We consider rigid rotation and tilting of the anion octahedra around the transition metal ions in $AB{\mathrm{O}}_3$ perovskites from the viewpoint of the Jahn-Teller problem and study the effect of the crystal field of the nearest $A$ and $B$ ion cages on the single electron spectrum of the central transition metal $B$ ions. While the crystal field of the octahedra, which creates degenerate manifolds $e_g$ and $t_{2g}$, does not change with such rigid rotations, the field of $A$ and $B$ cages deforms significantly from the ideal octahedral field in a way that leads to sizable splittings in these manifolds. The lowering of the ground state energy of a given many-electron configuration due to these splittings can thus be an important driving force behind the octahedral rotations and associated structural transformations. We find that the size of the splitting and the order of the orbitals are determined by a competition between the $A$ and $B$ cages, as well as a competition between the tilt and rotation angles.

Following a theoretical proposal on multi-impurity charge Kondo circuits [T. K. T. Nguyen and M. N. Kiselev, Phys. Rev. B 97, 085403 (2018)] and the experimental breakthrough in fabrication of the two-site Kondo simulator [W. Pouse et al., Nat. Phys. 19, 492 (2023)] we investigate a thermoelectric transport through a double-dot charge Kondo quantum nanodevice in the strong coupling operational regime. We focus on the fingerprints of the non-Fermi liquid and its manifestation in the charge and heat quantum transport. We construct a full-fledged quantitative theory describing crossovers between different regimes of the multichannel charge Kondo quantum circuits and discuss possible experimental realizations of the theory.

Long-distance superexchange coupling provides a powerful tool for scalable quantum processors required in quantum information and quantum computation. Quadripartite spin-crossover molecules are prototypical systems for studying novel quantum phenomena assisted by superexchange interactions. In this paper, we consider a quadripartite molecule where two central monomers are connected to two electrodes, whereas another two are side coupled to their neighboring central monomers. Numerical renormalization group results demonstrate that, when the energy level spacing between two central monomers $\mathrm{\Delta }$ turns on, spins on the two side monomers are organized ferromagnetically. The effective antiferromagnetic superexchange interaction between the side units and the electrodes then induces a long-distance ferromagnetic Ruderman-Kittel-Kasuya-Yosida interaction between two side monomers. For intermediate $\mathrm{\Delta }$, the linear conductance tends to reach its unitary limit in a moderate low-temperature regime due to a superexchange two-stage Kondo effect. When the hopping integral between two central monomers sweeps upwards, where $\mathrm{\Delta }$ is fixed at a small value, a quantum phase transition occurs. A long-distance antiferromagnetic exchange coupling between two side units is clarified, isolating the molecule from the electrodes with zero conductance. If $\mathrm{\Delta }$ is set to be an intermediate value, applying the hopping integral then triggers a singular quantum phase, where two central monomers are antiparallelly aligned, whereas two side ones are organized ferromagnetically, revealing well the competition between the above two kinds of long-distance superexchange couplings. Temperature-dependent images and numerical simulations confirm the above conclusions.

In recent years, first-order and second-order topological phonons have been discovered in crystalline materials, which has aroused great interest. In the present work, through symmetry analysis and bulk polarization calculation, we demonstrate that semihydrogenated graphene (i.e., graphone) is an ideal platform intrinsically possessing hybrid-order (first-order and second-order) phonon topology. Based on the acoustic sum rule correction of the phonon tight-binding Hamiltonian, we show that the first-order optical and acoustic topological phonon edge states as well as second-order topological phonon corner states can coexist in graphone. Interestingly, the twofold degeneracy of topological acoustic phonon edge modes in graphene is eliminated due to spatial inversion breaking in graphone. In addition, there are three topological corner states within the wide gap of the phonon spectrum of the zigzag-edged nanodisk. Our results not only show hybrid-order topological phonon properties in graphone but also pave the way for a more complete understanding of topological phonons in other crystalline materials.

The Peltier effect, which is the reverse counterpart of the Seebeck effect, has been experimentally observed in nanojunctions. Despite its potential applications in cooling nanoelectronic devices, achieving significant figures of merit and cooling power remain challenging. Here, we propose a novel approach to enable substantial Peltier cooling by leveraging orbital-Kondo states formed through strong electron correlations. We demonstrate this approach by showcasing a reverse heat current and reduced local temperature in a serially connected two-impurity junction, resulting in appreciable thermoelectric efficiency and cooling power. The discovery of this unconventional Kondo cooling phenomenon presents a promising avenue for nanorefrigeration and underscores the important role that strong electron correlations play in nonequilibrium quantum systems.

