${\mathrm{MnBi}}_4{\mathrm{Te}}_7$ belongs to a family of antiferromagnetic topological insulators. It forms a natural heterostructure of magnetic (septuple) and nonmagnetic (quintuple) topological blocks. Here, we explore the magnetism and magnetic interactions in this compound using inelastic neutron scattering. We find that the interlayer magnetic coupling is much weaker in ${\mathrm{MnBi}}_4{\mathrm{Te}}_7$ as compared to ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ due to the insertion of nonmagnetic quintuple layers in the former. However, other key magnetic energy scales residing within a single septuple block, the single-ion anisotropy and long-range intralayer exchanges, are essentially the same. This identifies a transferable set of magnetic interactions applicable to the extended family of magnetic topological insulators based on ${\mathrm{MnBi}}_2{\mathrm{Te}}_4\text{−}{\mathrm{Bi}}_2{\mathrm{Te}}_3$ heterostructures.

Schmid predicted that a single Josephson junction coupled to a resistive environment undergoes a quantum phase transition to an insulating phase when the shunt resistance $R$ exceeds the resistance quantum $h/\left(4e^2\right)$. Recent measurements and theoretical studies have sparked a debate on whether the location of this transition depends on the ratio between the Josephson energy and the charging energy. We employ a combination of innovative analytical and numerical techniques to demonstrate decisively that the transition line between superconducting and insulating behavior is indeed independent of this energy ratio. First, we apply field-theoretic renormalization group methods and find that the $\beta$ function vanishes along the critical line up to the third order in the Josephson energy. We then identify a simple fermionic model that precisely captures the low-energy physics on the critical line, independent of the energy ratio. This conformally invariant fermionic model is verified by comparing the expected spectrum with exact diagonalization calculations of the resistively shunted Josephson junction, showing excellent agreement even for moderate-sized systems. Importantly, this identification provides a rigorous nonperturbative proof that the transition line remains fixed at $R=h/\left(4e^2\right)$ for all ratios of Josephson to charging energies. The line also remains resilient to other ultraviolet cutoffs, such as the plasma frequency of the resistive environment. Finally, we implement an adiabatic approach to validate the duality at high Josephson energy.

In condensed matter physics, various mechanisms give rise to distinct insulating phases. The competition and interplay between these phases remain elusive, even for the seemingly most distinguishable band and Mott insulators. In multilayer systems, such interplay is mediated by interlayer hopping, which competes with the Coulomb repulsion to determine the nature of insulators. The layered compound $1T\text{−}{\mathrm{TaS}}_2$ provides an ideal platform for investigating this phenomenon, as it naturally hosts coexisting Mott and band insulating states. However, distinguishing these distinct insulating states and characterizing the evolution remain challenging. In this study, we employ a dual approach utilizing surface-sensitive high-resolution electron energy loss spectroscopy and bulk-sensitive Fourier-transform infrared spectroscopy to investigate the electronic excitation spectrum of $1T\text{−}{\mathrm{TaS}}_2$. Our methodology effectively identifies the features originating from the Mott and band insulators by analyzing the differences in their bulk and surface spectral weights, along with their energy distinctions. Based on the previous identification, we further investigate the evolution of insulating state features in the homostructure as they are modulated by temperature. The measurements and dynamical mean-field theory calculations suggest that the softening and broadening of Hubbard excitations in the Mott state with increasing temperature result from enhanced interlayer hopping between the Mott and band insulators.

We show that in models with the Hatsugai-Kohmoto type of interaction that is local in momentum space thus infinite range in real space, Kubo formulas neither reproduce the correct thermodynamic susceptibilities, nor yield sensible transport coefficients. Using Kohn's trick to differentiate between metals and insulators by threading a flux in a torus geometry, we uncover the striking property that Hatsugai-Kohmoto models with an interaction-induced gap in the spectrum sustain a current that grows as the linear size at any nonzero flux and which can be either diamagnetic or paramagnetic.

