We investigate the impact of spin-phonon coupling on the $S=1/2$ Heisenberg model on the kagome lattice. For the pure spin model, there is increasing evidence that the low-energy properties can be correctly described by a Dirac spin liquid, in which spinons with a conical dispersion are coupled to emergent gauge fields. Within this scenario, the ground-state wave function is well approximated by a Gutzwiller-projected fermionic state [Ran, Hermele, Lee, and Wen, Phys. Rev. Lett. 98, 117205 (2007)]. However, the existence of $U\left(1\right)$ gauge fields may naturally lead to instabilities when small perturbations are included. Phonons are ubiquitous in real materials and may play a relevant role in the determination of the actual physical properties of the kagome antiferromagnet. Therefore, we perform a step forward in this direction, including phonon degrees of freedom (at the quantum level) and applying a variational approach based upon Gutzwiller-projected fermionic Ansätze. Our results suggest that the Dirac spin liquid is stable for small spin-phonon couplings, while valence-bond solids are obtained at large couplings. Even though different distortions can be induced by the spin-phonon interaction, the general aspect is that the energy is lowered by maximizing the density of perfect hexagons in the dimerization pattern.

The magnetic phases of electron-doped infinite-layer ${\mathrm{Sr}}_{1-x}{\mathrm{La}}_x{\mathrm{CuO}}_2$ are elucidated by first-principles density functional calculations. The antiferromagnetic parent state, metallic transition, as well as lattice evolution with doping and pressure are found to be consistent with experiments. The specific heat coefficient $\gamma$, magnetic exchange coupling $J$, as well as the density of states at Fermi level $N\left(0\right)$ of low-energy states with multiple magnetic configurations are investigated. We highlight a subset of such states in which we note an increase in $N\left(0\right)$ to suggest the interesting effects of magnetic fluctuations and La substitution on the electronic structure of this material.

Identifying and controlling decoherence in single-electron sources (SESs) is important for their applications in quantum information processing. The recent experiments with ultrashort electron pulses [J. D. Fletcher et al., Nat. Commun. 10, 5298 (2019)] demonstrate strong decoherence that cannot be caused by traditional mechanisms such as electron-electron or electron-phonon interactions. Here we propose timing noise as a universal model, consistent with existing experimental data, to explain strong decoherence of ultrafast SES pulses, without resorting to any specific microscopic mechanism for such decoherence. We also propose a protocol to filter out timing noise which works even in the presence of other decoherence effects, such as those present in, e.g., low-energy SESs.

The immunity of Ising superconductors to external magnetic fields originates from a spin locking of the paired electrons to an intrinsic Zeeman-like field. The spin-momentum locking in non-centrosymmetric crystalline materials leads to type-I Ising pairing in which the direction of the intrinsic field can be deduced from the spin expectation values. Conversely, in centrosymmetric crystals the electron spins locked to the orbitals can form Ising type-II pairs consisting of spin-orbit split doublets. Due to time-reversal symmetry, the doublets are spin degenerate, making it difficult to read the spin polarization of bands and the direction of spin-orbit fields. Here we present an efficient approach to determine the direction of the intrinsic field using the spin-mixing parameter $b^2$. Using first-principles calculations based on the density functional theory, we study monolayer transition metal dichalcogenide superconductors ${\mathrm{PdTe}}_2, {\mathrm{NbTe}}_2$, and ${\mathrm{TiSe}}_2$ with the 1T structure. We calculate $b^2$ for individual Fermi pockets and provide a general picture of possible Ising type-II pairing within the full Brillouin zone. To complement our first-principles results, we use group theory to provide a detailed picture of spin-orbit coupling and spin mixing in the relevant bands forming Fermi pockets. We demonstrate that contrary to the anticipated effects of spin-orbit locking, not every spin-orbit split spin doublet actively participates in Ising pairing. Finally, by connecting the spin-mixing parameter $b^2$ with the intrinsic out-of-plane Zeeman field, we estimate the upper in-plane critical magnetic field.

Charge-carrier transport is investigated theoretically in a high-band-gap dielectric material excited by intense ultrashort laser pulses. Simulations of fused silica reveal that the photo-Dember effect causes a ${10}^7\mathrm{V}{\mathrm{m}}^{-1}$ electric field to build up inside the material. It is shown that this electric field can induce a significant transient optical anisotropy in the dielectric, which may help reconcile earlier reports of an ultrafast onset of birefringence in highly excited fused silica. The time-dependent electric field will also lead to emission of a broadband THz pulse propagating out from and along the dielectric surface.

