The sign-problematic generalized Baxter-Wu (GBW) model with asymmetric complex couplings is mapped onto a one-dimensional quantum model. Utilizing the model's exactly known critical properties, we study the relation between the conventional and the modified average signs and the phase transitions in the GBW model. We find that the average sign develops a negative peak near the critical point, but it is not a unique indicator of phase transition, as similar features can appear in noncritical regions. While the average modified sign provides a viable probe for the phase transition, the practical effectiveness of this method is limited by the exponential scaling of computational cost with the system's volume. We propose that the universal properties of the original model can be investigated through simulating the related reference model, based on the universality assumption. Using finite-size scaling analysis based on Monte Carlo simulations, we confirm the validity of this method, which thereby provides a novel framework for investigating phase transitions in systems plagued by the sign problem.

Nodal-sphere semimetals (NSSMs), featuring spherical band degeneracies in momentum space, constitute a fascinating class of topological materials. However, their realization in real materials is severely hampered by discrete crystallographic symmetry constraints, often resulting in gapped “pseudo” nodal spheres. Here, combining first-principles calculations and symmetry analysis, we predict a new three-dimensional boron allotrope, ${\text{CT-B}}_{24}$, as a nearly ideal NSSM. Its structural stability is systematically confirmed by phonon calculations, ab initio molecular dynamics simulations at 600 K, and elastic constant analysis. Notably, the electronic structure of ${\text{CT-B}}_{24}$ exhibits two bands crossing linearly near the Fermi level, forming a quasinodal sphere around the $\mathrm{\Gamma }$ point. The maximum energy gap is merely 0.008 meV, which is two orders of magnitude smaller than the gaps reported in previous pseudo-NSSMs. Furthermore, the (001) surface hosts pronounced drumheadlike surface states located outside the projected nodal sphere, providing distinct signatures detectable by angle-resolved photoemission spectroscopy. The nodal sphere also demonstrates remarkable robustness and tunability under external strain, driving a topological phase transition from an NSSM to a critical transition state and finally to a trivial insulator. Our work not only presents a superior material platform for exploring nodal-sphere physics but also suggests potential for strain-tunable topological devices.

In CuCl semiconductor microcavities with large decay of the cavity photon (CP), we have studied the dependence of the coupling energy between excitons and CPs on the position of the active layer in the cavity layer, using angle-resolved transmission (ART) spectroscopy. The ART spectra show that the characteristic energies and spectral widths of three cavity polariton branches depend on the incident angle of the probe light, and the spectral profiles of the respective branches also change between the symmetric and asymmetric structures. The incident-angle dependence of the characteristic energies and spectral widths of the respective branches extracted by the asymmetric spectral function is interpreted by the complex eigenvalues of the non-Hermitian effective Hamiltonian. This analysis in terms of an open quantum system allows us to estimate the coupling energy even in the weak coupling regime by considering the decay rates of CPs and excitons. From the analytical results, it is found that the coupling energies of the cavity polaritons depend on the position of the active layer, which indicates that the exciton-photon coupling changes between the strong and weak coupling regimes. Moreover, we discuss the active-layer position dependence of the coupling energy by considering the overlap integral of the exciton polarization and electric field of the CP.

We investigate the magnetotransport properties of a two-dimensional electronic system with unconventional Rashba spin-orbit coupling. We obtain the Landau levels analytically and find that intra-spin and/or inter-spin Landau-level crossing occurs. We compute the longitudinal conductivity and quantum Hall conductivity using the Kubo formalism based on linear response theory. We find that the usual Shubnikov-de Haas (SdH) oscillation in longitudinal conductivity appears, which is purely spin polarized, provided the Fermi level is placed suitably. Interestingly, we observe a beating pattern in the SdH oscillation in the intra-spin branches, which arises due to the superposition of SdH oscillations corresponding to two bands in each spin branch. This is contrary to the conventional Rashba system, where such beating is due to the superposition of SdH oscillation corresponding to two spin-branches. On the other hand, we note that quantum-Hall conductivity exhibits usual quantization in units of $e^2/h$. However, the Landau-level crossing gives rise to the double jump in the Hall conductivity if the Fermi level is placed precisely at the crossing point.

