We derive a general relation between the stacking vector $\mathbf{u}$ describing the relative shift of two layers of bilayer graphene and the Chern index. We find $C=\nu (1-\text{sign}(|V_{\text{AB}}|-|V_{\text{BA}}\left|\right))$, where $\nu$ is a valley index and $|V_{\alpha \beta }|$ the absolute value of the $\mathbf{u}$ dependent stacking potentials that uniquely determine the interlayer interaction; AA stacking plays no role in the topological character. With this expression we show that while ideal and relaxed minimally twisted bilayer graphene appear so distinct as to be almost different materials, their Chern index maps are, remarkably, identical. The topological physics of this material is thus strongly robust to lattice relaxations.

Quantum phenomena at the ultimate atomic-scale two-dimensional limit attract increasing interest due to the emergence of exotic physics and potential applications in prospective electronic devices. In this work, we carried out a theoretical and experimental study of the atomic, electronic, and spin structures of the single-atomic-layer PbBi compounds grown on silicon surface. We found that adsorption of half-monolayer of Bi and half-monolayer of Pb onto Si(111) surface leads to the formation of an atomic layer consisting of rhombuslike motifs. The rhombuslike motifs are arranged into the $2\sqrt{3}\times 2\sqrt{3}$ and $2\times 2$ periodical structures. In the $2\sqrt{3}\times 2\sqrt{3}$-PbBi phase, the motifs are oriented according to the $C_3$ symmetry and this phase is characterized as a trivial insulating spin-split system. In the $2\times 2$-PbBi phase, the motifs are oriented along a single direction giving rise to a one-dimensional metallic spin-polarized electron channel, which is of great relevance for spintronic-based applications. We traced the origin of the one-dimensional electronic states in the two-dimensional PbBi compound and discussed the interplay of these one-dimensional states with the geometry of the phase.

We study transport properties and the formation of bound states in the continuum (BIC) in asymmetric quantum mechanical and electromagnetic waveguides. An analytical model for an arbitrary asymmetric two-terminal quantum mechanical waveguide is proposed, and conditions of BIC formation are formulated. We show that the Friedrich-Wintgen mechanism of BIC formation in a system coupled to two continua takes place regardless of the symmetry of the system as long as the proportionate coupling condition is fulfilled. This result is illustrated by numerical simulation of two-dimensional quantum billiard and optical waveguide with a cavity. Due to the universal wave nature of BIC, the proposed BIC formation mechanism allows one to obtain BICs in the broader class of quantum mechanical, electromagnetic, acoustic, and other types of structures.

We report a unique type of gap in a magnetic Weyl semimetal ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ where the electrons are spin polarized and preserve the spin-momentum locking feature of Weyl fermions. Such spin-polarized gaps are associated with the Weyl node annihilation, where a pair of Weyl nodes with opposite chirality touch each other at ∼210 meV above the Fermi energy. These are revealed by both time- and spin-resolved ultrafast spectroscopy experiments, combined with first-principles calculations. The spin-polarized gap opening is accompanied by a topological phase transition, and the gap magnitude exhibits an unconventional temperature dependence originated from the Weyl physics. Furthermore, we propose possible circularly polarized terahertz-midinfrared radiation from such a spin-polarized gap. Our results shed light on exploring the topological properties of excited states and endow application potentials for gapped materials based on chirality.

Lattice vibration in solids may carry angular momentum. But unlike the intrinsic spin of electrons, the lattice vibration is rarely rotational. To induce angular momentum, one needs to find a material that can accommodate a twisted normal mode, two orthogonal modes, or excitation of magnons. If excitation is too strong, one may exceed the Lindemann limit, so the material melts. Therefore these methods are not ideal. Here, we theoretically propose a route to phonon angular momentum in a molecular crystal ${\mathrm{C}}_{60}$. We find that a single laser pulse is able to inject a significant amount of angular momentum to ${\mathrm{C}}_{60}$, and the momentum transfer is helicity dependent. Changing from right-circularly polarized light to left-circularly polarized light switches the direction of phonon angular momentum. On the ultrafast timescale, the orbital angular momentum change closely resembles the displacive excitation of coherent phonons, with a cosine-function dependence on time, different from the spin counterpart. Atomic displacements, even under strong laser excitation, remain far below the Lindemann criterion. Under thermal excitation, spinning ${\mathrm{C}}_{60}$ even at room temperature generates a huge angular momentum close to several hundred $\hslash$. Our finding opens the door to a large group of fullerenes, from ${\mathrm{C}}_{60},{\mathrm{C}}_{70}$ to their endohedral derivatives, where angular momentum can be generated through light or temperature. This paves the way to the phononic control electronic spin and harvesting thermal energy through phonon angular momentum.