Topological invariants are a significant ingredient in the study of topological phases of matter that intertwines the supposedly contradicting concepts of bulk and boundary. The nature of the invariants differs depending on the dimensionality of the boundary at which the topologically nontrivial states manifest themselves. The primary motivation of this work is to study two distinct scenarios of topological phases, differing in the dimensionality of their boundary states, and study the associated bulk topological invariants that characterize them. In this regard, we study the band-engineered Kane-Mele model which originally is a prototypical example of a system that hosts quantum spin Hall effect on a honeycomb lattice. Under a smooth band deformation caused by varying one of the nearest-neighbor hopping amplitudes (say $t_1$) as compared to the other two (say $t$), we observe that the system transits from its first-order topological insulating state (or quantum spin Hall state) to a second-order topological insulating (SOTI) state via a gap-closing transition. This transition occurs when the system crosses a particular threshold of the deformation parameter $t_1/t$ (namely, $t_1/t=2$), known as the semi-Dirac limit in literature. We show the presence of edge and corner modes as a signature of first- and second-order topology, respectively. Further, we observe the evolution of the Wannier charge center (WCC), a bulk property as a function of the deformation parameter $t_1/t$. It is seen that the behavior of the WCC is entirely different in the quantum spin Hall (QSH) phase as compared to the second-order topological state. We also find that, while the ${\mathbb{Z}}_2$ invariant successfully characterizes the QSH state, it cannot characterize higher-order topology (second order here). The model being mirror invariant, we also calculate mirror winding number to show that it is rendered trivial in the SOTI phase as well, while being nontrivial in the QSH phase. Finally, we observe that the spin resolved bulk polarization correctly establishes the appearance of second-order topological corner modes and thus categorizes this phase as an obstructed atomic insulator.

We introduce a simplified model for a three-terminal quantum thermocouple consisting of two strongly coupled quantum dots. To elucidate spin-dependent Seebeck and Peltier effects, we employ a microscopic Hamiltonian and map the Lindblad master equation onto a quantum transition network, capturing the key working principles for both reciprocal effects. Our analysis reveals quantum thermodynamic networks encompassing both Coulomb interaction and spin-flipping processes, lead to the emergence of spin-thermolectric effects. Using algebraic graph theory, we recover the phenomenological law of irreversible thermodynamics from the stochastic version of the entropy production rate expressed in terms of cycle flux and cycle forces. Remarkably, Onsager reciprocity and Kelvin relation for transport coefficients find their premises in the properties of cycle flux trajectories within the quantum transition network. This underscores the universal generality of thermodynamic principles across classical and quantum realms, despite their fundamentally different basis from classical laws of irreversible thermodynamics relying on local equilibrium assumptions.

We develop a diagrammatic Monte Carlo method for the real-time dynamics of dissipative quantum impurity models. These are small open quantum systems with interaction and local Markovian dissipation, coupled to a large quantum bath. Our algorithm samples the hybridization expansion formulated on a single real-time contour, rather than on the double Keldysh one, as it naturally arises in the thermofield/vectorized representation of the Lindblad dynamics. We show that local Markovian dissipation generally helps the convergence of the diagrammatic Monte Carlo sampling by reducing the sign problem, thus allowing to reach longer timescales as compared to the conventional unitary case. We apply our method to an Anderson impurity model in the presence of local dephasing and discuss its effect on the charge and spin dynamics of the impurity.

Recent improvements in the energy resolution of resonant inelastic x-ray scattering experiments (RIXS) at the Cu-$L_3$ edge have enabled the study of lattice, spin, and charge excitations. Here, we report on the detection of a low-intensity signal at 140 meV, twice the energy of the bond-stretching (BS) phonon mode, in the cuprate superconductor ${\text{Bi}}_2{\text{Sr}}_2\text{Ca}{\text{Cu}}_2{\text{O}}_{8+x}$ (Bi-2212). Ultrahigh-resolution polarimetric RIXS measurements allow us to resolve the outgoing polarization of the signal and identify this feature as a two-phonon excitation. Further, we study the connection between the two-phonon mode and the BS one-phonon mode by constructing a joint density of states toy model that reproduces the key features of the data.

We study a partially disordered one-dimensional system with interacting particles. Concretely, we impose a disorder potential to only every other site, followed by a clean site. Our numerical analysis of eigenstate properties is based on the entanglement entropy and density distributions. Most importantly, at large disorder, there exist eigenstates with large entanglement entropies and significant correlations between the clean sites. These states have volume-law scaling, embedded into a sea of area-law states, reminiscent of inverted quantum-scar states. These eigenstate features leave fingerprints in the nonequilibrium dynamics even in the large-disorder regime, with a strong initial-state dependence. We demonstrate that certain types of initial charge-density-wave states decay significantly, while others preserve their initial inhomogeneity, the latter being the typical behavior for many-body localized systems. This initial-condition-dependent dynamics may give extra control over the delocalization dynamics at large disorder strength and should be experimentally feasible with ultracold atoms in optical lattices.