Weak localization theory is developed for graphene heterostructures with transition metal dichalcogenides and topological insulators where the Rashba and valley Zeeman spin splittings of the energy spectrum are large enough. The anomalous magnetoresistance in low fields caused by weak localization is calculated. It is shown that the valley Zeeman splitting has no effect on weak localization in the absence of Rashba splitting, but it results in the change of the magnetoconductivity sign in the Rashba-coupled graphene. Intervalley scattering also affects the quantum correction to the conductivity resulting in its sign reversal. Analytical expressions are obtained for the anomalous magnetoconductivity at arbitrary relations between the Rashba and valley Zeeman splittings as well as the intervalley scattering rates.

We present a theoretical study of terahertz radiation induced transitions between attractive and repulsive Fermi polaron states in monolayers of transition metal dichalcogenides. Going beyond the simple few-particle trion picture, we develop a many-body description that explicitly accounts for correlations with the Fermi sea of resident charge carriers. We calculate the rate of the direct optical conversion process which has a threshold where the terahertz photon energy equals to the Fermi polaron binding energy. This process features a characteristic frequency dependence near the threshold, due to final-state electron-exciton scattering related to the trion correlation with the Fermi sea hole. Furthermore, we demonstrate that intense terahertz pulses can significantly heat the electron gas via Drude absorption enabling an additional, indirect conversion mechanism through collisions between hot electrons and polarons, which exhibits a strong exponential dependence on the electron temperature. Our results reveal the important role of many-body correlations and thermal effects in the terahertz-driven dynamics of excitonic complexes in two-dimensional semiconductors.

The in-plane anomalous Hall effect (IPAHE) with planar Hall current and magnetization/magnetic fields in various quantum materials has received increasing attention. Most of the current efforts are devoted to the intrinsic part due to the Berry curvature of electronic bands, however, how disorder scattering affects the extrinsic part (the skew scattering and side jump) remains largely elusive. Here we theoretically investigate the three universal classes of disorder scattering (scalar, spin-conserving, and spin-flipping) for the IPAHE, based on the prototypical two-dimensional massive Dirac fermion model with warping term under generic Zeeman fields. We find that the different disorder scattering results in a distinct dependence of the anomalous Hall conductivity on disorder strength, and we recover previously known results within some limits. Remarkably, the spin-flipping scattering could give rise to nontrivial contributions featuring sinusoidal oscillations with periods of $\pi$ and $2\pi$ to the extrinsic part, in contrast to the standard two-dimensional massive Dirac fermions. Our work unveils the rich features of anomalous transport in planar Hall geometry in the presence of disorder scattering and provides some useful insights into the magnetotransport phenomena.

Recent advances in moiré engineering motivate the study of lattice models of strongly correlated electrons subjected to substantial orbital magnetic flux. We analyze the triangular-lattice Hofstadter-Hubbard model at one-quarter flux quantum per plaquette and a density of one electron per site, where a chiral spin liquid phase may exist between weak-coupling integer quantum Hall and strong-coupling ${120}^{\circ }$ antiferromagnetic phases. We use matrix product state methods and analytical arguments to investigate this model compactified to cylinders of finite circumference. We uncover a glide particle-hole symmetry operation which, we argue, is spontaneously broken at the quantum Hall to spin liquid transition on odd-circumference cylinders. We numerically verify the spontaneous symmetry breaking and further demonstrate that this transition is associated with algebraic long-range correlations of various spin-singlet, charge-neutral operators. For even-circumference cylinders, the transition becomes a crossover associated with a large correlation length that grows substantially with circumference. Our findings suggest that in the two-dimensional limit, the transition to a chiral spin liquid phase is continuous and features critical fluctuations of the current.

