Band topology has been studied as a design principle of realizing robust boundary modes. Here, by exploring non-Hermitian topology, we propose a three-dimensional topological laser that amplifies surface modes. The topological surface laser is protected by nontrivial topology around branchpoint singularities known as exceptional points. In contrast to two-dimensional topological lasers, the proposed three-dimensional setup can realize topological boundary modes without judicious gain at the edge or symmetry protection, which are thus robust against a broad range of disorders. We also propose a possible optical setup to experimentally realize the topological surface laser. Our results provide a general guiding principle to construct non-Hermitian topological devices in three-dimensional systems.

We investigate the electronic structure of alternating-twist triple Bernal-stacked bilayer graphene (t3BG) as a function of interlayer coupling $\omega$, twist angle $\theta$, interlayer potential difference $\mathrm{\Delta }$, and top-bottom bilayers sliding vector $\mathit{\tau }$ for three possible configurations AB/AB/AB, AB/BA/AB, and AB/AB/BA. The parabolic low-energy band dispersions in a Bernal-stacked bilayer and gap opening through a finite interlayer potential difference $\mathrm{\Delta }$ allows the flattening of bands in t3BG down to $\sim 20$ meV for twist angles $\theta \lesssim 2^{\circ }$ regardless of the stacking types. The easier isolation of the flat bands and associated reduction of Coulomb screening thanks to the intrinsic gaps of bilayer graphene for finite $\mathrm{\Delta }$ facilitate the formation of correlation-driven gaps when it is compared to the metallic phases of twisted trilayer graphene under electric fields. We obtain the stacking dependent Coulomb energy versus bandwidth $U/W\gtrsim 1$ ratios in the $\theta$ and $\mathrm{\Delta }$ parameter space. We also present the expected $K$-valley Chern numbers for the lowest-energy nearly flat bands.

We investigate electric polarizations that emerge perpendicular to an applied thermal gradient in insulating systems. The thermally induced electric polarization, known as thermopolarization, has been studied conventionally in the case where an electric polarization appears along the thermal gradient. Here, we focus on the antisymmetric component of the thermopolarization tensor, and we reveal that it becomes nonzero due to the ferrotype order for electric-toroidal dipole moments. To describe local electric polarizations originating from the disproportionation of localized electronic clouds, we introduce a two-dimensional three-orbital model with localized $s$ and two $p$ orbitals, where the electric polarization at each site interacts with the neighboring one as dipole-dipole interactions. We find that a vortex-type configuration of local electric polarizations appears as a mean-field ground state, corresponding to a ferrotype electric-toroidal dipole order. By taking account of collective modes from this ordered state, we calculate the coefficient of the thermopolarization based on the linear-response theory. The antisymmetric component is nonzero in the presence of the electric-toroidal dipole order. We clarify that fluctuations in the $p$ orbitals are crucial in enhancing the antisymmetric thermopolarization. We discuss the appearance conditions based on the symmetry argument and the relevance to real materials.

The observation of a strong and tunable spin-orbit interaction (SOI) in surface-conducting diamond opens up a new avenue for building diamond-based spintronics. Herein we provide a comprehensive method to analyze the magnetotransport behavior of surface-conducting hydrogen-terminated diamond (H-diamond) Hall bar devices and $\mathrm{Al}/{\mathrm{Al}}_2{\mathrm{O}}_3/{\mathrm{V}}_2{\mathrm{O}}_5/\mathrm{H}$-diamond metal-oxide semiconductor field-effect transistors, respectively. By adopting a significantly improved theoretical magnetotransport model, the reduced magnetoconductance can be accurately explained both within and outside the quantum diffusive regime. The model is valid for all doping strategies of surface-conducting diamond tested. From this analysis, we find that the orbital magnetoresistance, a classical effect distinct from the SOI, dominates the magnetotransport in surface-conducting diamond at high magnetic fields. Furthermore, local hole mobilities as high as $1000\text{–}3000\mathrm{c}{\mathrm{m}}^2/\mathrm{V}\mathrm{s}$ have been observed in this work, indicating the possibility of diamond-based electronics with ultrahigh hole mobilities at cryogenic temperatures.

