In this work, we demonstrate that accurate ground-state wave functions may be constructed for polarons in a fully ab initio setting across the wide range of couplings associated with both the large and small polaron limits. We present a single general unitary transformation approach which encompasses an ab initio version of the Lee-Low-Pines theory at weak coupling and the coherent-state Landau-Pekar framework at strong coupling, while interpolating between these limits in general cases. We show that perturbation theory around these limits may be performed in a facile manner to assess the accuracy of the approach, as well as provide an independent route to the ab initio properties of polarons. We test these ideas on the case of LiF, where the electron polaron is expected to be large and relatively weakly coupled, while the hole polaron is expected to be a strongly coupled small polaron.

The Kitaev model, renowned for its exact solvability and potential to host non-Abelian anyons, remains a focal point in the study of quantum spin liquids and topological phases. While much of the existing literature has employed Majorana fermion techniques to analyze the model, particularly at zero temperature, its finite-temperature behavior has been less thoroughly explored via alternative methods. In this paper, we investigate the finite-temperature properties of the Kitaev model using the spin Green's function formalism. This approach provides a unified framework for computing key physical quantities, such as spin correlations, magnetic susceptibility, and the dynamical spin structure factor, offering valuable insights into the system's thermal dynamics. To solve the equation of motion for the spin Green's function, we truncate the hierarchy of multispin Green's functions using the Tyablikov decoupling approximation, which is particularly accurate at high temperatures. Our results show several similarities with Majorana-based numerical simulations, though notable differences emerge. Specifically, both static and dynamical spin-spin correlation functions capture not only ${\mathbb{Z}}_2$ flux excitations but also simple spin-flip excitations, with the latter dominating the response. Additionally, without explicitly assuming fractionalization, our results for the spin susceptibility and spin relaxation rate suggest the presence of fermionic degrees of freedom at low temperatures. Unlike Majorana-based approaches, which rely on exact solvability and are inherently limited to the pure Kitaev model, the spin Green's function formalism is well-suited for studying more realistic systems. It can naturally incorporate the effects of magnetic fields, including linear-order terms, and non-Kitaev interactions, making it applicable to a broader range of materials and experimental conditions. By providing an alternative method for analyzing the finite-temperature properties of the Kitaev model, this study complements existing approaches and lays the groundwork for future investigations into real materials and extended models.

The spin transport is carried by spin-polarized electrons in the metal and by magnons in the insulator. We compute the spin current in the presence of a spin accumulation in the metal, caused by the spin Hall effect, and a thermal gradient using Fermi's golden rule in the presence of interfacial disorder. For a perfectly clean interface, the in-plane momentum is conserved by the electron-magnon scattering events that govern the spin transport through the interface. We calculate how the disorder-induced broadening of scattering matrix elements with respect to the in-plane momentum influences the spin current. As a general result, we observe that, specifically for high temperatures, one should expect a rather small effect of interface disorder on the measured spin current, while for small temperatures there is a significant reduction of a spin current with increasing disorder.

We report on the properties of ${\mathrm{SrTb}}_2{\mathrm{O}}_4$, a frustrated zigzag ladder antiferromagnet, studied by single crystal neutron diffraction (with polarized neutrons in zero field and unpolarized neutrons in an applied magnetic field), as well as by neutron spectroscopy on a polycrystalline sample. The neutron scattering results are supported by single-crystal magnetization and heat capacity measurements. In zero field, neutron diffraction data show no transition to a magnetically ordered state down to the lowest experimentally available temperature of 35 mK, and the material remains magnetically disordered down to this temperature. Polarized neutron diffraction measurements reveal the presence of a diffuse scattering signal suggesting only very weak spin-spin correlations in the ground state. For $H\parallel c$ (the easy magnetization direction), we followed the magnetization process using neutron diffraction measurements and observed the appearance of field-induced magnetic Bragg peaks with integer $h$ and $k$ indices in the $\left(hk0\right)$ scattering plane. No magnetic peaks with a nonzero propagation vector were detected. The observed in-field data fit well to a simple two-sublattice model with magnetic moments aligned along the field direction but being significantly different in magnitude for the two inequivalent ${\mathrm{Tb}}^{3+}$ sites in the unit cell. Overall, the collected data point to a nonmagnetic ground state in ${\mathrm{SrTb}}_2{\mathrm{O}}_4$ despite the presence of strong interactions.

