We examine the pinning and dynamics of Abrikosov vortices interacting with pinning centers placed in a moiré pattern for varied moiré lattice angles. We find a series of locking angles at which the critical current shows a pronounced dip corresponding to lattices in which the vortices can flow along quasi-one-dimensional channels. At these locking angles, the vortices move with a finite Hall angle. Additionally, for some lattice angles there are peaks in the critical current produced when the substrate has a quasiperiodic character that strongly reduces the vortex channeling. Our results should be general to a broad class of particlelike assemblies moving on moiré patterns.

The electronic band structure of Bi is calculated using state of the art electronic structure methods, including density functional theory and $G_0{W}_0$ quasiparticle approximations. The delicate ordering of states at the $L$ point of the Brillouin zone, which determines the topological character of the electronic bands, is investigated in detail. The effect on the bands of strain, changing the structural parameters of the rhombohedral crystal structure, is shown to be important in determining this ordering and the resulting topological character.

We show by advanced electronic structure calculations that ${\mathrm{NbO}}_2$ essentially is a Peierls-type material. After simulating the rutile as well as the body-centered tetragonal phase with the Bethe-Salpeter equation, we are able to reproduce the experimental values for the electronic properties without adding correlations. Our calculation includes only excitonic corrections and no further interactions. The principal indirect gap is between $N$ and $\mathrm{\Gamma }$ and is found to be 0.98 eV, the direct gap at the $\mathrm{\Gamma }$ point amounts to 1.35 eV. We found the rutile structure to be anisotropic, with nesting vectors in the Fermi surfaces in the $\mathrm{\Gamma }MAZ$ and $\mathrm{\Gamma }XRZ$ planes.

We investigate a spatial subsystem entropy extracted from the one-particle density matrix (OPDM) of one-dimensional disordered interacting fermions that host a many-body localized (MBL) phase. Deep in the putative MBL regime, this OPDM entropy exhibits the salient scaling features of localization, even though it provides only an upper bound to the von Neumann entropy. First, we numerically show that the OPDM entropy of the eigenstates obeys an area law. Second, like the von Neumann entropy, the OPDM entropy grows logarithmically with time after a quantum quench, albeit with a different prefactor. Both these features survive at moderately large interactions and well toward the transition into the ergodic phase. We discuss prospects for calculating the OPDM entropy using approximate numerical methods and for its measurement in quantum gas experiments.

Close to the Fermi energy, nodal loop semimetals have a torus-shaped, strongly anisotropic Fermi surface, which affects their transport properties. Here we investigate the non-equilibrium dynamics of nodal loop semimetals by going beyond linear response and determine the time evolution of the current after switching on a homogeneous electric field. The current grows monotonically with time for electric fields perpendicular to the nodal loop plane however it exhibits nonmonotonical behavior for field orientations aligned within the plane. After an initial nonuniversal growth $\sim Et$, the current first reaches a plateau $\sim E$. Then, for perpendicular directions, it increases while for in-plane directions it decreases with time to another plateau, still $\sim E$. These features arise from interband processes. For long times or strong electric fields, the current grows as $\sim E^{3/2}t$ or $\sim E^3{t}^2$ for perpendicular or parallel electric fields, respectively. This nonlinear response represents an intraband effect where the large number of excited quasiparticles respond to the electric field. Our analytical results are benchmarked by the numerical evaluation of the current from continuum and tight-binding models of nodal loop semimetals.

Superconductors with kagome lattices have been identified for over 40 years, with a superconducting transition temperature $T_c$ up to 7 K. Recently, certain kagome superconductors have been found to exhibit an exotic charge order, which intertwines with superconductivity and persists to a temperature being one order of magnitude higher than $T_c$. In this work, we use scanning tunneling microscopy to study the charge order in kagome superconductor $\mathrm{Rb}{\mathrm{V}}_3{\mathrm{Sb}}_5$. We observe both a $2\times 2$ chiral charge order and nematic surface superlattices (predominantly $1\times 4$). We find that the $2\times 2$ charge order exhibits intrinsic chirality with magnetic field tunability. Defects can scatter electrons to introduce standing waves, which couple with the charge order to cause extrinsic effects. While the chiral charge order resembles that discovered in $\mathrm{K}{\mathrm{V}}_3{\mathrm{Sb}}_5$, it further interacts with the nematic surface superlattices that are absent in $\mathrm{K}{\mathrm{V}}_3{\mathrm{Sb}}_5$ but exist in $\mathrm{Cs}{\mathrm{V}}_3{\mathrm{Sb}}_5$.