We discuss the Lieb-Schultz-Mattis (LSM) theorem in two-dimensional spin systems with onsite $\mathrm{U}\left(1\right)⋊{\mathbb{Z}}_2$ spin-rotation symmetry and point group $C_{2v}$ symmetry about a site. We “twist” the point group symmetry by introducing a small uniform U(1) flux to obtain a projective symmetry, similarly to the familiar magnetic translation symmetry. The LSM theorem is proved in presence of the flux and then it is demonstrated that the theorem holds also for the flux-free system. Besides, the uniform flux enables us to show the LSM theorem for the time-reversal symmetry and the site-centered $C_2$-rotation symmetry.

Magnets with controllable magnetization and high critical temperature are essential for practical spintronics devices, among which the two-dimensional $1T\text{−}{\mathrm{CrTe}}_2$ stands out because of its high experimental critical temperature up to $\sim 300\mathrm{K}$ down to the single-layer limit. By using ab initio density functional theory, we investigate the magnetic properties of monolayer and bilayer $1T\text{−}{\mathrm{CrTe}}_2$ and demonstrate that the magnetic properties, such as the magnetocrystalline anisotropy, critical Curie temperature, and magnetizations, can be influenced by strain or electric fields.

Hybrid magnet-superconductor heterostructures attract significant interest for their potential to host unconventional superconductivity, topological superconductivity, and Majorana physics. Transition-metal dihalides ($\mathrm{M}{X}_2, \mathrm{M}=\mathrm{transition}$ metal, $X=\mathrm{Cl}$, Br, I) are compelling magnetic candidates due to their novel magnetic structures and possible ferroelectricity. Here, we employ low-temperature scanning tunneling microscopy/spectroscopy to investigate the interfaces fabricated by growing helimagnet candidate ${\mathrm{CrBr}}_2$ on $s$-wave superconductor ${\mathrm{NbSe}}_2$. Our results reveal that the monolayer ${\mathrm{CrBr}}_2$ is insulating; the measured low-energy electronic states on it derive entirely from the ${\mathrm{NbSe}}_2$ substrate. The superconducting properties of $\mathrm{CrB}{\mathrm{r}}_2/\mathrm{NbS}{\mathrm{e}}_2$ are nearly identical to the bare ${\mathrm{NbSe}}_2$, manifested by the superconducting gap spectra and their temperature and magnetic-field dependence, as well as the spatial distribution and bound states of magnetic vortices. Furthermore, in-gap excitations appear only at the dirty edges of ${\mathrm{CrBr}}_2$ islands and are absent from clean edges, suggesting the lack of intrinsic edge states. Taken together, these findings establish the topologically trivial nature of the helimagnetic insulator/$s$-wave superconductor system $\mathrm{CrB}{\mathrm{r}}_2/\mathrm{NbS}{\mathrm{e}}_2$, attributable to the absence of interfacial superconducting proximity and weak magnetic coupling.

The twofold symmetric vortex core structure is studied by Eilenberger theory in a type-II superconductor by assuming the nematic superconducting gap as well as the anisotropic effective mass model with an elliptic Fermi surface. We calculate the spatial structures of the pair potential, screening current, internal magnetic field, and local density of states (LDOS) around vortices. We estimate how the anisotropic Fermi surface and the pairing function contribute to the nematic anisotropy ratio of the twofold symmetric vortex core structure. Moreover, the LDOS in the nematic superconductor FeSe is evaluated using the Fermi surface obtained from the tight-binding model in order to reproduce the splitting of the LDOS peak into two parallel distributions with increasing energy. These results suggest that both the anisotropic Fermi surface and the anisotropic pairing function are necessary to understand the anisotropic vortex core structure in nematic superconductors.