The Lieb theorem is a cornerstone of quantum magnetism theory in condensed matter. In this work, we investigate the instability of the Lieb insulating ferrimagnetic phase in the extended Hubbard model on a trimer chain at half-filling, with one electron per site, under increasing the nearest-neighbor Coulomb coupling $V$. Our results show that despite a noticeable increase in doublon density with $V$, the ferrimagnetic insulating phase remains robust up to the phase separation (PS) line, which is observed at $V\gtrsim U/4$, where $U$ is the local Coulomb repulsion. Above the PS line, one of the coexisting phases is primarily populated by doublons on one of the two sublattices of the chain. This phase coexists with a metallic, unsaturated ferromagnetic phase for $U\gtrsim t$, and with a singlet phase for $U\lesssim t$, where $t$ is the intratrimer hopping amplitude. We estimate the PS and the crossover lines with the help of density matrix renormalization group calculations.

Amorphous solids, as long-range disorder materials, have complex order on multiple length scales. The regulation of the degree of order is generally achieved through annealing or static compression. Unlike previous studies, this work has triggered the disorder-to-order transition in amorphous selenium ($a$-Se) by cyclic compression-decompression in a dynamic diamond anvil cell. We found that the degree of order in the directionally bonded $a$-Se positively correlates with the number of compression cycles, and long-range order was achieved in a relatively low-pressure range. The possible transformation mechanism is discussed on the basis of medium-range structural parameter regulation. This work provides an understanding of the relaxation and extension of the order length scale by unconventional mechanical stimulation.

We investigate electrical transport in a quantum wire of $N$ sites connected to an equal number $(N_i/2)$ of sources and drains of charges in bulk. Each source and drain injects and extracts charges at the same rate, respectively. We show that the linear-response electrical current is nonreciprocal in such a system when the arrangement of sources and drains breaks the system's parity. We prove that inelastic scattering is essential for nonreciprocity in this system. For this, we invoke a master equation description of classical charge transport in a similar system. The nonreciprocal current in quantum wire matches that in the classical model for $N_i/N\sim 1$, generating a finite scattering length much smaller than the length of the wire. The nonreciprocity in the quantum wire oscillates with wire length when $N_i/N\ll 1$, and it can vanish at specific lengths.

The kagome ferrimagnet family $R\mathrm{M}{\mathrm{n}}_6{\mathrm{Sn}}_6$ ($R$ = Gd - Er) features Chern-gapped Dirac points in the band structure, theoretically allowing a large anomalous Hall effect. However, the experimentally observed anomalous Hall effect in $R\mathrm{M}{\mathrm{n}}_6{\mathrm{Sn}}_6$ is only comparable to those found in conventional ferromagnets. Here, we report a significant enhancement of anomalous Hall effect in the kagome ferrimagnet ${\mathrm{TbMn}}_6{\mathrm{Sn}}_6$ induced by local disorder through Mg substitution on Tb. By electrical transport and anomalous Nernst effect measurements, combined with first-principles calculations, we demonstrate that the Mg substitution modulates the band structure of ${\mathrm{TbMn}}_6{\mathrm{Sn}}_6$, leading to an increase in intrinsic anomalous Hall conductivity. Additionally, the modulated band structure enhances the longitudinal conductivity, further boosting the extrinsic anomalous Hall conductivity dominated by skew scattering. Consequently, the total anomalous Hall conductivity reaches $950{\mathrm{\Omega }}^{-1}\mathrm{c}{\mathrm{m}}^{-1}$, entering the regime of topological magnets. Both of the large intrinsic and extrinsic anomalous Hall conductivity can be attributed to the nontrivial band structure near the Fermi level.

Inversion asymmetry, combined with spin-orbit interaction, leads to Rashba or Dresselhaus effects or combinations that are promising for technologies based on antiferromagnetic spintronics. Since understanding the exact nature of spin-orbit interaction is crucial for developing a technology based on it, mapping the nature of inversion asymmetry with the type of spin-orbit interaction is the key. We simulate the perovskite oxide heterostructure ${\mathrm{LaAlO}}_3\left|{\mathrm{SrIrO}}_3\right|{\mathrm{SrTiO}}_3$, preserving the inversion symmetry within density functional theory to demonstrate the relation between the nature of inversion asymmetry and the corresponding Rashba- or Dresselhaus-type interaction. With progressive distortion in the heterostructure, we determine how the structure inversion asymmetry sets in with distorted bond lengths and angles, leading to the Rashba effect in the system in the presence of a microscopic electric field. Further, the introduction of tilted ${\mathrm{IrO}}_6$ octahedra leads to bulk inversion asymmetry, helping a combined Rashba-Dresselhaus interaction to occur. A comparison of the spin textures obtained with our density functional theory calculations and theoretical modeling helps us identify the exact nature of the interactions. In the context of perovskite heterostructures, our work demonstrates what types of distortions lead to bulk inversion asymmetry and what other distortions can lead only to structure inversion asymmetry by mapping different types of spin-orbit interactions to different distortions. Additionally, our work emphasizes the importance of spin texture in identifying the nature of spin-orbit interaction. It may serve as a guide for identifying different types of Rashba-like spin-orbit interactions.