The Kitaev chain can be engineered within a quantum dot-superconductor array, hosting Majorana zero modes at fine-tuned sweet spots. In this work, we propose and simulate the occurrence of Rabi and Ramsey oscillations to feasibly construct a minimal Majorana qubit in the quantum dot setup. Our real-time results incorporate realistic effects, e.g., charge noise and leakage, reflecting the latest experimental progress. We demonstrate that Majorana qubits with larger energy gaps exhibit significantly enhanced performance—longer dephasing times, higher quality factors, reduced leakage probabilities, and improved visibilities—compared to those with smaller gaps and with conventional quantum-dot-based charge qubits. We introduce a method for reading out Majorana qubits via quantum capacitance measurements. Our work paves the way for future experiments on realizing Majorana qubits in quantum dot-superconductor arrays.

By harnessing the unique properties of bilayer graphene, we present a flexible platform for achieving electrically tunable exciton polaritons within a microcavity. Using a semiclassical approach, we solve Maxwell's equations within the cavity, approximating the optical conductivity of bilayer graphene through its excitonic response as described by the Elliott formula. Transitioning to a quantum mechanical framework, we diagonalize the Hamiltonian governing excitons and cavity photons, revealing the resulting polariton dispersions, Hopfield coefficients, and Rabi splittings. Our analysis predicts that, under realistic exciton lifetimes, the exciton-photon interaction reaches a strong coupling regime. Furthermore, we explore the integration of an epsilon-near-zero material within the cavity, demonstrating that such a configuration can further enhance the light-matter interaction.

The insulating mixed-valent ${\mathrm{Ir}}^{+3.66}$ compound ${\mathrm{Ba}}_4{\mathrm{NbIr}}_3{\mathrm{O}}_{12}$ hosts two holes per ${\mathrm{Ir}}_3{\mathrm{O}}_{12}$ trimer unit. We address the electronic structure via resonant inelastic x-ray scattering (RIXS) at the Ir $L_3$ edge and exact diagonalization. The holes occupy quasimolecular orbitals that are delocalized over a trimer. This gives rise to a rich intra-$t_{2g}$ excitation spectrum that extends from 0.5 eV to energies larger than 2 eV. Furthermore, it yields a strong modulation of the RIXS intensity as a function of the transferred momentum q. A clear fingerprint of the quasimolecular trimer character is the observation of two modulation periods, $2\pi /d$ and $2\pi /2d$, where $d$ and $2d$ denote the intratrimer Ir-Ir distances. We discuss how the specific modulation reflects the character of the wave function of an excited state. Our quantitative analysis shows that spin-orbit coupling $\lambda$ of about 0.4 eV is decisive for the character of the electronic states, despite a large hopping $t_{a_{1g}}$ of about 0.8 eV. The ground state of a single trimer is described very well by both holes occupying the bonding $j=\frac{1}{2}$ orbital, forming a vanishing quasimolecular moment with $J=0$.

The interplay between non-Hermiticity and disorder gives rise to unique universality classes of Anderson transitions. Here, we develop a field-theoretical description of non-Hermitian disordered systems based on fermionic replica nonlinear sigma models. We classify the target manifolds of the nonlinear sigma models across all the 38-fold symmetry classes of non-Hermitian systems and corroborate the correspondence of the universality classes of Anderson transitions between non-Hermitian systems and Hermitized systems with additional chiral symmetry. We apply the nonlinear sigma model framework to study the spectral properties of non-Hermitian random matrices with particle-hole symmetry. Furthermore, we demonstrate that the Anderson transition unique to nonreciprocal disordered systems in one dimension, including the Hatano-Nelson model, originates from the competition between the kinetic and topological terms in a one-dimensional nonlinear sigma model. We also discuss the critical phenomena of non-Hermitian disordered systems with symmetry and topology in higher dimensions.