Quantum spin liquids (QSLs) have generated considerable excitement as phases of matter with emergent gauge structures and fractionalized excitations. In this context, phase transitions out of QSLs have been widely discussed as Higgs transitions from deconfined to confined phases of a lattice gauge theory. However, the possibility of a wider range of novel phases, occurring between these two limits, has yet to be systematically explored. In this Research Letter, we develop a formalism which allows for interactions between fractionalized quasiparticles coming from the constraint on the physical Hilbert space and can be used to search for exotic, hidden phases. Taking pyrochlore spin ice as a starting point, we show how a U(1) QSL can give birth to abundant daughter phases, without the need for fine-tuning of parameters. These include a (charged) ${\mathbb{Z}}_2$ QSL and a supersolid. We discuss implications for experiment, and numerical results which support our analysis. These results are of broad relevance to QSLs subject to a parton description and offer a useful perspective in the search for exotic hidden phases in quantum magnets.

We introduce the magnon circular photogalvanic effect enabled by two-magnon Raman scattering. This provides an all-optical pathway to the generation of directed magnon currents with circularly polarized light in honeycomb antiferromagnetic insulators. The effect is the leading order contribution to magnon photocurrent generation via optical fields. Control of the magnon current by the polarization and angle of incidence of the laser is demonstrated. Experimental detection by sizable inverse spin Hall voltages in platinum contacts is proposed.

Optical spectral weight transfer associated with the onset of superconductivity at high-energy scales compared with the superconducting gap has been observed in several systems such as high-$T_c$ cuprates. While there are still debates on the origin of this phenomenon, a consensus is that it is due to strong correlation effects beyond the BCS theory. Here, we show that there is another route to a nonzero spectral weight transfer based on the quantum geometry of the conduction band in multiband systems. We discuss applying this idea to the cuprates and twisted multilayer graphene.

So far, topological band theory has been discussed mainly for systems described by eigenvalue problems. Here, we develop a topological band theory described by a generalized eigenvalue problem (GEVP). Our analysis elucidates that non-Hermitian topological band structures may emerge for systems described by a GEVP with Hermitian matrices. The above result is verified by analyzing a two-dimensional toy model where symmetry-protected exceptional rings (SPERs) emerge although the matrices involved are Hermitian. Remarkably, these SPERs are protected by emergent symmetry, which is unique to the systems described by the GEVP. Furthermore, these SPERs elucidate the origin of the characteristic dispersion of hyperbolic metamaterials which is observed in experiments.

We investigate the spin Seebeck effect and spin pumping in a junction between a ferromagnetic insulator and a magnetic impurity deposited on a normal metal. By performing a numerical renormalization group calculation, we show that spin current is enhanced by the Kondo effect. This spin current is suppressed by an increase in temperature or a magnetic field comparable to the Kondo temperature. Our results indicate that spin transport can be a direct probe of spin excitation in strongly correlated systems.

We study the response of a thermal state of the Hubbard-like system to either global or local non-Hermitian perturbation, which coalesces the degenerate ground state within the U(1) symmetry breaking phase. We show that the dynamical response of the system is strongly sensitive to the underlying quantum phase transition (QPT) from a Mott insulator to a superfluid state. The Uhlmann fidelity in the superfluid phase decays to a steady value determined by the order of the exceptional point (EP) within the subspace spanned by the degenerate ground states, but remains almost unchanged in the Mott insulating phase. It demonstrates that the phase diagram at zero temperature is preserved even though a local probing field is applied. Specifically, two celebrated models, including the Bose-Hubbard model and the Jaynes-Cummings-Hubbard model, are employed to demonstrate this property in the finite-size system, wherein fluctuations of the boson and polariton number are observed based on EP dynamics. This work presents an alternative approach to probe the superfluid-insulator QPT at nonzero temperature.

The topological Dirac semimetal ${\mathrm{Cd}}_3{\mathrm{As}}_2$ has drawn great attention for its novel physics and technical applications in optoelectronic devices operating in the infrared and THz regimes. Exploring the photoexcited carrier dynamics in ${\mathrm{Cd}}_3{\mathrm{As}}_2$ is helpful to understand the transient carrier occupation and cooling processes that are closely associated with its unconventional band characteristic. Here, the photoexcited carrier dynamics in the ${\mathrm{Cd}}_3{\mathrm{As}}_2$ thin film epitaxially grown on the $\mathrm{GaAs}\left(111\right)B$ substrate was investigated by employing the time-resolved midinfrared transient reflectance measurements, along with a theoretical modeling concerning the electron-polar optical ($e$-PO) phonon interaction. The measured photoexcited electron relaxation time of ${\mathrm{Cd}}_3{\mathrm{As}}_2$ increases with the probe wavelength, and increases with temperature at temperatures higher than 30 K. Theoretical modeling under the framework of the two-temperature model can give qualitatively good description on both the temperature and probe wavelength dependence of the carrier relaxation time. Additionally, theoretical modeling shows that charge screening of the interaction of electrons with polar optical phonons can greatly reduce the hot carrier relaxation rate. The measured transient reflectance analyses indicate that the hot phonon effect plays a negligible role on the carrier relaxation in ${\mathrm{Cd}}_3{\mathrm{As}}_2$. Our findings provide further insight into the photoexcited carrier dynamics in ${\mathrm{Cd}}_3{\mathrm{As}}_2$ and valuable reference for developing its high-performance infrared optoelectronic devices.

