Breaking down the traditional perception on phonons which are achiral, the existence of a chiral phonon carrying angular momentum provides possible ways to couple electrons, photons, spins, magnons, and excitons, etc. We theoretically proposed an electron-chiral phonon interaction with a two-phonon process, in contrast to a conventional electron-phonon interaction, and a kind of effective Hubbard interaction through exchanging two chiral phonons is proposed. Taking a two-dimensional diatomic honeycomb lattice as an example, we found this repulsive Hubbard interaction mediated by chiral phonons induces unconventional and high-temperature superconductivity. Moreover, the numerical calculations show an inverse isotope effect which is consistent with experimental observations in high-$T_c$ superconductors. Our finding on an electron-chiral phonon and the associated Cooper pair provides a path to understand the high-$T_c$ superconductivity.

Unconventional quasiparticles of a twofold band degeneracy with $\left|C\right|=4$, named as charge-four Weyl point (CFWP), have been revealed in bosonic and artificial systems, while it is challenging in fermionic systems because of nonnegligible spin-orbit coupling. Herein, we propose a carbon allotrope, termed cP-C24, as an ideal platform to realize CFWP in the nonrelativistic limit. Besides one CFWP ($C=+4$), there are also eight type-I Weyl points ($C=+1$) and twelve type-II Weyl nodes ($C=-1$), making the topological charge in the whole Brillouin zone to be neutral. The characteristic quadruple helicoid surface states of CFWP are presented. Our work offering the avenue of CFWP in fermionic systems would absolutely advance the research on the chirality-dependent physical properties associated with unconventional topological quasiparticles.

We analyze the Hilbert space connectivity of the $L$ site PXP model by constructing the Hamiltonian matrices via a Gray code numbering of basis states. The matrices are all formed out of a single Hamiltonian-path backbone and entries on skew diagonals. Starting from this observation, we construct an ensemble of related Hamiltonians based on random graphs with tunable constraint degree and variable network topology. We study the entanglement structure of their energy eigenstates and find two classes of weakly entangled midspectrum states. The first class contains scars that are approximate products of eigenstates of the subsystems. Their origin can be traced to the near orthogonality of random vectors in high-dimensional spaces. The second class of scars has $log2$ entanglement entropy and is tied to the occurrence of special types of subgraphs. The latter states have some resemblance to the Lin-Motrunich $\sqrt{2}$ scars.

We reveal in this study the fundamental low-energy landscape in the ferrimagnetic ${\mathrm{Sr}}_2\mathrm{CrRe}{\mathrm{O}}_6$ double perovskite and describe the underlying mechanisms responsible for the three low-energy excitations below 1.4 eV. Based on resonant inelastic x-ray scattering and magnetic dynamics calculations, and experiments collected from both ${\mathrm{Sr}}_2\mathrm{CrRe}{\mathrm{O}}_6$ powders and epitaxially strained thin films, we reveal a strong competition between spin-orbit coupling, Hund's coupling, and the strain-induced tetragonal crystal field. We also demonstrate that a spin-flip process is at the origin of the lowest excitation at 200 meV, and we bring insights into the predicted presence of orbital ordering in this material. We study the nature of the magnons through a combination of ab initio and spin-wave theory calculations, and show that two nondegenerate magnon bands exist and are dominated either by rhenium or chromium spins. The rhenium band is found to be flat at about 200 meV (±25 meV) through X-L-W-U high-symmetry points and is dispersive toward Γ.

Heterostructures between two-dimensional quantum spin Hall insulators (QSHIs) and superconducting materials can allow for the presence of Majorana fermions at their conducting edge states. Although a strong interface hybridization helps induce a reasonable superconducting gap on the topological material, the hybridization can modify the material's electronic structure. In this work, we utilize a realistic low-energy model with tunable interlayer hybridization to study the edge-state physics in a heterostructure between monolayer quantum spin Hall insulator $1T^{\prime }{\text{-WTe}}_2$ and $s$-wave superconductor $2H{\text{-NbSe}}_2$. We find that even in the presence of strong interlayer hybridization that renders the surface to become conducting, the edge state shows a significantly enhanced local density of states and induced superconductivity compared to the surface. We provide an alternate heterostructure geometry that can utilize the strong interlayer hybridization and realize a spatial interface between a regime with a clean QSHI gap and a topological conducting edge state.

