In search of materials with three-dimensional flat band dispersions, using ab initio computations we investigate how topological phases evolve as a function of hydrostatic pressure and uniaxial strain in two types of superlattices: HgTe/CdTe and HgTe/HgSe. In short-period HgTe/CdTe superlattices, our analysis unveils the presence of isoenergetic nodal lines, which could host strain-induced three-dimensional flat bands at the Fermi level without requiring doping, when fabricated, for instance, as core-shell nanowires. In contrast, HgTe/HgSe short-period superlattices are found to harbor a rich phase diagram with a plethora of topological phases. Notably, the unstrained superlattice realizes an ideal Weyl semimetal with Weyl points situated at the Fermi level. A small-gap topological insulator with multiple band inversions can be obtained by tuning the volume: under compressive uniaxial strain, the material transitions sequentially into a Dirac semimetal to a nodal-line semimetal, and finally into a topological insulator with a single band inversion.

In a layered and strongly anisotropic superconductor, the hybrid modes provided by the propagation of electromagnetic waves in the matter identify two well-separated energy scales connected to the large in-plane plasma frequency and to the soft out-of-plane Josephson plasmon. Despite wide interest in their detection and manipulation by means of different experimental protocols, the physical ingredients underlying a unified description of plasma waves valid at arbitrary energy and momentum are still poorly understood. Here we provide a complete description of generalized plasma waves in layered superconductors in terms of the gauge-invariant superconducting phase by including both the Coulomb interaction and the relativistic effects. We show that the anisotropy of the superfluid response leads to two intertwined hybrid light-matter modes with mixed longitudinal and transverse character, while a purely longitudinal plasmon is only recovered for wave vectors larger than the crossover scale set in by the plasma-frequency anisotropy. Interestingly, below such scale both modes appear with equal weight in the physical density response. Our results open a promising perspective for plasmonic applications made possible by next-generation spectroscopic techniques able to combine submicron momentum resolution with THz energy resolution.

We study the creation of quantum skyrmions in quadratic nanoscopic lattices of quantum spins coupled by Dzyaloshinkii-Moriya and exchange interactions. We numerically show that different kinds of quantum skyrmions, characterized by the magnitude of their spin expectation values and strong differences in their stability, can appear as ground state and as metastable excitations. In dependence on the coupling strengths and the lattice size, the adiabatic rotation of magnetic control fields at the boundary allows for the creation of quantum skyrmions.

The ruthenate family of layered perovskites has been a topic of intense interest, with much work dedicated to the superconducting state of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$. Another long-standing puzzle is the lack of superconductivity in its sister compound, ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$, which constrains the possible mechanisms of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$. Here we address a microscopic mechanism that unifies the orders in these materials. Beginning from a model of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ featuring interorbital spin-triplet pairing via Hund's and spin-orbit couplings, we find that bilayer coupling alone enhances, while staggered rotations destroy interorbital superconductivity. A magnetic field then shifts van Hove singularities, allowing intraorbital spin-density wave order to form in ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$. Our theory predicts that ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ without staggered rotations exhibits interorbital superconductivity with a possibly higher transition temperature.

Charge-transfer insulators are the parent phase of a large group of today's unconventional high-temperature superconductors. Here we study experimentally and theoretically the interband excitations of the charge-transfer insulator silver fluoride ${\mathrm{AgF}}_2$, which has been proposed as an excellent analog of oxocuprates. Optical conductivity and resonant inelastic x-ray scattering on ${\mathrm{AgF}}_2$ polycrystalline sample show a close similarity with that measured on undoped ${\mathrm{La}}_2{\mathrm{CuO}}_4$. While the former shows a charge-transfer gap $\sim 3.4$ eV, larger than in the cuprate, $dd$ excitations are nearly at the same energy in the two materials. Density functional theory and exact diagonalization cluster computations of the multiplet spectra show that ${\mathrm{AgF}}_2$ is more covalent than the cuprate, in spite of the larger fundamental gap. Furthermore, we show that ${\mathrm{AgF}}_2$ is at the verge of a charge-transfer instability. The overall resemblance of our data on ${\mathrm{AgF}}_2$ to those published previously on ${\mathrm{La}}_2{\mathrm{CuO}}_4$ suggests that the underlying charge-transfer insulator physics is the same, while ${\mathrm{AgF}}_2$ could also benefit from a proximity to a charge density wave phase as in ${\mathrm{BaBiO}}_3$. Therefore, our work provides a compelling support to the future use of fluoroargentates for materials' engineering of novel high-temperature superconductors.

We propose a correspondence between the description of emergent Lorentz symmetry in condensed-matter systems and the established general effective field theory for Lorentz violation in fundamental theories of spacetime and matter. This correspondence has potential implications in both directions. We illustrate the proposal by investigating its consequences for the spectral and transport properties of Dirac and Weyl semimetals. Particular realizations of this framework give rise to Dirac nodal spectra with nodal lines and rings. We demonstrate a bulk-boundary correspondence between bulk topological invariants and drumhead surface states of these Dirac nodal semimetals. We calculate their transport coefficients in leading-order perturbation theory, thereby characterizing the unconventional electromagnetic response due to small deviations from emergent Lorentz invariance. Some prospective future applications of the correspondence are outlined.

