We construct a hydrodynamic framework describing plastic deformations in electronic crystals. The framework accounts for pinning, phase, and momentum relaxation effects due to translational disorder, diffusion due to the presence of interstitials and vacancies, and strain relaxation due to plasticity and dislocations. We obtain the hydrodynamic mode spectrum and correlation functions in various regimes in order to identify the signatures of plasticity in electronic crystal phases. In particular, we show that proliferation of dislocations depins the spatially resolved conductivity until the crystal melts, after which point a new phase of a pinned electronic liquid emerges. In addition, the mode spectrum exhibits a competition between pinning and plasticity effects, with the damping rate of some modes being controlled by pinning-induced phase relaxation and some by plasticity-induced strain relaxation. We find that the recently discovered damping-attenuation relation continues to hold for pinned-induced phase relaxation even in the presence of plasticity and dislocations. We also comment on various experimental setups that could probe the effects of plasticity. The framework developed here is applicable to a large class of physical systems including electronic Wigner crystals, multicomponent charge density waves, and ordinary crystals.

Quantum spin hall (QSH) effect, a class of quantum state, is promising for dissipationless transport with topologically protected helical edge state. Based on first-principles calculations, we theoretically demonstrate the topological physics of $1\mathrm{H}\text{−}\mathrm{Pd}{\mathrm{Se}}_2$ with a large gap up to 0.24 eV. The band inversion takes place either among the $p_z$ and $p_{x,y}$ orbitals of Se atom or due to spin-orbital coupling under compressive strains. The edge modes exist in the energy gap and can be characterized by an effective edge state Hamiltonian. Based on the derived two-band $\mathbf{k}·\mathbf{p}$ model, we demonstrate the profound phenomena induced by the applied magnetic field effects and optical transitions for chiral fermions, which lead to complicated excitonic selection rules in this topologically nontrivial two-dimensional material. Our results will pave the way for future theoretical and experimental studies on $\mathrm{Pd}{X}_2$ $(X=\mathrm{S},\mathrm{Se},\mathrm{Te})$ monolayers.

Multilayer stacks of twisted optical metasurfaces are considered as a prospective platform for chiral nanophotonic devices. Such structures are primarily used for the realization of circularly polarized light sources, artificial optical rotation, and circular dichroism. At the same time, the behavior of their hybrid photonic modes is strongly affected by the moiré pattern of superimposed periodic constituents. In this paper, we show that moiré periodicity in bilayer dielectric photonic crystal slabs (PCSs) leads to a rise of unlimitedly narrow optical resonances, which are very sensitive to the relative twist and gap width between the sublayers. We demonstrate the structure providing twist-tuning of the hybrid mode wavelength in the range of 300–600 nm with the quality factor varying from ${10}^2$ up to ${10}^5$ correspondingly. The obtained results pave the way for the utilization of moiré-assisted effects in multilayer PCSs.

We provide a formal derivation of a reduced model for twisted bilayer graphene (TBG) from Density Functional Theory. Our derivation is based on a variational approximation of the TBG Kohn-Sham Hamiltonian and asymptotic limit techniques. In contrast with other approaches, it does not require the introduction of an intermediate tight-binding model. The so-obtained model is similar to that of the Bistritzer-MacDonald (BM) model but contains additional terms. Its parameters can be easily computed from Kohn-Sham calculations on single-layer graphene and untwisted bilayer graphene with different stackings. It allows one in particular to estimate the parameters $w_{\mathrm{AA}}$ and $w_{\mathrm{AB}}$ of the BM model from first principles. The resulting numerical values, namely $w_{\mathrm{AA}}=w_{\mathrm{AB}}\simeq 126\mathrm{meV}$ for the experimental interlayer mean distance are in good agreement with the empirical values $w_{\mathrm{AA}}=w_{\mathrm{AB}}=110\mathrm{meV}$ obtained by fitting to experimental data. We also show that if the BM parameters are set to $w_{\mathrm{AA}}=w_{\mathrm{AB}}\simeq 126\mathrm{meV}$, the BM model is an accurate approximation of our reduced model.