Relaxometry schemes employing nitrogen-vacancy (NV) centers in diamonds are essential in biology and physics to detect a reduction of the color centers' characteristic spin relaxation ($T_1$) time caused by, e.g., paramagnetic molecules in proximity. However, while only the negatively charged NV center is to be probed in these pulsed-laser measurements, an inevitable consequence of the laser excitation is the conversion to the neutrally charged NV state, interfering with the result for negatively charged NV centers' $T_1$ time or even dominating the response signal. In this paper, we perform relaxometry measurements on an NV ensemble in nanodiamond combining a $520\mathrm{n}\mathrm{m}$ excitation laser and microwave excitation while simultaneously recording the fluorescence signals of both charge states via independent beam paths. Correlating the fluorescence intensity ratios to the fluorescence spectra at each laser power, we monitor the ratios of both charge states during the $T_1$-time measurement and systematically disclose the excitation-power-dependent charge conversion. Even at laser intensities below saturation, we observe charge conversion, while at higher intensities, charge conversion outweighs spin relaxation. These results underline the necessity of low excitation power and fluorescence normalization before the relaxation time to accurately determine the $T_1$ time and characterize paramagnetic species close to the sensing diamond.

The topological property of a system is a static property in general. For instance, the topological edge state is observed by measuring the local density of states. In this paper we propose a system whose topological property is only revealed by dynamics. As a concrete example, we explore a nonlinear dynamical topological phase transition revealed by a quench dynamics of a Cooper-pair box array connected with capacitors, where the dynamics is driven by giving a flux to the left-end Cooper-pair box. It is described by coupled nonlinear differential equations due to the Josephson effect. It is trivial as far as the static system is concerned. However, the wave propagation induced by the quench dynamics demonstrates a rich topological phase diagram in terms of the strength of the input. It consists of topological, trivial, trapped, and dimer phases.

We perform ab initio many-body calculations to investigate the exciton shift current in small-diameter zigzag BN nanotubes and also single BN sheet, using the $GW$ plus Bethe-Salpeter equation ($GW$-BSE) method with the newly developed efficient algorithms. Our $GW$-BSE calculations reveal a large in-gap peak in the shift current spectrum in all the studied BN systems due to the $A$ exciton. The peak value of the excitonic shift current is more than three times larger than that of the quasiparticle shift current, and is attributed to the significant enhancement of the optical dipole matrix element by the $A$ exciton. The effective exciton shift current conductivity is nearly 10 times larger than the largest shift conductivity observed in ferroelectric semiconductors. Importantly, the direction of the shift current in the BN nanotubes is found to be independent of the tube chirality ($n,0$) (or diameter), contrary to the simple rule of $\text{sgn}\left(J_{\text{shift}}\right)=\text{mod}(n,3)$ predicted by previous model Hamiltonian studies. Finally, our ab initio calculations also show that the exciton excitation energies decrease significantly with the decreasing diameter due to the curvature-induced orbital rehybridization in small-diameter zigzag BN nanotubes.

Magnetization reversal of a soft ferromagnetic Fe layer coupled to a ${\left[\mathrm{Co}/\mathrm{Pt}\right]}_{\mathrm{ML}}$ multilayer (ML) with perpendicular magnetic anisotropy (PMA) has been studied in situ with an aim to understand the origin of exchange bias (EB) in orthogonal magnetic anisotropic systems. The interface remanent state of the ML is modified by magnetic field annealing, and its effect on the soft Fe layer is monitored using the in situ magneto-optical Kerr effect (MOKE). A considerable shift in the Fe layer hysteresis loop from the center and an unusual increase in the coercivity, similar to EB phenomena, is attributed to the exchange coupling at the ${\left[\mathrm{Co}/\mathrm{Pt}\right]}_{\mathrm{ML}}$ and Fe interface. The effect of the coupling on spin orientation at the interface is further explored precisely by performing an isotope selective grazing incident nuclear resonance scattering (GINRS) technique. Here, the interface selectivity is achieved by introducing a 15 Å-thick ${\mathrm{Fe}}^{57}$ marker between the ${\left[\mathrm{Co}/\mathrm{Pt}\right]}_{\mathrm{ML}}$ and Fe layers. The interface signal from ${\mathrm{Fe}}^{57}$ is further enhanced by performing measurements under x-ray standing-wave conditions. The combined MOKE and GINRS analysis revealed a unidirectional pinning of the Fe layer due to the net in-plane magnetic spin at the interface caused by magnetic field annealing. Unidirectional exchange coupling or pinning at the interface, which may be due to the formation of asymmetrical closure domains, is found to be responsible for the origin of EB with an unusual increase in coercivity.

Flat bands in twisted bilayer two-dimensional materials present a platform for investigating exotic phenomena. However, achieving flat bands typically necessitates small twist angles, resulting in large moiré superlattices that increase experimental complexity and hinder compact device integration. In this work, we discovered that multiple flat bands with extremely low bandwidth can form in twisted bilayer binary materials, such as AlN and GaN, at large twist angles. The atomic reconstruction, which is most pronounced in AlN and GaN among commonly used two-dimensional materials, can alter the electronic localization position and provide effective barriers among the localized states, playing a crucial role in the formation of these ultraflat bands. Our findings offer a unique approach for generating flat bands at large twist angles.