The anomalous Hall effect (AHE) is a key transport signature revealing the topological properties of magnetic compounds. In quantum materials, the classical linear dependence of the AHE on magnetization often breaks down, which is typically ascribed to the presence of topological magnetic or electronic textures. However, the complex electronic structure of these compounds may offer alternative, unexplored mechanisms. Here, we show that a giant nonlinear AHE can originate from a series of magnetic-field-induced Lifshitz transitions in the spin-dependent band structure. In our experiments on ${\mathrm{EuCd}}_2{\mathrm{As}}_2$ the AHE contributes to 97% of the total Hall response, corresponding to a record anomalous Hall angle of 21%. Our scaling analysis and first-principles calculations demonstrate that the electronic structure is extremely sensitive to spin canting, with the magnetic field causing band crossing and band inversion and introducing a band gap when oriented along specific directions. Our results not only provide an ideal platform for Berry curvature engineering but reveal a general effect that may be applied to other material systems.

We investigate wave-vortex interaction emerging from an expanding compact vortex cluster in a two-dimensional Bose-Einstein condensate. We adapt techniques developed for compact gravitational objects to derive the characteristic modes of the wave-vortex interaction perturbatively around an effective vortex flow field. We demonstrate the existence of orbits or sound rings, in analogy to gravitational light rings, and compute the characteristic spectrum for the out-of-equilibrium vortex cluster. The spectrum obtained from numerical simulations of a stochastic Gross-Pitaevskii equation exhibiting an expanding vortex cluster is in excellent agreement with analytical predictions. Our findings are relevant for two-dimensional quantum turbulence, the semiclassical limit around fluid flows, and rotating compact objects exhibiting discrete circulation.

We report a comprehensive study of magnetic correlations in ${\mathrm{LaNiO}}_2$, a parent compound of the recently discovered family of infinite-layer (IL) nickelate superconductors, using multiple experimental and theoretical methods. Our specific heat, muon-spin rotation ($\mu \mathrm{SR}$), and magnetic susceptibility measurements on polycrystalline ${\mathrm{LaNiO}}_2$ show that long-range magnetic order remains absent down to 2 K. Nevertheless, we detect residual entropy in the low-temperature specific heat, which is compatible with a model fit that includes paramagnon excitations. The $\mu \mathrm{SR}$ and low-field static and dynamic magnetic susceptibility measurements indicate the presence of short-range magnetic correlations and glassy spin dynamics, which we attribute to local oxygen nonstoichiometry in the average infinite-layer crystal structure. This glassy behavior can be suppressed in strong external fields, allowing us to extract the intrinsic paramagnetic susceptibility. Remarkably, we find that the intrinsic susceptibility shows non-Curie-Weiss behavior at high temperatures, in analogy to doped cuprates that possess robust nonlocal spin fluctuations. The distinct temperature dependence of the intrinsic susceptibility of ${\mathrm{LaNiO}}_2$ can be theoretically understood by a multimethod study of the single-band Hubbard model in which we apply complementary cutting-edge quantum many-body techniques (dynamical mean-field theory, cellular dynamical mean-field theory, and the dynamical vertex approximation) to investigate the influence of both short- and long-ranged correlations. Our results suggest a profound analogy between the magnetic correlations in parent (undoped) IL nickelates and doped cuprates.

There is a growing interest in applying different external potentials other than the conventional uniform disorder for inhibiting thermalization and realizing the many-body localization (MBL) phase. Various types of external potentials have been explored, e.g., linear/quasiperiodic potentials and discrete disorder, but what type of potentials is the most “effective” for inducing MBL remains elusive. In this work, we formally introduce the problem of the optimal randomized potential from two aspects, which is to find the optimal on-site energy distribution so that (1) the required random field strength for the onset of MBL (i.e., $W_c$ in the literature) is the minimal, and (2) the thermalization occurs the slowest after the system is prepared in an out-of-equilibrium state. We choose the spinless fermionic chain to study the problem using two complementary methods. For short chains ($L<20$), we employ exact numerical approaches for (1) diagnosing MBL transitions and (2) simulating the dynamics of the entanglement entropy. For longer chains [$L\sim O\left(100\right)$], we employ a modified real-space renormalization group (RSRG) approach to trade off accuracy for simulation sizes and scaling properties. Both methods predict that random potentials with a certain degree of polarization towards binary disorder will favor the MBL more than the uniform distribution. For some studied cases, the optimal random distribution lies in between the uniform and binary disorder. We attribute that to the competition between increased on-site energy fluctuation and enhanced degeneracy near the distribution edge ($\pm W$). While precisely determining the functional form of the optimal random distribution is not feasible at present, our results could be useful for engineering more efficient potentials for realizing MBL in mesoscopic systems.