The interaction of laser pulses with condensed matter forms the basis of light-wave-driven electronics potentially enabling tera- and petahertz switching rate applications. Carrier control using near- and midinfrared pulses is appealing for integration into existing platforms. Toward this end, a fundamental understanding of the complexity of phenomena concerning sub-band-gap driven semiconductors such as high harmonic generation, carrier excitation due to multiphoton absorption, and interband tunneling as well as carrier-carrier interactions due to strong acceleration in infrared transients is important. Here, stimulated emission from polycrystalline ZnO thin films for pump wavelengths between 1.2 μm (1 eV) and 10 μm (0.12 eV) is observed. Contrary to the expected higher intensity threshold for longer wavelengths, the lowest threshold pump intensity for stimulated emission is obtained for the longest pump wavelength corroborating the importance of collisional excitation upon intraband electron acceleration.

We study the valence band structure of ${\mathrm{ReSe}}_2$ crystals with varying thickness down to a single layer using nanoscale angle-resolved photoemission spectroscopy and density functional theory. The width of the top valence band in the direction perpendicular to the rhenium chains decreases with decreasing number of layers, from $\sim 200$ meV for the bulk to $\sim 80$ meV for monolayer. This demonstrates increase of in-plane anisotropy induced by changes in the interlayer coupling and suggests progressively more one-dimensional character of electronic states in few-layer rhenium dichalcogenides.

We apply the finite-temperature variational cluster approach to a strongly correlated and spin-orbit coupled model for four electrons (i.e., two holes) in the $t_{2g}$ subshell. We focus on parameters suitable for antiferromagnetic Mott insulators, in particular, ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$, and identify a crossover from the low-temperature regime, where spin-orbit coupling is essential, to the high-temperature regime where it leaves few signatures. The crossover is seen in one-particle spectra, where $xz$ and $yz$ spectra are almost one dimensional (as expected for weak spin-orbit coupling) at high temperature. At lower temperature, where spin-orbit coupling mixes all three orbitals, they become more two dimensional. However, stronger effects are seen in two-particle observables like the weight in states with definite on-site angular momentum. We thus identify the enigmatic intermediate-temperature “orbital-order phase transition,” which has been reported in various x-ray diffraction and absorption experiments at $T\approx 260$ K, as the signature of the onset of spin-orbital correlations.

Single layer Pb on top of (111) surfaces of group IV semiconductors hosts charge density wave and superconductivity depending on the coverage and on the substrate. These systems are normally considered to be experimental realizations of single band Hubbard models and their properties are mostly investigated using lattice models with frozen structural degrees of freedom, although the reliability of this approximation is unclear. Here, we consider the case of Pb/Ge(111) at $1/3$ coverage, for which surface x-ray diffraction and angle-resolved photoemission spectroscopy data are available. By performing first-principles calculations, we demonstrate that the nonlocal exchange between Pb and the substrate drives the system into a $3\times 3$ charge density wave. The electronic structure of this charge ordered phase is mainly determined by two effects: The magnitude of the Pb distortion and the large spin-orbit coupling. Finally, we show that the effect applies also to the $3\times 3$ phase of Pb/Si(111) where the Pb-substrate exchange interaction increases the bandwidth by more than a factor $1.5$ with respect to density functional theory $+\mathrm{U}$, in better agreement with scanning tunneling spectroscopy data. The delicate interplay between substrate, structural, and electronic degrees of freedom invalidates the widespread interpretation available in literature considering these compounds as physical realizations of single band Hubbard models.

The exciton recombination and spin dynamics are studied in monolayer-thick (In,Al)As/AlAs quantum wells characterized by an indirect band gap and a type-I band alignment. The exciton recombination time and the photoluminescence intensity are strongly dependent on strength and orientation of an applied magnetic field. In contrast to no effect of an in-plane field, at a temperature of 1.8 K a magnetic field applied parallel to the growth axis drastically slows down the recombination and reduces the intensity of photoluminescence. The magnetic-field-induced circular polarization of photoluminescence is studied as a function of the magnetic field strength and direction, as well as sample temperature. The observed nonmonotonic behavior of these functions is provided by the interplay of bright and dark exciton states contributing to the emission. Taking into account the magnetic-field-induced redistribution of the indirect excitons between their bright and dark states, we evaluate the heavy-hole longitudinal $g$ factor of 3.6, the radiative recombination time for the bright excitons of 0.13 ms, and the nonradiative recombination time of the bright and dark excitons of 0.43 ms, as well as the spin relaxation times of electron of $25\mu \mathrm{s}$ and heavy hole of $16\mu \mathrm{s}$, bound in the exciton.