Including the effect of the trivial band near Weyl nodes, we evaluate the longitudinal magnetoconductivity (LMC) of Weyl semimetals along the magnetic field direction using the Boltzmann magnetotransport theory and study its dependence on the magnetic field, Fermi energy, and temperature. We find that for weak internode and node-trivial band scatterings, the LMC is quadratic in the magnetic field and is inversely proportional to the fourth power of the Fermi energy at high densities due to internode scatterings and to the square of the Fermi energy at low densities due to scatterings between a Weyl node and the trivial band. In the case of strong internode and node-trivial band scatterings, the magnetic field-driven anisotropy induced by the phase-space volume element and the orbital magnetic moment cannot be neglected. As a result, the LMC exhibits a significantly different trend compared to that in the weak internode and node-trivial band scattering limit. Finally, we calculate the temperature dependence of the LMC in the strong inelastic scattering limit and obtain its asymptotic behaviors at low and high temperatures, respectively, demonstrating that the temperature dependence is strongly affected by the existence of the trivial band.

We performed high-magnetic-field magnetization, polarization, and ultrasonic measurements in ${\mathrm{Ba}}_2{\mathrm{CuGe}}_2{\mathrm{O}}_7$ to investigate field-induced multiferroic properties arising from a cross-correlation between electric dipoles and electric quadrupoles in addition to cross-correlation between magnetic dipoles and electric dipoles. Magnetization $M$ shows saturation behavior above 20 T for several magnetic field directions, however, electric polarization $P_c$ exhibits an increase, and elastic constants show a softening above 20 T. Based on quantum states with a crystalline electric field for the $D_{2d}$ point group and $d\text{−}p$ hybridization between Cu-$3d$ and O-$2p$ electrons, we confirmed that the matrix of an electric dipole $P_z$ was proportional to that of an electric quadrupole $O_{xy}$. Furthermore, considering the spin-orbit coupling of $3d$ electrons and the Zeeman effect, we showed that $P_z$ and $O_{xy}$ simultaneously exhibited field-induced responses. These findings indicate that the orbital degrees of freedom, in addition to the spin degrees of freedom, contribute to the high-field multiferroicity in ${\mathrm{Ba}}_2{\mathrm{CuGe}}_2{\mathrm{O}}_7$.

A two-dimensional topological superconductor (TSC) represents an exotic quantum material with quasiparticle excitation manifesting in dispersive Majorana modes (DMMs) at the boundaries. A domain-wall DMM can arise at the boundary between two TSC domains with opposite Chern numbers or with a π-phase shift in their pairing gap, which can only be tuned by a magnetic field. Here we propose the concept of a ferroelectric (FE) TSC, which not only enriches the domain-wall DMMs but also significantly makes them electrically tunable. The π-phase shift of the pairing gap is shown to be attained between two TSC domains of opposite FE polarization, and switchable by reversing FE polarizations. In combination with ferromagnetic (FM) polarization, the domain wall can host helical, doubled chiral, and fused DMMs, which can be transferred into each other by changing the direction of the electrical and/or magnetic field. Furthermore, based on first-principles calculations, we demonstrate $\alpha \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$ to be a promising FE TSC candidate in proximity with a FM layer and a superconductor substrate. We envision that a FE TSC will significantly ease the manipulation of a DMM by electrical field to realize fault-tolerant quantum computation.

We consider an interacting quantum dot strongly coupled to two superconducting leads in a Josephson junction geometry. By defining symmetry-adapted superpositions of states from the leads, we formulate an effective Hamiltonian for the strong-hybridization regime with a single orbital directly coupled to the dot and three additional indirectly coupled orbitals. This minimal basis set allows one to account for the quasiparticles in the vicinity of the dot as well as those further away in the leads and to describe how their role evolves as a function of coupling strength and phase bias $\varphi$. This formulation also reveals the changing nature of the spin-doublet state for the experimentally relevant coupling strengths. The binding of a nearly decoupled quasiparticle in the vicinity of the quantum dot explains the “doublet chimney” in the phase diagram for $\varphi \sim \pi$, in contrast to $\varphi \sim 0$, where the residual quasiparticle escapes to infinity and plays no active role.

We demonstrate the possibility of the optical orientation of excitons in the near field of the metal grating that covers a semiconductor nanostructure. Excitons generated this way have the wave vector greater than the wave vector of the incident radiation. We suggest that optical orientation method is applicable to study the fine structure and kinetics of the hot excitons in semiconductor quantum well.