Spectroscopic data may often contain unwanted extrinsic signals. For example, in the angle-resolved photoemission spectroscopy (ARPES) experiment, a wire mesh is typically placed in front of the charge-coupled device to block stray photoelectrons but could cause a gridlike structure in the spectra during quick measurement mode. In the past, this structure was often removed using the mathematical Fourier filtering method by erasing the periodic structure. However, this method may lead to information loss and vacancies in the spectra because the grid structure is not strictly linearly superimposed. Here, we propose a deep learning method to overcome this problem effectively. Our method takes advantage of the self-correlation information within the spectra themselves and can greatly optimize the quality of the spectra while removing the grid structure and noise simultaneously. It has the potential to be extended to all spectroscopic measurements to eliminate other extrinsic signals and enhance the spectral quality based on the self-correlation of the spectra solely.

We present a high-resolution, angle-resolved photoemission spectroscopy study of the normal electronic state of the superconducting ${\mathrm{NiBi}}_3$. Our experimental results show a complex Fermi surface structure with many sheets along the $\mathrm{\Gamma }\text{−}X$ and $\mathrm{\Gamma }\text{−}Y$ directions of the Brillouin zone. The band structure presents a topological surface state (TSS) at the high symmetry $\mathrm{\Gamma }$ point with a surface Dirac point at the energy $-0.185$ eV. The Dirac-like cone presents a linear dispersion along ${\mathrm{k}}_x$ while it presents saddle-like states along ${\mathrm{k}}_y$ located in the vicinity of the surface Dirac point. Our results are in good agreement with results of density functional theory band structure calculations. We also discuss the topological band structure of the ${\mathrm{NiBi}}_3$ compound.

We report the evolution of structural, magnetic, transport, and electronic properties of bulk polycrystalline ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{Co}}_{1-x}{\mathrm{Nb}}_x{\mathrm{O}}_3$ ($x=$ 0.025–0.25) samples. The Rietveld refinement of the x-ray diffraction patterns with $R\overline{3}c$ space group reveals that the lattice parameters and rhombohedral distortion monotonously increase with the ${\mathrm{Nb}}^{5+}(4d^0$) substitution ($x$). The magnetic susceptibility exhibits a decrease in the magnetic ordering temperature and net magnetization with $x$, which manifests that the Nb substitution dilutes the ferromagnetic (FM) double-exchange interaction and enhances the antiferromagnetic (AFM) superexchange interaction. Interestingly, for the $x>$ 0.1 samples the FM order is completely suppressed and the emergence of a glassy state is clearly evident. Moreover, the decrease in the coercivity ($H_C$) and remanence ($M_r$) with $x$ in the magnetic isotherms measured at 5 K further confirms the dominance of AFM interactions and reduction of FM volume fraction for the $x>$ 0.1 samples. More interestingly, we observe resistivity minima for the $x=$ 0.025 and 0.05 samples, which are analyzed using the quantum corrections in the conductivity, and found that the weak localization effect dominates over the renormalized electron-electron interactions in the three-dimensional (3D) limit. Further, a semiconducting resistivity behavior is obtained for $x>$ 0.05, which follows the Arrhenius law at high temperatures ($\sim 160$–320 K), and the 3D variable range hopping prevails in the low-temperature region ($<160$ K). The core-level photoemission spectra confirm the valence state of constituent elements and the absence of ${\mathrm{Co}}^{2+}$ is discernible.

We study theoretically analog multiterminal Josephson junctions formed by gapped superfluids created upon resonant pumping of cavity exciton polaritons. We study the $p$-like bands of a five-terminal junction in the four-dimensional (4D) parameter space created by the superfluid phases acting as quasimomenta. We find 4/6 Weyl points in 3D subspaces with preserved/broken time-reversal symmetry. We link the real space topology (vortices) to the parameter space one (Weyl points). We derive an effective Hamiltonian encoding the creation, motion, and annihilation of Weyl nodes in 4D. Our work paves the way to the study of exotic topological phases in a platform allowing direct measurement of eigenstates and band topology.

Revealing the properties of single spin defects in solids is essential for quantum applications based on solid-state systems. However, it is intractable to investigate the temperature-dependent properties of single defects, due to the low precision for single-defect measurements in contrast to defect ensembles. Here we report that the temperature dependence of the Hamiltonian parameters for single negatively charged nitrogen-vacancy centers in diamond at room temperature is precisely measured and the results are in reasonable agreement with first-principles calculations. In particular, the hyperfine interactions with randomly distributed ${}^{13}\mathrm{C}$ nuclear spins are clearly observed to vary with temperature and the relevant coefficients are measured with hertz-level precision. The temperature-dependent behaviors are attributed to both thermal expansion and lattice vibrations by first-principles calculations. Our results pave the way for taking nuclear spins as more stable thermometers at nanoscale. The methods developed here for high-precision measurements and first-principles calculations can be further extended to other solid-state spin defects.