We propose to boost the performance of the density matrix renormalization group (DMRG) in two dimensions by using Gutzwiller projected states as the initialization Ansatz. When the Gutzwiller projected state is properly chosen, the notorious “local minimum” issue in DMRG can be circumvented and the precision of DMRG can be improved by orders of magnitude without extra computational cost. Moreover, this method allows one to quantify the closeness of the initial Gutzwiller projected state and the final converged state after DMRG sweeps, thereby shedding light on whether the Gutzwiller Ansatz captures the essential entanglement features of the actual ground state for a given Hamiltonian. Kitaev honeycomb model has been exploited to demonstrate and benchmark this method.

The effects of downfolding a Brillouin zone can open gaps and quench the kinetic energy by flattening bands. Quasiperiodic systems are extreme examples of this process, which leads to new phases and critical eigenstates. We analytically and numerically investigate these effects in a two-dimensional topological insulator with a quasiperiodic potential and discover a complex phase diagram. We study the nature of the resulting eigenstate quantum phase transitions; a quasiperiodic potential can make a trivial insulator topological and induce topological insulator-to-metal phase transitions through a unique universality class distinct from random systems. This wealth of critical behavior occurs concomitantly with the quenching of the kinetic energy, resulting in flat topological bands that could serve as a platform to realize the fractional quantum Hall effect without a magnetic field.

Based on the dimension of degeneracy, topological electronic systems can roughly be divided into three parts: nodal point, line, and surface materials corresponding to zero-, one-, and two-dimensional degeneracy, respectively. In parallel to electronic systems, the concept of topology was extended to phonons, promoting the birth of topological phonons. Till date, few nodal point, line, and surface phonon candidates have been predicted in solid-state materials. In this study, based on symmetry analysis and first-principles calculation, we prove that zero-, one-, and two-dimensional degeneracy co-exist in the phonon dispersion of one single realistic solid-state material ${\mathrm{SnO}}_2$ with $P{4}_2/mnm$ structure. In contrast to the previously reported electronic systems, the topological phonons observed in ${\mathrm{SnO}}_2$ are not restricted by the Pauli exclusion principle, and they experience negligible spin-orbit coupling effect. Hence, ${\mathrm{SnO}}_2$ with multiple dimensions of degeneracy phonons is a good platform for studying the entanglement among nodal point, line, and surface phonons. Moreover, obvious phonon surface states are visible, which is beneficial for experimental detection.

In this paper, we report that $\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$, portlandite, is predicted to be stable up to 100 GPa via an unbiased structural search method combined with first-principles calculations. Two pressure-induced structures with $P2{}_1/c$ and $Pnma$ symmetry are proposed at high pressure. The monoclinic $P2{}_1/c$ structure is the most stable phase after 23 GPa and will undergo a phase transition into a $Pnma$ phase at 78 GPa and 0 K. This phase transition is computed to occur at lower pressure as the temperature increases. The high-pressure phases of $\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$ that we proposed are stable against decomposition into CaO and ${\mathrm{H}}_2\mathrm{O}$ at the conditions corresponding to lower mantle geotherms. Our results shed light on the possible presence of $\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$ in the Earth's mantle as well as its water transportation in the Earth's interior.

We study pressure-induced structural evolution of vanadium diselenide (${\mathrm{VSe}}_2$), a $1T$ polymorphic member of the transition metal dichalcogenide (TMD) family, using synchrotron-based powder x-ray diffraction (XRD) and first-principles density functional theory (DFT). Our XRD results reveal anomalies at $P\sim 4\mathrm{GPa}$ in the c/a ratio, V-Se bond length, and Se-V-Se bond angle, signaling an isostructural transition. This transition is followed by a first-order structural transition from the $1T$ (space group $P\overline{3}m1$) phase to a $3R$ (space group $R\overline{3}m)$ phase at $P\sim 11\mathrm{GPa}$ due to sliding of adjacent Se-V-Se layers. Both the transitions at $\sim 4$ and 11 GPa are cognate with associated changes in the Debye-Waller factors not reported so far. We present various scenarios to understand the experimental results within DFT and find that the $1T$ to $3R$ transition is captured using spin-polarized calculations with Hubbard correction ($U_{\mathrm{eff}}=U-J=8\mathrm{eV}$), giving a transition pressure of $\sim 9$ GPa, close to the experimental value.