Two-dimensional van der Waals heterostructures (vdWHs) have been extensively studied for their excellent physical characteristics. In this paper, two twisted $\mathrm{Sb}/\mathrm{W}{\mathrm{Te}}_2$ vdWHs are respectively constructed by stacking of Sb and $\mathrm{W}{\mathrm{Te}}_2$ monolayers with different interlayer rotation angles, and their electronic properties are studied by first-principles calculation. Firstly, the effects of spin-orbit coupling on the electronic structure of $\mathrm{Sb}/\mathrm{W}{\mathrm{Te}}_2$ vdWHs, such as band gap and band alignment, are addressed in detail. Furthermore, for the $\mathrm{Sb}/\mathrm{W}{\mathrm{Te}}_2$ vdWH with an interlayer rotation angle of ${30}^{\circ }$, its band gap, band alignment, and spin splitting are investigated by adjusting the external electric field, biaxial strain, and interlayer coupling, respectively. Under an applied external electric field, the band structure of the $\mathrm{Sb}/\mathrm{W}{\mathrm{Te}}_2$ vdWH undergoes a transition from a direct band gap to an indirect band gap, and a semiconductor-metal transition occurs at $\pm 0.7\mathrm{V}/\AA$ along with the transition of types I, II, and III band alignments. Similarly, the band gap and band alignment of the $\mathrm{Sb}/\mathrm{W}{\mathrm{Te}}_2$ vdWH can also be modulated by biaxial strain and interlayer coupling. In addition, the calculated electronic structure present that the Rashba- and Zeeman-type spin splitting are dependent on the external electric field, biaxial strain, and interlayer coupling. Thus, the controllable electronic properties of $\mathrm{Sb}/\mathrm{W}{\mathrm{Te}}_2$ vdWHs have great application potential for spintronic and optoelectronic devices.

Photonic crystals (PhCs) have emerged as a popular platform for realizing various topological phases due to their flexibility and potential for device applications. In this article, we present a comprehensive classification of topological bands in one- and two-dimensional photonic crystals, with and without time-reversal symmetry. Our approach exploits the symmetry representations of field eigenmodes at high-symmetry points in momentum space, allowing for the efficient design of a wide range of topological PhCs. In particular, we show that the complete classification provided here is useful for diagnosing photonic crystal analogs of obstructed atomic limits, fragile phases, and stable topological phases that include bands with Dirac points and Chern numbers.

We revisit low-temperature optical spectra of transition-metal dichalcogenide monolayers and point to a possible crystallization of electrons (or holes) at low to moderate charge densities. To calculate the excitonic spectra under such conditions, we introduce the recursion method and compute how the charge density affects the energies, linewidths, and oscillator strengths of exciton and trion complexes. Equally important, we study how excitons and trions in the monolayer evolve when the charge particles gradually transition to a periodic Wigner lattice. The results provide valuable information on the ability to detect whether the particles are ordered through the exciton spectrum. Finally, we calculate the change in exciton energy in cases where the added charge particles have similar and dissimilar quantum numbers (spin and valley) to those of the electron or hole in the exciton. The results of this work shed new light on the important optical properties of monolayer semiconductors.

Kelvin probe force microscopy (KPFM) has been employed to probe charge carriers in a graphene/hexagonal boron nitride (hBN) heterostructure [Nano Lett. 21, 5013 (2021)]. We propose an approach for operating valley filtering based on the KPFM-induced potential $U_0$ instead of using external or induced pseudomagnetic fields in strained graphene. Employing a tight-binding model, we investigate the parameters and rules leading to valley filtering in the presence of a graphene quantum dot (GQD) created by the KPFM tip. This model leads to a resolution of different transport channels in reciprocal space, where the electron transmission probability at each Dirac cone (${\mathbf{K}}_{\mathbf{1}}=-\mathbf{K}$ and ${\mathbf{K}}_{\mathbf{2}}=+\mathbf{K}$) is evaluated separately. The results show that $U_0$ and the Fermi energy $E_F$ control (or invert) the valley polarization, if electrons are allowed to flow through a given valley. The resulting valley filtering is allowed only if the signs of $E_F$ and $U_0$ are the same. If they are different, the valley filtering is destroyed and might occur only at some resonant states affected by $U_0$. Additionally, there are independent valley modes characterizing the conductance oscillations near the vicinity of the resonances, whose strength increases with $U_0$ and are similar to those occurring in resonant tunneling in quantum antidots and to the Fabry-Perot oscillations. Using KPFM, to probe the charge carriers, and graphene-based structures to control valley transport, provides an efficient way for attaining valley filtering without involving external or pseudomagnetic fields as in previous proposals.