The excellent magnetic entropy change ($\mathrm{\Delta }{S}_T$) in the temperature range of 20 $\sim$ 77 K due to the first-order phase transition makes ${\mathrm{Pr}}_2\mathrm{In}$ an intriguing candidate for magnetocaloric hydrogen liquefaction. As an equally important magnetocaloric parameter, the adiabatic temperature change ($\mathrm{\Delta }{T}_{ad}$) of ${\mathrm{Pr}}_2\mathrm{In}$ associated with the first-order phase transition has not yet been reported. In this work, the $\mathrm{\Delta }{T}_{ad}$ of ${\mathrm{Pr}}_2\mathrm{In}$ is obtained from heat capacity measurements: 2 K in fields of 2 T and 4.3 K in fields of 5 T. While demonstrating a $\mathrm{\Delta }{T}_{ad}$ that is not as impressive as its remarkable $\mathrm{\Delta }{S}_T, {\mathrm{Pr}}_2\mathrm{In}$ exhibits a low Debye temperature ($T_D$) of around 110 K. Based on these two observations, an approach that combines the mean-field and Debye models is developed to study the correlation between $\mathrm{\Delta }{T}_{ad}$, one of the most important magnetocaloric parameters, and $T_D$, one important property of a material. The role of $T_D$ in achieving large $\mathrm{\Delta }{T}_{ad}$ is revealed: materials with higher $T_D$ tend to exhibit larger $\mathrm{\Delta }{T}_{ad}$, particularly in the cryogenic temperature range. This discovery explains the absence of an outstanding $\mathrm{\Delta }{T}_{ad}$ in ${\mathrm{Pr}}_2\mathrm{In}$ and can serve as a tool for designing or searching for materials with both a large $\mathrm{\Delta }{S}_T$ and a $\mathrm{\Delta }{T}_{ad}$.

Relaxation rates in nearly integrable systems usually increase quadratically with the strength of the perturbation that breaks integrability. We show that the relaxation rates can be significantly smaller in systems that are integrable along two intersecting lines in the parameter space. In the vicinity of the intersection point, the relaxation rates of certain observables increase with the fourth power of the distance from this point, whereas for other observables one observes standard quadratic dependence on the perturbation. As a result, one obtains exceedingly long-living prethermalization but with a reduced number of the nearly conserved operators. We show also that such a scenario can be realized in spin ladders.

The interplay between a topological degeneracy and the residue degeneracy (also known as the residue entropy) of quantum criticality remains an important but not thoroughly understood topic. We find that this topological degeneracy, provided by a Majorana zero mode pair, relaxes the otherwise strictly requested symmetry requirement, to observe the boundary Berezinskii-Kosterlitz-Thouless (BKT) quantum phase transition (QPT) of a dissipative resonant level. Our work indicates that the topological degeneracy can be potentially viewed as an auxiliary symmetry that realizes a robust boundary QPT. The relaxation of the symmetry requirement extends the transition from a point to a finite area, thus greatly reducing the difficulty to experimentally observe the QPT. This topology-involved exotic BKT phase diagram, on the other hand, provides another piece of evidence that can further confirm the existence of a Majorana zero mode.

Skyrmion-hosting multilayer stacks are promising avenues for applications, although little is known about the depth dependence of the magnetism. We address this by reporting the results of circular dichroic resonant elastic x-ray scattering (CD-REXS), micromagnetic simulations, and low-energy muon-spin rotation (LE-${\mu }^{+}\mathrm{SR}$) measurements on a stack comprising $[\mathrm{Ta}$/CoFeB/${\mathrm{MgO}]}_{16}$/Ta on a Si substrate. Energy-dependent CD-REXS shows a continuous, monotonic evolution of the domain-wall helicity angle with incident energy, consistent with a three-dimensional hybrid domain-wall-like structure that changes from Néel-like near the surface to Bloch-like deeper within the sample. LE-${\mu }^{+}\mathrm{SR}$ reveals that the magnetic field distribution in the trilayers near the surface of the stack is distinct from that in trilayers deeper within the sample. Our micromagnetic simulations support a quantitative analysis of the ${\mu }^{+}\mathrm{SR}$ results. By increasing the applied magnetic field, we find a reduction in the volume occupied by domain walls at all depths, consistent with a crossover into a region dominated by skyrmions above approximately 180 mT.