Twisted bilayer graphene possesses intrinsic chirality, which has received less attention among the extensive research conducted on twistronics. In this paper, the optical absorption theory beyond the common linear perturbation is developed with more sophisticated light-electron interactions considered. Numerical calculations based on the theory found that reducing the twisted angle produces a redshift of absorption in the high-frequency range, while increasing absorption strength in the low-frequency domain, which can be further significantly enhanced when applying an interlayer bias. More importantly, the absorption spectroscopy is shown to be an effective way to detect both the chirality and the anisotropy of the twisted bilayer structure.

Recent progress in material design enables the study of correlated, low-temperature phases and associated anomalous transport in two-dimensional kagome systems. Here, we show that unconventional spin transport can arise in such systems even at elevated temperatures due to emergent dynamical constraints. To demonstrate this effect, we consider a strong-coupling limit of an extended Hubbard model on the kagome lattice with a density of $2/3$. We numerically investigate the charge and spin transport by a cellular automaton circuit, allowing us to perform simulations on large systems to long times while preserving the essential conservation laws. The charge dynamics reflects the constraints and can be understood by a Gaussian field theory of a scalar height field. Moreover, the system exhibits a hidden spin conservation law with a dynamic sublattice structure, which enables additional slow relaxation pathways for spin excitations. These features can be directly tested by measuring the dynamic spin structure factor with neutron scattering.

${\mathrm{Ni}}_3\mathrm{In}$ is a paramagnetic intermetallic consisting of $AB$-stacked Ni-kagome networks. Correlated electron behaviors deviating from the Fermi-liquid form have recently been observed in ${\mathrm{Ni}}_3\mathrm{In}$ bulk single crystals, attributed to stabilization of a partially flat electronic band near the Fermi level. Synthesis of this system in thin-film form offers unique opportunities for tuning of materials that could aid in identifying the microscopic origin of the non-Fermi-liquid response and exploring the suspected quantum criticality therein. Here, we report the realization of (001)-oriented epitaxial thin films of ${\mathrm{Ni}}_3\mathrm{In}$ on single-crystal ${\mathrm{SrTiO}}_3$ (111) substrates by molecular beam epitaxy. Via control of growth conditions, we fabricate high-quality films with quantum fluctuations strongly influencing the physical properties of the system. Analysis of the electrical transport response reveals that intrinsic spin fluctuations in ${\mathrm{Ni}}_3\mathrm{In}$ may account for the observed non-Fermi-liquid behavior. Such structures may facilitate driving ${\mathrm{Ni}}_3\mathrm{In}$ across a potential quantum critical phase transition and uncover the role of unusual flat bands in triggering correlated phenomena.

In a bulk GaAs crystal, an unusual magnetoresistance effect, which takes place when a spin-polarized current flows through the sample, was detected. Under conditions of optical pumping of electron spins, an external magnetic field directed along the electric current and perpendicular to the oriented spins decreases the resistance of the material. The phenomenon is due to the spin-dependent scattering of electrons by neutral donors. It was found that the sign of the magnetoresistance does not depend on the sign of the exciting light circular polarization; the effect is even with respect to the sign of the spin polarization of the carriers. The effect is absent when the excitation is linear polarized, which indicates a correlation between the spins of optically oriented free electrons and electrons localized on donors.

The few-body problem for $N=4$ fermionic charge carriers in a double-well moiré quantum dot (MQD), representing the first step in a bottom-up strategy to investigate formation of molecular supercrystals in transition metal dichalcogenide (TMD) moiré superlattices with integral fillings, $\nu >1$, is solved exactly by employing large-scale exact-diagonalization via full configuration interaction (FCI) computations. A comparative analysis with the mean-field solutions of the often used spin-and-space unrestricted Hartree-Fock (sS-UHF) method demonstrates the limitations of the UHF method (by itself) to provide a proper description of the influence of the interdot Coulomb interaction. In particular, it is explicitly shown for $\nu =2$ that the exact charge densities (CDs) within each MQD retain the ringlike shape characteristic (for a wide range of relevant parameters) of a fully isolated MQD, as was found for sliding Wigner molecules (WMs). This deeply quantum-mechanical behavior contrasts sharply with the UHF CDs that portray solely orientationally pinned and well-localized dumbbell dimers. An improved CD, which agrees with the FCI-calculated one, derived from the restoration of the sS-UHF broken parity symmetries is further introduced, suggesting a beyond-mean-field methodological roadmap for correcting the sS-UHF results. It is conjectured that the conclusions for the $\nu =2$ moiré TMD superlattice case extend to all cases with integral fillings that are associated with sliding WMs in isolated MQDs. The case of $\nu =3$, associated with a pinned WM in isolated MQDs, is an exception.