The dynamics of non-Hermitian quantum systems have taken on an increasing relevance in light of quantum devices which are not perfectly isolated from their environment. The interest in them also stems from their fundamental differences from their Hermitian counterparts, particularly with regard to their spectral and eigenvector correlations. These correlations form the fundamental building block for understanding the dynamics of quantum systems as all other correlations can be reconstructed from it. In this Letter, we study such correlations across a localization transition in non-Hermitian quantum systems. As a concrete setting, we consider non-Hermitian power-law banded random matrices which have emerged as a promising platform for studying localization in disordered, non-Hermitian systems. We show that eigenvector correlations show marked differences between the delocalized and localized phases. In the delocalized phase, the eigenvectors are strongly correlated as evinced by divergent correlations in the limit of vanishingly small complex eigenvalue spacings. On the contrary, in the localized phase, the correlations are independent of the eigenvalue spacings. We explain our results in the delocalized phase by appealing to the Ginibre random-matrix ensemble. On the other hand, in the localized phase, an analytical treatment sheds light on the suppressed correlations, relative to the delocalized phase. Given that eigenvector correlations are fundamental ingredients towards understanding real- and imaginary-time dynamics with non-Hermitian generators, our results open an avenue for characterizing dynamical phases in non-Hermitian quantum many-body systems.

Within the Floquet theory of periodically driven quantum systems, we demonstrate that a circularly polarized off-resonant electromagnetic field can destroy the electron states bound by three-dimensional attractive potentials. As a consequence, the optically induced delocalization of bound electrons appears. The effect arises from the changing of topological structure of a potential landscape under a circularly polarized off-resonant electromagnetic field which turns simply connected potentials into doubly connected ones. Possible manifestations of the effect are discussed for conduction electrons in condensed-matter structures.

We show that the transport integrals of the two-site charge Kondo circuits connecting various multichannel Kondo simulators satisfy the generalized Wiedemann-Franz law with the universal Lorenz ratios all greater than 1. The magic Lorenz ratios are directly related to the Anderson's orthogonality catastrophe in quantum simulators providing some additional universal measure for the strong electron-electron correlations. We present a fullly fledged theory for the magic Lorenz ratios and discuss possible routes for the experimental verifications of the theory.

We prove a few spectral theorems for the density of states of a Weyl node with arbitrary topology. We show that the density of extended states of a Weyl node with random impurity potentials remains gapless in the presence of a magnetic field. Therefore, a magnetic field precludes Anderson localization in Weyl semimetals, when internode transitions are suppressed for smooth enough potentials. We also provide a rigorous quantum mechanical proof of the chiral magnetic effect for arbitrary topology of a Weyl node.

The topological phase in amorphous systems adds a new dimension to the topological states of matter. Here, we present an interesting phenomenon dubbed the topological Anderson amorphous insulator (TAAI). Anderson disorder can drive topologically trivial amorphous systems with structural disorders into noncrystalline topological insulators. The gap closing and reopening, spin Bott index, robust edge states, and quantized conductance characterize the Anderson disorder-induced nontrivial topology in amorphous systems. More importantly, phase diagrams are given for the topological phase transition (TPT). It is found that amorphous structural disorder and Anderson disorder are synergistic to drive the $s\text{−}p$ band inversion of the system and hence the TPT, which is further confirmed by the effective medium theory. Our findings report a disorder-induced topological phenomenon in noncrystalline systems and shed light on the physical understanding of the interplay between the coexistence of two types of disorder effects and topology.

The energy band structure of excitons is studied in periodic potentials produced by the short-range interaction between the exciton and electrons of Wigner lattices. Treating the exciton as a pointlike dipole that interacts with the periodic potential, we can solve a simple one-body problem that provides valuable information on excitons in many-body problem settings. By employing group theory, we identify the excitonic energy bands that can couple to light and then quantify their energy shifts in response to a change in the period of the potential. This approach allows us to emulate the response of optically active exciton and trion states to a change in electron density. In agreement with experimental results, the exciton (trion) state shows a relatively strong (weak) energy shift when the electron density increases. This behavior stems from the difficulty of exciton states to remain extended, whereas the trion state remains largely the same as long as its radius is smaller than the average distance between resident electrons. We identify an important relation between the energy blueshift of bright excitons and the presence of a Wigner crystal, where the blueshift magnitude is proportionate to the symmetry of the electron lattice.