Quantum measurements allow us to observe quantum many-body systems consisting of a multitude of microscopic degrees of freedom. However, the quantum uncertainty and the exponentially large Hilbert space pose natural barriers to simple interpretations of the quantum measurement outcomes. We propose a nonlinear “measurement energy” based upon the measurement outcomes and a general approach akin to quantum machine learning to extract the most probable states (maximum likelihood estimates), naturally reconciling noncommuting observables and getting more out of the quantum measurements. Compatible with established quantum many-body ansatzes and efficient optimization, our strategy offers state-of-art capacity with control and full information. We showcase the versatility and accuracy of our strategy on random long-range fermion and Kitaev quantum spin-liquid models, where smoking-gun signatures were lacking.

Recently, intercalation emerges as an effective way to manipulate ground-state properties and enrich quantum phase diagrams of layered transition metal dichalcogenides (TMDCs). In this work, we focus on fully Ta-intercalated bilayer $2H\text{−}{\mathrm{TaS}}_2$ with a stoichiometry of ${\mathrm{Ta}}_3{\mathrm{S}}_4$, which has recently been experimentally synthesized. Based on first-principles calculations, we computationally show the suppression of an intrinsic $3\times 3$ charge-density wave (CDW) in the ${\mathrm{TaS}}_2$ layer, and the emergence of a $2\times 1$ CDW in intercalated Ta layer. The formation of the CDW in ${\mathrm{Ta}}_3{\mathrm{S}}_4$ is triggered by strong electron-phonon coupling (EPC) between the $d$-like orbitals of intercalated Ta atoms via the imaginary phonon modes at $M$ point. A $2\times 1$ CDW structure is identified, featuring quasi-one-dimensional Ta chains, attributable to the competition between the CDW displacements associated with potential CDW vectors (${\mathit{q}}_{\text{CDW}}\mathrm{s}$). Superconductivity is found to coexist with the $2\times 1$ CDW in ${\mathrm{Ta}}_3{\mathrm{S}}_4$, with an estimated superconducting transition temperature ($T_{\mathrm{c}}$) of 3.0 K, slightly higher than that of bilayer ${\mathrm{TaS}}_2$. The ${\mathrm{Ta}}_3{\mathrm{S}}_4$ structures of non-CDW, $2\times 1$ CDW, and $2\times 2$ CDW can be switched by strain. Our work enriches the phase diagram of ${\mathrm{TaS}}_2$, offers a candidate material for studying the interplay between CDW and superconductivity, and highlights intercalation as an effective way to tune the physical properties of layered materials.

Recently, the possibility of a low-temperature noncubic phase in $\mathrm{Ba}\mathrm{Zr}{\mathrm{O}}_3$ has generated engaging discussions about its true ground state and the consequences on its physical properties. In this paper, we investigate the microscopic behavior of the $\mathrm{Ba}\mathrm{Zr}{\mathrm{O}}_3$ cubic phase by developing a shell model from ab initio calculations and by performing molecular dynamics simulations at finite temperature and under negative pressure. We study three different scenarios created by tuning the intensities of the antiferrodistortive (AFD) instabilities and, consequently, the sequence of phase transitions with temperature. From a detailed analysis of the cubic phase at atomic scale, we find that oxygen octahedra are barely distorted, present rotation angles that may oscillate with significant amplitudes, are AFD correlated with their closest neighbors on the plane perpendicular to the pseudocubic rotation axis exhibiting $\left(00a^{-}\right)$-type ordering, and form instantaneous, dynamic, and unstable domains over time. Our simulations support the existence of nanoregions with short-range ordering in cubic $\mathrm{Ba}\mathrm{Zr}{\mathrm{O}}_3$ associated with experimentally observed anomalies and unveil that they can exist regardless of whether or not structural phase transitions related with AFD distortions occur at lower temperatures.

We report simulations of exciton-exciton annihilation (EEA) in dye molecular aggregates with explicit laser pulse excitation in the framework of reduced density matrix theory. We use a supramolecular complex model Hamiltonian, in which each monomer of an aggregate is described by three electronic levels. An expectation value approximation is used to avoid the large amount of matrix elements for various exciton configuration distributions in molecular aggregates. Quantum fluctuations truncation is applied to derive the closed equations of motion for single- and two-site functions. Using these equations, we simulate the laser pulse driven EEA dynamics, and compare it with rate equations to highlight the importance of quantum coherence effects. We also discuss the effects of the aggregate configuration, the molecular transition dipoles and the duration of laser pulses on EEA. Furthermore, we analyze the relation between the coherence time of fusion and the annihilation process. Our study indicates that EEA process is more efficient for coherent excitation at the lowest energy level in J aggregates. Our simulation of the normalized population of the first excited state for various field amplitudes can qualitatively reproduce time-resolved experimental emission spectra. This confirms that the fundamental description of laser pulse induced EEA is correct within our model.