The chirality-dependent asymmetric light-matter interaction can be enhanced using various artificial photonic structures as well as engineered incident field. Compared with chiral photonic structures, the engineered optical field is less explored and recently recognized as a new regime in tailoring light-matter interaction. In this work, we demonstrate that weakly chiral spheres can exhibit the maximum scattering circular dichroism (CD) by tailoring the incident field to construct chirality-sensitive anapole states. By considering the chiral terms radiated from Mie nanospheres as perturbation, a multiscattering model is established to predict the chiroptical Mie scattering response of these nanospheres. This model provides a specific guideline to tailor the incident field to realize the upper limit of the achievable scattering CD.

A spin-triplet superconducting state mediated by ferromagnetic (FM) spin fluctuations has been suggested to occur in the newly discovered heavy-fermion superconductor ${\mathrm{UTe}}_2$. However, the recent neutron scattering measurements revealed the presence of antiferromagnetic (AFM) spin fluctuations in ${\mathrm{UTe}}_2$. Here, we report the ${}^{125}\mathrm{Te}$ nuclear magnetic resonance studies of a single-crystal ${\mathrm{UTe}}_2$, suggesting the coexistence of FM and AFM spin fluctuations in ${\mathrm{UTe}}_2$. Owing to the two different Te sites in the compound, we conclude that the FM spin fluctuations are dominant within ladders and the AFM spin fluctuations originate from the interladder magnetic coupling. Although AFM spin fluctuations exist in the system, the FM spin fluctuations in the ladders may play an important role in the appearance of the spin-triplet superconducting state of ${\mathrm{UTe}}_2$.

Since the realization of high-quality microwave cavities coupled to quantum dots, one can envisage the possibility to investigate the coherent interaction of light and matter in semiconductor quantum devices. Here we study a parallel double quantum dot device operating as single-electron splitter interferometer, with each dot coupled to a local photon cavity. We explore, how quantum correlation and entanglement between the two separated cavities are generated by the coherent transport of a single electron passing simultaneously through the two different dots. We calculate the covariance of the cavity occupations by using a diagrammatic perturbative expansion based on Keldysh Green's functions to fourth order in the dot-cavity interaction strength, taking into account vertex diagrams. Furthermore, we demonstrate the creation of entanglement by showing that the classical Cauchy-Schwarz inequality is violated if the energy levels of the two dots are almost degenerate. For large level detuning or a single dot coupled to two cavities, we show that the inequality is not violated.

Recent examination of the cerium Hugoniot with pyrometry and x-ray diffraction (XRD) has revealed a narrow solid-liquid two-phase region. It has been suggested that nonequilibrium melting may be occurring along the Ce Hugoniot, with either melt kinetics or a sluggish α-ε transition impeding the transition. In particular, the kinetics of the α-ε is unknown and the location of the phase boundary is in dispute. Static measurements suggest a nearly vertical phase boundary that intersects the Hugoniot at 6–7 GPa. This lies in direct conflict with dynamic measurements along the Hugoniot observing α-Ce through incipient melt. This work presents dynamic experiments using XRD to examine the behavior of the α-ε phase transition. The results show that the α-ε phase transition occurs through a tetragonal distortion, with the transition beginning at temperatures below the solid Hugoniot. Following the initial deviation from an ideal fcc structure, the $c/a$ ratio is found to gradually increase with no steady value observed in the ε phase within the range of these experiments (below 17 GPa). Multiple diffraction patterns captured during the peak stress state show no significant change in $c/a$ ratio prior to uniaxial release, upon which Ce reverts to an fcc structure. The results indicate that the α-ε transition occurs rapidly, both on loading and release. An examination of the $c/a$ ratio with increasing temperatures suggests 11.5 GPa as a lower bound for the location of the α-ε-liquid triple point.

Although a large class of topological materials has uniformly been identified using symmetry properties of wave functions, the past two years have seen the rise of multigap topologies beyond this paradigm. Given recent reports of unexplored features of such phases, platforms that are readily implementable to realize them are therefore desirable. Here we demonstrate that multigap topological phase transitions of non-Abelian charged nodal lines arise in classical phonon waves. By adopting a simple spring-mass system, we construct nodal lines of a three-band system. The braiding process of the nodal lines is readily performed by adjusting the spring constants. The generation and annihilation of the nodal lines are then analyzed using the Euler class. Finally, we retrieve topological transitions from trivial nodal lines to a nodal link. Our work provides a simple platform that can offer diverse insights to not only theoretical but also experimental studies on multigap topology.