The essential building blocks of cuprate superconductors are two-dimensional ${\mathrm{CuO}}_2$ sheets interspersed with charge reservoir layers. In bilayer cuprates, two closely spaced ${\mathrm{CuO}}_2$ sheets are separated by a larger distance from the subsequent pair in the next unit cell. In contrast to single-layer cuprates, prior theoretical work on bilayer systems has predicted two distinct acoustic plasmon bands for a given out-of-plane momentum transfer. Here we report random phase approximation (RPA) calculations for bilayer systems which corroborate the existence of two distinct plasmon bands. We find that the intensity of the lower-energy band is negligibly small in most parts of the Brillouin zone, whereas the higher-energy band carries significant spectral weight. We also present resonant inelastic x-ray scattering (RIXS) experiments at the O $K$-edge on the bilayer cuprate ${\mathrm{Y}}_{0.85}{\mathrm{Ca}}_{0.15}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ (Ca-YBCO), which show only one dispersive plasmon branch, in agreement with the RPA calculations. In addition, the RPA results indicate that the dispersion of the higher-energy plasmon band in Ca-YBCO is not strictly acoustic but exhibits a substantial energy gap of approximately 250 meV at the two-dimensional Brillouin zone center.

Gapless topological boundary states characterize nontrivial topological phases that arise from the bulk-boundary correspondence in symmetry-protected topological materials. However, symmetry-breaking perturbations gap these edge bands, resulting in the loss of these crucial boundary states. In this connection, we systematically examine the robustness of the bulk-boundary correspondence in the case of a quantum spin Hall insulator via the feature-spectrum topology approach. Our findings provide a comprehensive understanding of feature-energy duality and show that the aggregate number of gapless edge states in the energy-momentum (E-k) map and the nontrivial edge states in the ${\widehat{S}}_z$ feature spectrum equals the spin-Chern number of a multilayer quantum spin Hall insulator. We identify a van der Waals material bismuth bromide $\left({\mathrm{Bi}}_4{\mathrm{Br}}_4\right)$ as a promising candidate through our first-principles calculations. Our paper not only unravels the intricacies of the bulk-boundary correspondence, but it also provides a pathway for exploring quantum spin Hall insulators with high spin-Chern numbers.

We study a periodically driven one-dimensional Kitaev model in the presence of disorder. In the clean limit our model exhibits four topological phases corresponding to the existence or nonexistence of edge modes at zero and $\pi$ quasienergy. When potential disorder is added, the system parameters get renormalized and the system may exhibit a topological phase transition. When starting from the Majorana $\pi$ mode (MPM) phase, which hosts only edge Majoranas with quasienergy $\pi$, disorder induces a transition into a neighboring phase with both $\pi$ and zero modes on the edges. We characterize the disordered system using (i) exact diagonalization, (ii) Arnoldi mapping onto an effective tight-binding chain, and (iii) topological entanglement entropy.

We study the hydrodynamic flow of electrons through a smooth potential energy landscape in two dimensions, for which the electrical current is concentrated along thin channels that follow percolating equipotential contours. The width of these channels, and hence the electrical resistance, is determined by a competition between viscous and thermoelectric forces. For the case of periodic (moiré) potentials, we find that hydrodynamic flow provides a route to linear-in-$T$ resistivity. We calculate the associated prefactors for potentials with $C_3$ and $C_4$ symmetry. On the other hand, for a random potential the resistivity has qualitatively different behavior because equipotential paths become increasingly tortuous as their width is reduced. This effect leads to a resistivity that grows with temperature as $T^{10/3}$.

The topological quantum states of matter can be characterized by the geometric form and number of symmetry-enforced band degeneracy, such as nodal points, lines, and surfaces from twofold up to eightfold degeneracy. Here, we report the observation of double Dirac (fourfold) nodal lines stabilized by multiple nonsymmorphic symmetries in puckered honeycomb systems, black phosphorus, and metal monochalcogenides. By angle-resolved photoemission spectroscopy, we found a fourfold (Dirac) nodal line running along the armchair zone boundary of black phosphorus, reproduced by first-principles band calculations and proven by our symmetry analysis to be protected by space-time inversion and glide-mirror symmetries. The presence of the multiple glide-mirror symmetries in its binary counterpart, GeS, diversifies into five Dirac nodal lines, two of which come closer at the zigzag zone boundary to form a nearly eightfold nodal line. Our results demonstrate the correspondence between nonsymmorphicity and band degeneracy with puckered honeycomb systems.