The variational autoregressive network is extended to the Euclidean path integral representation of a quantum partition function. An essential challenge is adapting the sequential process of sample generation by an autoregressive network to a nonlocal constraint due to the periodic boundary condition in the path integral. An add-on oracle is devised for this purpose, which accurately identifies and stalls unviable configurations as soon as they occur. The oracle enables rejection-free sampling conforming to the periodic boundary condition. As a demonstration, the oracle-guided autoregressive network is applied to obtain variational solutions of quantum spin chains at finite temperatures with relatively large system sizes and numbers of time slicing, and to efficiently compute thermodynamic quantities.

Recent experiments suggest a multicomponent pairing function in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, which appears to be inconsistent with the absence of an apparent cusp in the transition temperature $T_c$ as a function of the uniaxial strain. We show, however, that the theoretical cusp in $T_c$ for a multicomponent pairing can be easily smeared out by the spatial inhomogeneity of strain, and the experimental data can be reproduced qualitatively by a percolation model. These results shed light on multicomponent pairings. We then perform a thorough group-theoretical classification of the pairing functions, taking the spin-orbit coupling into account. We list all 13 types of two-component spin-singlet pairing functions, with 8 of them belonging to the $E_g$ representation. In particular, we find two types of intraorbital pairings in the $E_g$ representation $(k_x{k}_z,k_y{k}_z)$ are favorable in view of most existing experiments.

We develop a fast and accurate machine-learned interatomic potential for the Mo-Nb-Ta-V-W quinary system and use it to study segregation and defects in the body-centered-cubic refractory high-entropy alloy MoNbTaVW. In the bulk alloy, we observe clear ordering of mainly Mo-Ta and V-W binaries at low temperatures. In damaged crystals, our simulations reveal clear segregation of vanadium, the smallest atom in the alloy, to compressed interstitial-rich regions such as radiation-induced dislocation loops. Vanadium also dominates the population of single self-interstitial atoms. In contrast, due to its larger size and low surface energy, niobium segregates to spacious regions such as the inner surfaces of voids. When annealing samples with supersaturated concentrations of defects, we find that in complete contrast to W, interstitial atoms in MoNbTaVW cluster to create only small $(\sim 1 \mathrm{nm})$ experimentally invisible dislocation loops enriched by vanadium. By comparison to W, we explain this by the reduced but three-dimensional migration of interstitials, the immobility of dislocation loops, and the increased mobility of vacancies in the high-entropy alloy, which together promote defect recombination over clustering.

We theoretically investigate the high-order harmonic generation (HHG) from ZnO crystals driven by orthogonally polarized two-color (OTC) laser fields. By solving the semiconductor Bloch equation (SBE) in two dimensions, it is found that the HHG yields show a high sensitivity to the relative phase between the two fields. By changing the laser intensity, the yields of HHG show two maxima in the high-order region. To reveal the physical origin of these phenomena, we analyze the behavior of the electron-hole pair in the real and reciprocal spaces by the recollision model, respectively. It could qualitatively interpret the results by SBE simulations. Our analysis demonstrates that the OTC laser fields could control the electron-hole dynamics and HHG processes efficiently.

The extremely large magnetoresistance (XMR) observed in many topologically nontrivial and trivial semimetals has attracted much attention in relation to its underlying physical mechanism. In this paper, by combining the band structure and Fermi surface (FS) calculations with the Hall resistivity and de Haas–van Alphen (dHvA) oscillation measurements, we studied the anisotropy of magnetoresistance (MR) of ${\mathrm{ReO}}_3$ with a simple cubic structure, an “ordinary” nonmagnetic metal considered previously. We found that ${\mathrm{ReO}}_3$ exhibits almost all the characteristics of XMR semimetals: the nearly quadratic field dependence of MR, a field-induced upturn in resistivity followed by a plateau at low temperatures, and high mobilities of charge carriers. It was found that for magnetic field H applied along the $c$ axis, the MR exhibits an unsaturated H${}^{1.75}$ dependence, which was argued to arise from the complete carrier compensation supported by the Hall resistivity measurements. For H applied along the direction of ${15}^{\circ }$ relative to the $c$ axis, an unsaturated H${}^{1.90}$ dependence of MR up to $(9.43\times {10}^3)\%$ at 10 K and 9 T was observed, which was explained by the existence of electron open orbits extending along the $k_x$ direction. Two mechanisms responsible for XMR observed usually in the semimetals occur also in the simple metal ${\mathrm{ReO}}_3$ due to its peculiar FS (two closed electron pockets and one open electron pocket), once again indicating that the details of FS geometrical configuration are a key factor for the observed XMR in materials.