We describe a relation between the light-cone velocities after a quantum quench and the internal structure of the initial state, in the particular case of free fermions on a chain at half filling. The considered states include short-range valence bond solids, i.e., dimerized states, and long-range states such as the rainbow. In all the considered cases the correlations spread into one or a few well-defined light cones, each of them presenting an effective velocity which can be read from the form factor. Interestingly, we find that the observed velocities range from zero to the Fermi velocity and may not always be obtained from the dispersion relation for valid momenta.

The extended nature of atomic $5d$ orbitals, together with a relatively short $\sim 3\AA$ Ir-Ir distance across edge-shared octahedra in honeycomb iridate lattices, leads to a tendency to disrupt the local, spin-orbit-entangled $j_{\mathrm{eff}}=\frac{1}{2}$ moments of ${\mathrm{Ir}}^{4+}$ ions in favor of dimerization and the formation of molecular orbitals, especially upon lattice compression. The sensitivity of Ir $L$-edge spectroscopy to both spin-orbit entanglement in $j_{\mathrm{eff}}$ states and the quenching of orbital degrees of freedom in dimerized states results in a peculiar evolution of x-ray absorption spectra across dimerization transitions, including energy shifts in the opposite direction for $L_3$ and $L_2$ leading absorption edges and substantial changes in their isotropic branching ratio. We present a theoretical description of the evolution of $5d$ electronic states, and related x-ray absorption spectra, in going from the single ion to the dimerized limit. The calculations reproduce the experimental results for hyper-honeycomb $\beta \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$ [L. Veiga et al., Phys. Rev. B 100, 064104 (2019); L. Veiga et al., Phys. Rev. B 96, 140402(R) (2017)] and shed light into the weakening of the coupling between spin and orbital degrees of freedom as the strength of dimerization increases. The results provide a basis for the interpretation of $L$-edge x-ray absorption spectra in $5d$ systems where competition between the formation of local $j_{\mathrm{eff}}$ states and molecular orbitals is at play.

Highly excited eigenstates of quantum many-body systems are typically featureless thermal states. Some systems, however, possess a small number of special, low-entanglement eigenstates known as quantum scars. We introduce a quantum-inspired machine-learning platform based on a quantum variational autoencoder (QVAE) that detects families of scar states in spectra of many-body systems. Unlike a classical autoencoder, QVAE performs a parametrized unitary operation, allowing us to compress a single eigenstate into a smaller number of qubits. We demonstrate that the autoencoder trained on a scar state is able to detect the whole family of scar states sharing common features with the input state. We identify families of quantum many-body scars in the PXP model beyond the ${\mathbb{Z}}_2$ and ${\mathbb{Z}}_3$ families and find dynamically decoupled subspaces in the Hilbert space of disordered, interacting spin-ladder model. The possibility of an automatic detection of subspaces of scar states opens new pathways in studies of models with a weak breakdown of ergodicity and fragmented Hilbert spaces.

Lattice gauge theories form an intriguing class of theories highly relevant to both high-energy particle physics and low-energy condensed matter physics with the rapid development of engineered quantum devices providing new tools to study, e.g., dynamics of such theories. The massive Schwinger model is known to exhibit intricate properties of more complicated theories and has recently been shown to undergo dynamical quantum phase transitions out of equilibrium. With current technology, noise is inevitable and potentially fatal for a successful quantum simulation. This paper studies the dynamics subject to noise of a $(1+1)\mathrm{D}\mathrm{U}\left(1\right)$ quantum link model following a quench of the sign of the mass term. We find that not only is the system capable of handling noise at rates realistic in NISQ-era devices, promising the possibility to study the target dynamics with current technology, but the effect of noise can be understood in terms of simple models. Specifically the gauge-breaking nature of bit flip channels results in exponential dampening of state amplitudes, and thus observables, which does not affect the structures of interest. This is especially important as it demonstrates that the gauge theory can be successfully studied with devices that only exhibit approximate gauge invariance.

We consider an implementation of the adiabatic spin dynamics approach in a tight-binding description of the electronic structure. The adiabatic approximation for spin degrees of freedom assumes that the faster electronic degrees of freedom are always in a quasiequilibrium state, which significantly reduces the numerical complexity in comparison to the full electron dynamics. Noncollinear magnetic configurations are stabilized by a constraining field, which allows us to directly obtain the effective magnetic field from the negative of the constraining field. While the dynamics are shown to conserve energy, we demonstrate that adiabatic spin dynamics does not conserve the total spin angular momentum when the lengths of the magnetic moments are allowed to change, which is confirmed by numerical simulations. Furthermore, we develop a method to extract an effective two-spin exchange interaction from the energy curvature tensor of noncollinear states, which we calculate at each time step of the numerical simulations. We demonstrate the effect of noncollinearity on this effective exchange and limitations due to multispin interactions in strongly noncollinear configurations beyond the regime where the Heisenberg model is valid. The relevance of the results are discussed with respect to experimental pump-probe experiments that follow the ultrafast dynamics of magnetism.