The experimental observation of an in-plane Hall effect (IPHE) in ${\mathrm{V}}_5{\mathrm{S}}_8$ has sparked interest in exploring more materials and mechanisms for this unexpected phenomenon. Several theoretical studies have predicted an IPHE due to Berry curvature in a prototypical magnetic Weyl semimetal, ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$. Here, we report the observation of an IPHE in ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$. The field dependence exhibits a dominant $B^3$ behavior. The field angle and temperature dependencies are obtained, which imply the crucial role played by the Lorentz force. Considering the band structure of ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$, a multiband model is applied to account for the observed IPHE, which is further confirmed by the good agreement between the experimental data and the model. Our analysis depicts how anisotropy, combined with multibands, can give rise to a Lorentz-force-induced in-plane Hall effect. Such an insight is useful in future investigations of the IPHE in various materials. Moreover, our results show that the IPHE may be used to reveal the conductivity anisotropy of individual Fermi pockets in multiband systems.

We report a theoretical formalism that describes a dynamic magnetic response of rare-earth orthoferrites, particularly those with non-Kramers rare-earth (R) ions, when driven by strong terahertz fields. We derive a total thermodynamic potential for the exchange coupled R-Fe system by constructing an effective Hamiltonian and employing a mean-field theory approximation. We investigate static properties of the R and Fe subsystems across the spin-reorientation phase transitions and obtain resonance frequencies for Fe and R magnetic sublattices as a function of temperature. Taking an example of the archetypical orthoferrite $\mathrm{TmFe}{\mathrm{O}}_3$, we perform numerical modeling to accurately describe the behavior of its anisotropy functions vs temperature. Finally, we analyze switching dynamics of Fe spins and nonlinear effects in the R subsystem of $\mathrm{TmFe}{\mathrm{O}}_3$ driven by strong terahertz radiation.

Linear temperature dependence of transport coefficients in metals is often ascribed to non-Fermi-liquid physics. Here we demonstrate the $T$-linear behavior of nonlocal conductivity in a clean 2D electron fluid, where carrier collisions assist conduction and lead to hydrodynamic transport with conductance rather than resistance growing with temperature. The key aspect is the occurrence of multiple hydrodynamic modes representing odd-parity modulations of the Fermi surface evolving in space and time. A cascade of such modes results in a linear $T$ dependence that extends down to lowest temperatures, as well as a Kolmogorov-like fractional power $-5/3$ scaling of conductivity vs wave number. This behavior serves as a clear signature of tomographic hydrodynamics driven by such modes, and is expected to be a generic feature of 2D electron fluids with simple near-circular Fermi surfaces.

We demonstrate that dynamical electron correlation and fluctuating local magnetic moments are crucial for the phonon spectra of the infinite-layer nickelate superconductor ${\mathrm{LaNiO}}_2$ using density functional theory plus dynamical mean-field theory ($\mathrm{DFT}+\mathrm{DMFT}$) calculations. We find significant hardening of optical Ni-O bond-stretching phonons when going from a nonmagnetic to paramagnetic state, and increasing Coulomb interaction will make them even harder. The electron correlation is found to be sensitive to the Ni-O bond-stretching distortions, indicating a strong interplay between the electron correlation and lattice. We find that the strong local electron correlation will not favor charge orders that couple to the Ni-O bond-stretching phonons, in support of the recent experiment that a $3a_0$ charge order is absent in the infinite-layer nickelates. Our results emphasize that the effects of local magnetic fluctuations should be fully taken into account when describing the lattice dynamics of the infinite-layer nickelates without long-range magnetic orders, and also provide evidence to ascribe the kink observed in the recent angle-resolved photoemission experiment to possible strong electron coupling to the Ni-O bond-stretching phonons.