We show that the many-body features of graphene band structure and electronic response can be accurately evaluated by applying many-body perturbation theory to a tight-binding (TB) model. In particular, we compare TB results for the optical conductivity with previous ab initio calculations, showing nearly perfect agreement both in the low-energy region near the Dirac cone ($\sim 100$ meV) and at the higher energies of the $\pi$ plasmon ($\sim 5$ eV). A reasonable agreement is reached also for the density-density response at the Brillouin zone corner. With the help of the reduced computational cost of the TB model, we study the effect of self-consistency on the screened interaction ($W$) and on the quasiparticle corrections, a task that is not yet achievable in ab initio frameworks. We find that self-consistency is important to reproduce the experimental results on the divergence of the Fermi velocity, as confirmed by previous studies, while it marginally affects the optical conductivity. Finally, we study the robustness of our results against doping or the introduction of a uniform dielectric environment.

Chiral crystals have recently garnered significant interest in condensed matter physics due to their unique electronic and optical properties. In this paper, we explore the connection between the chirality of crystal structures and the chirality of topological quasiparticles. We specifically predict and analyze several chiral enantiomeric materials, such as ${\mathrm{SrSi}}_2$ and ${\mathrm{BaSi}}_2$, which crystallize in the chiral space groups $P4{}_{3}32$ and $P4{}_{1}32$. Based on first-principles calculations and theoretical analysis, we reveal that the phonon spectra of these materials host various topological phononic quasiparticles, including charge-2 triple points, charge-2 Dirac points, charge-2 Weyl points, and charge-1 Weyl points. Our paper shows that in these enantiomeric materials, the opposite chirality of the crystal structure results in topological quasiparticles with opposite chiral topological charges and distinct topological surface states. Our paper elucidates the intrinsic relationship between the chirality of crystal structures and the chirality of topological quasiparticles, providing promising theoretical guidance and a material platform for investigating the physical properties of chiral crystals.

We investigate the non-Hermitian Su-Schrieffer-Heeger (SSH) model on the Bethe lattice, revealing a novel localization phenomenon coined the inner non-Hermitian skin effect. This effect is featured by the localization of all eigenstates within the bulk of the lattice, diverging from the conventional skin effect observed in general non-Hermitian systems. The analytical treatment of the model demonstrates that the Hamiltonian can be decoupled into a series of one-dimensional chains, with one end fixed at the bottom boundary while the other ends are positioned at varying generations within the bulk. This configuration leads to the emergence of the inner non-Hermitian skin effect, which is further validated by performing circuit simulations. Our findings provide new insights into the interplay between non-Hermitian physics and the self-similar structure on the Bethe lattice.

Spin-momentum locking is an intrinsic property of evanescent electromagnetic waves, which implies that the transverse spin is locked with the dispersive momentum obeying the left-hand screw rule. More importantly, it fundamentally governs the polarization and transport of surface modes, resulting in the discovery of efficient spin-direction couplers, unidirectional guided waves, and photonic spin lattices. Modeling of time-varying media involves the dispersion effect, which is widely available in materials capable of dynamic modulation, potentially redefining a spin-momentum locking phenomenon. In this work, by introducing a nonlocal dispersive photonic time crystal interface model, we unveil the anomalous spin-momentum locking of surface electromagnetic waves due to time modulation. It is found that the interaction between multiple surface modes occurs under slight modulation, which is due to the hybridization of the spin angular momentum induced by the strong dispersion properties of the surface waves. In addition, we also consider the modulation amplitude and the modulation frequency as new degrees of freedom in the model, which enables the evolution of anomalous spin-momentum locking of surface plasmon polariton modes. Our work provides a unique perspective for a deeper understanding of spin-polarized transport induced by anomalous spin-momentum locking in time-varying photonic material systems, paving the way for engineering the spin structure and topological properties of electromagnetic waves with promising applications in the fields of spin optics and topological photonics.