In some non-Hermitian systems, the eigenstates in the bulk are localized at the boundaries of the systems. This is called the non-Hermitian skin effect, and it has been studied mostly in discrete systems. In the present work, we study the non-Hermitian skin effect in a continuous periodic model. In a one-dimensional system, we show that the localization lengths are equal for all the eigenstates. Moreover, the localization length and the eigenspectra in a large system are independent of the types of open boundary conditions. These properties are also found in a non-Hermitian photonic crystal. Such remarkable behaviors in a continuous periodic model can be explained in terms of the non-Bloch band theory. By constructing the generalized Brillouin zone for a complex Bloch wave number, we derive the localization length and the eigenspectra under an open boundary condition. Furthermore, we show that the generalized Brillouin zone also has various physical properties, such as bulk-edge correspondence.

In this work, the generalization of the Friedel formula and Krein's theorem in complex potential scattering theory is presented. The consequences of various symmetry constraints on dynamical systems are discussed. In addition, the Muskhelishvili-Omnès representation of Krein's theorem is also given and discussed.

Designing unconventional interactions between single phonons and spins is fascinating for its applications in quantum phononics. Here we propose a reliable scheme for coupling spins and phonons in phononic dimer and trimer lattices, with the combination of solid-state defects and diamond phononic (optomechanical) crystals. The dimer and trimer lattices used are an array of coupled phononic cavities with spatially modulated hopping rates. We predict a series of unconventional sound-matter interaction phenomena in this hybrid quantum system. In the dimer lattice we show the formation of chiral spin-phonon bound states and topology-dependent phononic collective radiation. While in the trimer lattice, chiral bound states still exist and the spin relaxation is sublattice dependent. The chiral bound states existing in both types of lattices are robust to a large amount of disorder, which can mediate chiral and robust spin-spin interactions. This work provides a promising platform for phonon-based quantum information processing and quantum simulation.

The complex dynamical phases of quantum systems are dictated by atomic interactions that usually evoke an emergent periodic order. Here, we study a quantum many-body system with two competing and substantially different long-range interaction potentials where the dynamical instability towards density order can give way to a disordered amorphous solid, which exhibits local density modulations but no long-range periodic order. We consider a two-dimensional Bose-Einstein condensate in the Rydberg-dressing regime coupled to an optical standing wave resonator. The dynamic pattern formation in this system is governed by the competition between the two involved interaction potentials: repulsive soft-core interactions arising due to Rydberg dressing and infinite-range sign changing interactions induced by the cavity photons. The amorphous phase is found when the two interaction potentials introduce incommensurate length scales. The dynamic formation of this peculiar phase can be attributed to frustration induced by the two competing interaction energies and length scales.

The rise of organic electronics calls for versatile modeling tools. In this context, we develop a semiclassical Boltzmann theory that describes transport and excitonic processes in crystalline organic semiconductors on equal footing. The generation of singlet and triplet excitons out of the ground state, their formation from free electrons and holes, the reverse processes, as well as the fusion and fission of excitons are included. The corresponding scattering integrals respect spin conservation, which requires matrix-valued distribution functions. They also include fermionic and bosonic many-particle effects such as Pauli blocking. We employ a multipole expansion of the distribution functions, where quadrupolar terms turn out to be essential for the triplet excitons. This work provides a basis for the modeling of organic solar cells, in which excitonic processes are crucial for the performance. Moreover, the theory is of general interest for transport and transitions of multiple (quasi-) particle species carrying spin in nonequilibrium systems.

We study the Coulomb glass emerging from the interplay of strong interactions and disorder in a model of spinless fermions on the Bethe lattice. In the infinite coordination number limit, strong interactions induce a metallic Coulomb glass phase with a pseudogap structure at the Fermi energy. Quantum and thermal fluctuations both melt this glass and induce a disordered quantum liquid phase. We combine self-consistent diagrammatic perturbation theory with continuous time quantum Monte-Carlo simulations to obtain the complete phase diagram of the electron glass and to characterize its dynamical properties in the quantum liquid, as well as in the replica symmetry broken glassy phase. Tunneling spectra display an Efros-Shklovskii pseudogap upon decreasing temperatures, but the density of states remains finite at the Fermi energy due to residual quantum fluctuations. Our results bear relevance to the metallic glass phase observed in Si inversion layers.

Rare-earth nickelates $R\mathrm{Ni}{\mathrm{O}}_2$ adopting an infinite-layer phase show superconductivity once La, Pr, or Nd is substituted by a divalent cation. Either in the pristine or doped form, these materials are reported to adopt a high-symmetry, perfectly symmetric, $P_4\text{/}mmm$ tetragonal cell. Nevertheless, bulk compounds are scarce, hindering a full understanding of the role of chemical pressure or strain on lattice distortions that in turn could alter magnetic and electronic properties of the two-dimensional nickelates. Here, by performing a full analysis of the prototypical ${\mathrm{YNiO}}_2$ compound with first-principles simulations, we identify that these materials are prone to exhibit ${\mathrm{O}}_4$ group rotations whose type and amplitude are governed by the usual $R$-to-Ni cation size mismatch. We further show that these rotations can be easily tuned by external stimuli modifying lattice parameters such as pressure or strain. Finally, we reveal that H intercalation is favored for any infinite-layer nickelate member and pushes the propensity of the compounds to exhibit octahedra rotations.