The efficient light-matter interaction in combination with the small volume occupied by monolayer transition metal dichalcogenides makes this material class a notable option as a gain layer in future optoelectronic devices. Many-body effects of excited carriers influence the emission dynamics due to the introduction of optical nonlinearities following excitation, but the exact mechanisms remain unexamined from a theoretical point of view. In this paper, we present a material-realistic microscopic theory of a device based on an ${\mathrm{MoS}}_2$ monolayer, which demonstrates stimulated emission activity at room temperature. The modeling procedure combines Coulomb and light-matter interaction matrix elements with doublet-level quantum laser equations. These give access to the dynamics of the photon-assisted polarization, populations, and photon number while allowing the solution of a multiscale problem ranging from femto- to nanoseconds. The input-output curve, hole burning, and spectral clamping obtained from this theory present strong indications of electron-hole-plasma-based lasing occurring at densities above $5\times {10}^{13}{\text{cm}}^{-2}$ in this device.

We study nonequilibrium spin dynamics in differentially rotating systems, deriving an effective Hamiltonian for conduction electrons in the comoving frame. In contrast to conventional spin current generation mechanisms that require vorticity, our theory describes spins and spin currents arising from differentially rotating systems regardless of vorticity. We demonstrate the generation of spin currents in differentially rotating systems, such as liquid metals with Taylor-Couette flow. Our alternative mechanism will be important in the development of nanomechanical spin devices.

Weak momentum dissipation in ultraclean metals gives rise to novel non-Ohmic current flow, including ballistic and hydrodynamic regimes. Recently, hydrodynamic flow has attracted intense interest because it presents a valuable window into the electronic correlations and the longest lived collective modes of quantum materials. However, diagnosing viscous flow is difficult as the macroscopic observables of ballistic and hydrodynamic transport such as the average current distribution can be deceptively similar, even if their respective microscopics deviate notably. Based on kinetic Boltzmann theory, here we propose to address this issue via the transverse channel voltage at zero magnetic field, which can efficiently detect hydrodynamic flow in a number of materials. To this end, we show that the transverse voltage is sensitive to the interplay between anisotropic fermiology and boundary scattering, resulting in a nontrivial behavior in narrow channels along crystalline low-symmetry directions. We discuss several materials where the channel-size dependent stress of the quantum fluid leads to a characteristic sign change of the transverse voltage as a new hallmark of the crossover from the ballistic to the hydrodynamic regime.

The detection of static charge density waves (CDWs) in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ (LSCO) with $x\sim 0.12$ at relatively high temperatures has raised the question of what lattice feature pins the CDWs. Some recent structural studies have concluded that some form of monoclinic distortion, indicated by the appearance of certain weak Bragg peaks (type M peaks) at otherwise forbidden positions, are responsible for CDW pinning. As a test of this idea, we present neutron diffraction results for a single crystal of ${\mathrm{La}}_{2-x}{\mathrm{Ba}}_x{\mathrm{CuO}}_4$ (LBCO) with $x=1/8$, which is known to undergo two structural transitions on cooling, from high-temperature tetragonal (HTT) to low-temperature orthorhombic (LTO) near 240 K, involving a collective tilt pattern of the corner-sharing ${\mathrm{CuO}}_6$ octahedra, and from LTO to low-temperature tetragonal (LTT) near 56 K, involving a new tilt pattern and the appearance of intensity at peaks of type T. We observe both type M and type T peaks in the LTT phase, while the type M peaks (but not type T) are still present in the LTO phase. Given that CDW order is observed only in the LTT phase of LBCO, it is apparent that the in-plane Cu-O bond anisotropy associated with the octahedral tilt pattern is responsible for charge pinning. We point out that evidence for a similar, but weaker, bond anisotropy has been observed previously in LSCO and should be responsible for CDW pinning there. In the case of LBCO, the monoclinic distortion may help to explain previously reported magneto-optical evidence for gyrotropic order.