In this work we revisit the problem of a Majorana box qubit (MBQ) readout by a point-contact (PC) detector. The logic states of the MBQ are associated with the combined fermion parities of the MBQ and its tunnel-coupled quantum dot, which is measured by a PC detector. Beyond the existing studies on limiting bias voltage regimes, we analyze the steady-state current and the current power spectrum across all bias voltages. Our results indicate that the MBQ readout via the parity-dependent detector current is effective only at low bias voltage regime and requires the dot energy level to be off-resonance with the Majorana qubit. In contrast, the current power spectrum allows MBQ readout through the parity-dependent Rabi oscillation peak signals for arbitrary bias voltages, without restrictions on the dot energy level. Particularly, with focus on the MBQ measurement visibility, we analyze the peak-to-pedestal ratio for each characteristic peak (associated with each logic state of the qubit) and the signal-to-noise ratio of the two peaks. By examining these two metrics, we identify the optimal bias voltage window for the PC detector at low temperature limit.

Honeycomb cobalt oxides containing $3d \mathrm{C}{\mathrm{o}}^{2+}$ ions might realize frustrated magnetism and novel quantum phases. Among candidate materials, $\mathrm{N}{\mathrm{a}}_3\mathrm{C}{\mathrm{o}}_2\mathrm{Sb}{\mathrm{O}}_6$ stands out for its distorted honeycomb lattice and significant in-plane anisotropy, motivating vector-field tuning inside the honeycomb plane. Here we use thermal transport down to the millikelvin regime to study twin-free crystals of $\mathrm{N}{\mathrm{a}}_3\mathrm{C}{\mathrm{o}}_2\mathrm{Sb}{\mathrm{O}}_6$ subject to in-plane vector fields. We find that the thermal conductivity $\kappa$ never exceeds the heat-transport capability of phonons, rendering its suppression primarily due to phonon scattering off magnetic excitations and/or domain boundaries. The system's field-driven quantum criticality manifests itself as an abundance of magnetic fluctuations hindering the heat transport, which further depends on the field direction in an intriguing manner. Our work establishes a powerful strategy of tuning magnetic quantum criticality with rotating fields and monitoring it with phonon heat transport.

Twisted bilayer graphene (TBG) has extraordinary electronic properties at the magic angle along with an isolated flat band at the magic angle. However, the non-Hermitian phenomena in twisted bilayer graphene remain unexplored. In this work, we study a non-Hermitian TBG formed by one-layer graphene twisted relative to another layer with gain and loss. Using a non-Hermitian generalization of the Bistritzer-MacDonald model, we find Dirac cones centered at only the $K_M$ ($K_M^{\prime }$) corner of the moiré Brillouin zone at the $K^{\prime }$ ($K$) valley deform into rings of exceptional points in the presence of non-Hermiticity, which is different from single-layer graphene with gain and loss, where exceptional rings appear in both $K$ and $K^{\prime }$ corners of the Brillouin zone. We show that the exceptional rings are protected by non-Hermitian chiral symmetry. More interestingly, at an “exceptional magic angle” larger than the Hermitian magic angle, the exceptional rings coincide and form non-Hermitian flat bands with zero energy and a finite lifetime. These non-Hermitian flat bands in the moiré system, which are isolated from dispersive bands, are distinguished from those in non-Hermitian frustrated lattices. In addition, we find that the non-Hermitian flat band has topological charge conserved in the moiré Brillouin zone, which is allowed for analogs of non-Hermitian fractional quantum Hall states.

Clifford circuits can be utilized to disentangle quantum states with polynomial cost, thanks to the Gottesman-Knill theorem. Based on this idea, the Clifford Circuits Augmented Matrix Product States (CAMPS) method, which is a seamless integration of Clifford circuits within the density-matrix renormalization group algorithm, was proposed recently and was shown to be able to reduce entanglement in various quantum systems. In this work, we further explore the power of the CAMPS method in critical spin chains described by conformal field theories (CFTs) in the scaling limit. We find that the optimized disentanglers correspond to duality transformations, which significantly reduce the entanglement entropy in the ground state. For the critical quantum Ising spin chain governed by the Ising CFT with self-duality, the Clifford circuits found by CAMPS coincide with the duality transformation, i.e., the Kramer-Wannier self-duality in the critical Ising chain. It reduces the entanglement entropy by mapping the free conformal boundary condition to the fixed one. In the more general case of the XXZ chain, the CAMPS gives rise to a duality transformation mapping the model to the quantum Ashkin-Teller spin chain. Our results highlight the potential of the framework as a versatile tool for uncovering hidden dualities and simplifying the entanglement structure of critical quantum systems.