Using a particle-based model we examine the depinning motion of solitons in skyrmion chains in quasi-one-dimensional (1D) and two-dimensional (2D) systems containing embedded 1D interfaces. The solitons take the form of a particle or hole in a commensurate chain of skyrmions. Under an applied drive, just above a critical depinning threshold, the soliton moves with a skyrmion Hall angle of zero. For higher drives, the entire chain depins, and in a 2D system we observe that both the solitons and chain move at zero skyrmion Hall angle and then transition to a finite skyrmion Hall angle as the drive increases. In a 2D system with a 1D interface that is at an angle to the driving direction, there can be a reversal of the sign of the skyrmion Hall angle from positive to negative. Our results suggest that solitons in skyrmion systems could be used as information carriers in racetrack geometries that would avoid the drawbacks of finite skyrmion Hall angles. The soliton states become mobile at significantly lower drives than the depinning transition of the skyrmion chains themselves.

We consider fractional quantum Hall states in systems where two flat Chern number $C=\pm 1$ bands are labeled by an approximately conserved valley index and interchanged by time reversal symmetry. At filling factor $\nu =1$ this setting admits an unusual hierarchy of correlated phases of excitons, neutral particle-hole pair excitations of a fully valley-polarized orbital ferromagnet parent state where all electrons occupy a single valley. Excitons experience an effective magnetic field due to the Chern numbers of the underlying bands. This obstructs their condensation in favor of a variety of crystalline orders and gapped and gapless liquid states. All these have the same quantized charge Hall response and are electrically incompressible, but differ in their edge structure, orbital magnetization, and hence valley and thermal responses. We explore the relevance of this scenario for moiré heterostructures of bilayer graphene on a hexagonal boron nitride substrate.

The quantum Hall effect has been predicted and discovered in various condensed-matter systems. A promising quantum material for such topological effects is graphene. We report the numerical observation of a curvature-induced spin-Hall effect in a monolayer graphene Möbius strip. The solution of the Dirac equation on the nontrivial and non-Euclidean manifold reveals that despite the absence of a Hall current, a spin-Hall current is a natural consequence for such a topology, as predicted from symmetry arguments.

In this work we get insight into the impact of reduced density matrix functionals on the quality of removal/addition energies obtained using the extended Koopmans' theorem (EKT). Within the reduced density matrix functional theory (RDMFT) the EKT approach reduces to a matrix diagonalization, whose ingredients are the one- and two-body reduced density matrices. A striking feature of the EKT within RDMFT is that it opens a band gap, although it is too large, in strongly correlated materials, which are a challenge for state-of-the-art methods such as $GW$. Using the one-dimensional Hubbard model and the homogeneous electron gas as test cases, we find that (i) with exact or very accurate density matrices the EKT systematically overestimates the band gap in the Hubbard model and the bandwidth in the homogeneous electron gas and (ii) with approximate density matrices, instead, the EKT can benefit from error cancellation. In particular we test a new approximation which combines random-phase approximation screening with the power functional approximation to the two-body reduced density matrix introduced by Sharma et al. [Phys. Rev. B 78, 201103 (2008)]. An important feature of this approximation is that it reduces the EKT band gap in the studied models; it is hence a promising approximation for correcting the EKT band-gap overestimation in strongly correlated materials.

In this work, we present a collection of three-dimensional higher-order symmetry-protected topological phases (HOSPTs) with gapless hinge modes that exist only in strongly interacting systems subject to subsystem symmetry constraints. We use a coupled wire construction to generate three families of microscopic lattice models: insulators with helical hinge modes, superconductors with chiral Majorana hinge modes, and fractionalized insulators with helical hinge modes that carry fractional charge. In particular, these HOSPTs do not require spatial symmetry protection, but are instead protected by subsystem symmetries, and support “fractonic” quasiparticle excitations that move within only a low-dimensional submanifold of the system. We analyze the anomaly structure for the boundary theory and the entanglement Hamiltonian, and show that the side surfaces of these HOSPTs, despite being partially gapped, exhibit symmetry anomalies, and can only be realized as the boundary of three-dimensional HOSPT phases.