In the era of two-dimensional (2D) and quasi-2D quantum materials, one needs to model strain at the level of the Hamiltonian as opposed to a semiclassical approach. Corrections to the electronic Hamiltonian due to strain arise from two sources: deformations of the lattice and changes in the hoppings. Here, we provide a general theory that takes into account the symmetry of the lattice and that of the bonds and allows us to write the strain corrections from all sources in any 2D lattice in terms of the band structure parameters like the velocity and the inverse mass tensor. We then use group theory to identify when strain can be described as a scalar and/or vector potential. We discuss the nature of the potentials that arise from in- and out-of-plane hoppings, allowing us to model multilayer systems. We also show that, in general, one encounters multiple vector potentials in different sectors of the Hilbert space, but one can derive a simpler effective low-energy-strained Hamiltonian via Hilbert space projections. We discuss several toy models of 2D systems and present strained bilayer graphene as a practical system that incorporates all the above features. We identify a strain-dependent energy scale in bilayer graphene above which we would no longer need to account for multiple vector potentials. The generality of this formulation allows for a wider range of materials to be investigated for quantum transport straintronics.

We report an ab initio, nonrelativistic QED method that couples light and matter self-consistently beyond the electric dipole approximation and without multipolar truncations. This method is based on an extension of the Maxwell-Pauli-Kohn-Sham approach to a full minimal coupling Hamiltonian, where the space- and time-dependent vector potential is coupled to the matter system, and its back reaction to the radiated fields is generated by the full current density. The implementation in the open-source code octopus is designed for massively parallel multiscale simulations considering different grid spacings for the Maxwell and matter subsystems. Here, we show applications of this framework to simulate renormalized Cherenkov radiation of an electronic wave packet, magneto-optical effects with nonchiral light in nonchiral molecular systems, and renormalized plasmonic modes in a nanoplasmonic dimer. We show that in some cases, the beyond-dipole effects cannot be captured by a multipolar expansion Hamiltonian in the length gauge. Finally, we discuss further opportunities enabled by the framework in the field of twisted light and orbital angular momentum, inelastic light scattering, and strong-field physics.

A rigorous proof of integrability or nonintegrability in quantum many-body systems is among the most challenging tasks, as it involves demonstrating the presence or absence of local conserved quantities and deciphering the complex dynamics of the system. In this paper, we establish a graph-theoretical analysis as a comprehensive framework for proving the nonintegrability of quantum systems. Exemplifying the PXP model, which is widely believed to be nonintegrable, this paper rigorously proves the absence of local conserved quantities, thereby confirming its nonintegrability. This proof for the PXP model gives several important messages not only that the system is nonintegrable but also the quantum many-body scarring observed in the model is not associated with the existence of local conserved quantities. From a graph-theoretical perspective, we also highlight its advantage, even in integrable systems, as the classification of local conserved quantities can be achieved by simply counting the number of isolated loops in the graphs. Our approach is broadly applicable for establishing proofs of (non)integrability in other quantum many-body systems, significantly simplifying the process of proving nonintegrability and giving numerous potential applications.

We analyze quantum-geometric bounds on optical weights in topological phases with pairs of bands hosting nontrivial Euler class, a multigap invariant characterizing non-Abelian band topology. We show how the bounds constrain the combined optical weights of the Euler bands at different dopings and further restrict the size of the adjacent band gaps. In this process, we also consider the associated interband contributions to dc conductivities in the flat-band limit. We physically validate these results by recasting the bound in terms of transition rates associated with the optical absorption of light, and demonstrate how the Euler connections and curvatures can be determined through the use of momentum and frequency-resolved optical measurements, allowing for a direct measurement of this multiband invariant. Additionally, we prove that the bound holds beyond the degenerate limit of Euler bands, resulting in nodal topology captured by the patch Euler class. In this context, we deduce optical manifestations of Euler topology within $\mathbf{k}·\mathbf{p}$ models, which include quantized optical conductivity, and third-order jerk photoconductivities. We showcase our findings with numerical validation in lattice-regularized models that benchmark effective theories for real materials and are realizable in metamaterials and optical lattices.