A nonmagnetic layer can inject spin-polarized currents into an adjacent ferromagnetic layer via spin-vorticity coupling (SVC), inducing spin wave resonance (SWR). In this work, we present the theoretical model of SWR generated by shear-horizontal surface acoustic wave (SH-SAW) via SVC, which contains distinct vorticities from well-studied Rayleigh SAW. Both Rayleigh- and SH-SAW delay lines have been designed and fabricated with a ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$/Cu bilayer integrated on ST-cut quartz. Given the same wavelength, the measured power absorption of SH-SAW is four orders of magnitudes higher than that of the Rayleigh SAW. In addition, a high-order frequency dependence of the SWR is observed in the SH-SAW, indicating that SVC can be strong enough to compare with magnetoelastic coupling.

The thermal phase transitions of a spin-1/2 Ising-Heisenberg model on the diamond-decorated square lattice in a magnetic field are investigated using a decoration-iteration transformation and classical Monte Carlo simulations. A generalized decoration-iteration transformation maps this model exactly onto an effective classical Ising model on the square lattice with temperature-dependent effective nearest-neighbor interactions and magnetic field strength. The effective field vanishes along a ground-state phase boundary of the original model, separating a ferrimagnetic and a quantum monomer-dimer phase. At finite temperatures this phase boundary gives rise to an exactly solvable surface of discontinuous (first-order) phase transitions, which terminates in a line of Ising critical points. The existence of discontinuous reentrant phase transitions within a narrow parameter regime is reported and explained in terms of the low-energy excitations from both phases. These exact results, obtained from the mapping to the zero-field effective Ising model, are corroborated by classical Monte Carlo simulations of the effective model.

From first-principles density functional theory calculations, we propose hexagonal layered ${\mathrm{MgB}}_3{\mathrm{C}}_3$ as a potential phonon-mediated superconductor at 59 K, which is far higher than the superconductivity of ${\mathrm{MgB}}_2$ ($\approx$ 39 K). The ${\mathrm{MgB}}_3{\mathrm{C}}_3$ is energetically and dynamically stable at ambient pressure in the P-62m hexagonal structure with c/a $\approx$ 0.79 and forms in stacks of honeycomb B-C layers with Mg as a space filler. Band structure calculations indicate that the bands at the Fermi level derive mainly from B and C orbitals in which two $\sigma$ and two $\pi$ bands both contribute to the total density of state. The $\pi$ bands are found to be strongly coupled with out-of-plane acoustic phonon mode, while the $\sigma$ bands coupled with the in-plane bond-stretching optical ${\mathrm{E}}^{\prime }$ phonon modes produces a sizable superconductivity in ${\mathrm{MgB}}_3{\mathrm{C}}_3$.

There have been several proposals for platforms supporting topological superconductivity in high-temperature superconductors, in order to make use of the larger superconducting gap and the expected robustness of Majorana zero modes towards perturbations. In particular, the iron-based materials offer relatively large $T_c$ and nodeless energy gaps. In addition, atomically flat surfaces enable the engineering of defect structures and the subsequent measurement of spectroscopic properties to reveal topological aspects. From a theory perspective, a materials-specific description is challenging due to the correlated nature of the materials and complications arising from the multiband nature of the electronic structure. Here we include both aspects in realistic interacting models, and find that the correlations themselves can lead to local magnetic order close to linear potential scattering defects at the surface of the superconductor. Using a self-consistent Bogoliubov-de Gennes framework in a real-space setup using a prototype electronic structure, we allow for arbitrary magnetic orders and show how a topological superconducting state emerges. The calculation of the topological invariant and the topological gap allows us to map out the phase diagram for the case of a linear chain of potential scatterers. While intrinsic spin-orbit coupling is not needed to enter the topological state in the presence of spin-spiral states, it enlarges the topological phase. We discuss the interplay of a triplet component of the superconducting order parameter and the spin spiral leading effectively to extended spin-orbit coupling terms, and connect our results to experimental efforts on the Fe(Se,Te) system.