodynamic theories we introduce describe thermalization in systems with fractonlike mobility constraints, including fluids where charge and dipole moment are both locally conserved, and fluids where charge is conserved along every line or plane of a lattice. Each of these fluids is subdiffusive and constitutes a new universality class of hydrodynamic behavior. There are infinitely many such classes, each with distinct subdiffusive exponents, all of which are captured by our formalism. Our framework naturally explains recent results on dynamics with constrained quantum circuits, as well as recent experiments with ultracold atoms in tilted optical lattices. We identify crisp experimental signatures of these novel hydrodynamics and explain how they may be realized in near term ultracold atom experiments.