Inelastic neutron scattering (INS) is a key method for studying magnetic excitations in spin systems, including molecular spin clusters. The method has significantly advanced in recent years and now permits us to probe the scattering intensity as a function of the energy transfer and the momentum-transfer vector Q. It was recently shown that high molecular symmetry facilitates the analysis of spectra. Point-group symmetry imposes selection rules in isotropic as well as anisotropic spin models. Furthermore, the Q dependence of the INS intensity may be completely determined by the point-group symmetry of the states involved in a transition, thereby affording a clear separation of dynamics (energies, transition strengths) and geometrical features (Q dependencies). This paper addresses this issue for anisotropic spin models. We identify several cases where the Q dependence is completely fixed by the point-group symmetry. For six- and eight-membered planar spin rings and two polyhedra (the cube and the icosahedron), we tabulate and plot the corresponding powder-averaged universal intensity functions. The outlined formalism straightforwardly applies to other highly symmetric systems and should be useful for future analyses of INS spectra by focusing on those features that contain information on either spin dynamics or the point-group symmetry of states.

We have studied the magnetic torque in uranium monosulfide (US) single crystals to explore the magnetic anisotropy in this material. Uranium monosulfide crystallizes in a cubic, NaCl-type of crystal structure and exhibits the largest magnetocrystalline anisotropy observed in cubic systems. By performing detailed torque measurements we observe a strongly anisotropic behavior in the paramagnetic phase due to crystal defects and quadrupolar pair interactions. Our studies also confirm the presence of a large anisotropy in the ferromagnetic state of the US system with the $\langle 100\rangle , \langle 111\rangle$, and $\langle 110\rangle$ directions being the hard, easy, and intermediate axes, respectively. Furthermore, the anisotropy in the paramagnetic phase shows similar characteristics to the anisotropy observed in the ferromagnetic phase, as characterized by second and fourth rank susceptibility terms. The similarity of the anisotropic behaviors in paramagnetic and ferromagnetic phases is the consequence of strong magnetoelastic properties in this system, which possibly lead to the rhombohedral structural distortion, not only in the ferromagnetic phase but also in the paramagnetic phase (induced by applied magnetic field).

We study correlated switchings of a superconducting nanobridge probed with a train of current pulses. For pulses with a low repetition rate each pulse transits the superconducting bridge to the normal state with probability $P$ independent of the outcomes in the preceding pulses. We show that with the reduction of the time interval between pulses long-range correlation between pulses occurs: stochastic switching in a single pulse raises the temperature of the bridge and affects the outcome of the probing for the next pulses. As a result, an artificial intricate stochastic process with an adjustable strength of correlation is produced. We identify the regime where apparent switching probability exhibits the thermal hysteresis with discontinuity at a critical current amplitude of the probing pulse. This engineered stochastic process can be viewed as an artificial phase transition and provides an interesting framework for studying correlated systems. The process resembles the familiar transition from the superconducting to normal state in the current-bias nanowire, proceeding through a phase slip avalanche. Due to its extreme sensitivity to the control parameter, i.e., electric current, temperature, or magnetic field, it offers the opportunity for ultrasensitive detection.

We reproduce the electronic properties of FeSe in the high-temperature phase within an ab initio framework that includes screened Fock exchange and local dynamical correlations. We robustly capture the experimental band structure, as long as the system is in the Hund metal phase. In particular, we account for the shrinking of the Fermi pockets and the sinking below the Fermi level of the hole pocket with $xy$ orbital character. This entails the elusive correct estimate of the Sommerfeld coefficient, and supports the interpretation of noncompensated Fermi pockets seen in ARPES in terms of surface electron doping. More stringently, our modeling matches well the experimental interband optical spectrum, and captures qualitatively the temperature dependence of the thermoelectric power, extremely sensitive to the details of the bands around the Fermi level.