Recent simulation work on the moiré formed by small angle crystalline twist grain boundaries (TGBs) revealed an unexpected load-induced first-order structural transition. Most likely universal, this unconventional transition drives a frictionally superlubric-to-locked transformation. It involves moiré reconstruction, with emergence of locally commensurate mini-domains, the formation of which implies broader consequences for interfacial phenomena beyond friction, which we study by nonequilibrium molecular dynamics simulations and nonequilibrium Green's function transport calculations. We show that, for small angle ($\theta \lesssim 4^{\circ }$) Au(111) TGBs at the critical load of a few gigapascals, the transition is accompanied by singular responses, including a sharp dip in mechanical compliance and a $\approx 20–30\%$ increase of ballistic electrical resistance as well as of interfacial thermal resistance of mixed phononic and electronic origin. Both reflect the reconstruction of interlayer registry and coupling induced by the emergence of mini-domains. These structurally driven singularities provide experimentally accessible fingerprints of the transition and reveal a unified coupling between mechanical, dissipative, and transport responses at TGBs, opening alternative routes toward the active control of moiré interfacial functionalities.

In the context of an ever-expanding experimental and theoretical interest in the magnetization dynamics of mesoscopic magnetic structures, both in the classical and quantum regimes, we formulate a low-energy field theory for the linear spin waves in finite and textured ferromagnets and we perform its constrained canonical quantization. The introduction of a manifestly gauge invariant Lagrangian enables a straightforward application of the Noether's theorem. Taking advantage of this in the context of a broad class of axisymmetric ferromagnets of special conceptual and experimental relevance, a general expression of the conserved and quantized spin-wave total angular momentum is rigorously derived, while separate conservation and quantization of its orbital and spin components are established for a more restricted class of uniaxial exchange ferromagnets. Further particularizing this general framework to the case of axially saturated magnetic thin disks, we develop a semianalytic theory of the low-frequency part of the exchange-dipole azimuthal spin-wave spectrum, providing a powerful theoretical platform for the analysis and interpretation of magnetic resonance experiments on magnetic microdots as further demonstrated in a joint paper [Phys. Rev. B 113, L100410 (2026)].

We consider a lattice model of twisted bilayer graphene (TBG) for incommensurate twist angles, focusing on the role of large-momentum-transfer umklapp terms. These terms, which nearly connect the Fermi points of different layers, are typically neglected in effective continuum descriptions but could, in principle, destroy the Dirac cones; they are indeed closely analogous to those appearing in fermions within quasiperiodic potentials, where they play a crucial role. We prove that, for small but finite interlayer coupling, the semimetallic phase is stable provided the angles belong to a fractal set of large measure (which decreases with the hopping strength) characterized by a number-theoretic Diophantine condition. In particular, this set excludes the (zero measure) commensurate angles. Our method combines a renormalization group (RG) analysis of the imaginary-time, zero-temperature Green's functions, with number theoretic properties, and it is similar to the technique used in the Lindstedt series approach to Kolmogorov-Arnold-Moser (KAM) theory. The convergence of the resulting series allows us to rule out nonperturbative effects. The result provides a partial justification of the effective continuum description of TBG in which such large-momentum interlayer hopping processes are neglected.

Previous extended x-ray absorption fine structure (EXAFS) studies have shown evidence of anomalous contributions in the atomic background function whose origin was debated in the literature. The present study presents advanced multiple scattering calculations aimed to put to a test the existence and origin of the so-called atomic XAFS (AXAFS) contribution, discussed in details in some previous works. We show with specific examples that the muffin-tin approximation normally used in EXAFS calculations is inappropriate to study this phenomenon as spurious oscillations in the atomic background are related to the potential truncation at the border of the muffin-tin spheres. We thus employed a more sophisticated approach beyond the muffin-tin approximation based on the full-potential multiple scattering theory and self-consistent charge densities. The comparison of EXAFS calculations with previous experimental results allowed us to exclude the presence of the AXAFS as due to modulations of the charge density as previously suggested. This study confirms the importance of a proper account of the atomic background in EXAFS data analysis, which may contain effects beyond the single-electron approximation but are found not affected by AXAFS. We conclude that the contribution of AXAFS in EXAFS experiments, if existing at all, is negligible and should not be considered in EXAFS data analysis.