van der Waals (vdW) layered semiconductors exhibiting large nonlinear-optical (NLO) effects have substantial potential for developing nanoscale quantum optical devices. ${\mathrm{NbOCl}}_2$, an ultrathin quantum light source, has recently displayed a large second harmonic effect. However, its small bandgap (∼1.8 eV) impedes transparent light conversion across wide spectral regions, especially when utilizing practical 1-µm lasers. In this paper, we found that ${M\mathrm{O}}_2{X}_2$ $(M=\mathrm{W}, \mathrm{Mo}; X=\mathrm{Cl}, \mathrm{Br})$ effectively widens the bandgap by using ${\mathrm{W}}^{6+}$ or $\mathrm{M}{\mathrm{o}}^{6+}$ instead of $\mathrm{N}{\mathrm{b}}^{4+}$ to eliminate in-gap nonbonding orbitals. Meanwhile, the octahedral $\left[{M\mathrm{O}}_4{X}_2\right]$ motifs display polar Jahn-Teller distortions as large as $\left[{\mathrm{NbO}}_2{X}_4\right]$ to demonstrate comparable NLO polarization effects. Based on first-principles calculations, ${\mathrm{WO}}_2{\mathrm{Cl}}_2$ and ${\mathrm{MoO}}_2{\mathrm{Br}}_2$ are predicted to possess wider bandgaps (2.9 and 2.2 eV), giant birefringence (0.5–0.7), and significant second harmonic and ferroelectric photovoltaic effects competitive with $\mathrm{NbO}{X}_2$. These findings provide strong incentives to explore new vdW materials with superior NLO properties.

An accurate description of the low-energy electronic bands in twisted bilayer graphene (tBLG) is of great interest due to their relation to correlated electron phases such as superconductivity and Mott-insulator behavior at half-filling. The paradigmatic model of Bistritzer and MacDonald [Proc. Natl. Acad. Sci. USA 108, 12233 (2011)], based on the moiré pattern formed by tBLG, predicts the existence of “magic angles” at which the Fermi velocity of the low-energy bands goes to zero, and the bands themselves become dispersionless. Here, we reexamine the low-energy bands of tBLG from the ab initio electronic structure perspective, motivated by features related to the atomic relaxation in the moiré pattern, namely, circular regions of AA stacking, triangular regions of AB/BA stacking and domain walls separating the latter. We find that the bands are never perfectly flat and the Fermi velocity never vanishes, but rather a “magic range” exists where the lower band becomes extremely flat and the Fermi velocity attains a nonzero minimum value. We propose a simple $(2+2)$-band model, comprised of two different pairs of orbitals, both on a honeycomb lattice: the first pair represents the low-energy bands with high localization at the AA sites, while the second pair represents highly dispersive bands associated with domain-wall states. This model gives an accurate description of the low-energy bands with few (13) parameters that are physically motivated and vary smoothly in the magic range. In addition, we derive an effective two-band Hamiltonian which also gives an accurate description of the low-energy bands. This minimal two-band model affords a connection to a Hubbard-like description of the occupancy of subbands and can be used a basis for exploring correlated states.

The Hatsugai-Kohmoto interaction model has gained a lot of attention in recent years, due to the fact it is exactly solvable in momentum space in any dimension while capturing some key features of the Mott phase. Here a one-dimensional lattice model with this interaction is approached from the real-space perspective, to explore how breaking the translation invariance of a lattice affects the intuition built by studying the exact solution in $k$ space. The ground state properties of chains with periodic and open boundary conditions are calculated and compared with both the exact solution in momentum space as well as with analogous solutions of the Hubbard model. The results show that introducing hard edges enhances the ferromagnetic correlations and the system undergoes a magnetic transition before reaching the strong coupling limit. Understanding the impact of hard edges is a crucial step toward answering the looming question of the existence of edge states and other topological phenomena in systems with this type of interaction.

We introduce a machine learning approach for solving ill-posed inverse problems, specifically addressing the Fredholm integral equation of the first kind. Harnessing the powerful capabilities of normalizing flows to approximate data distributions, combined with a robust probabilistic framework, our approach stands out by delivering robust solutions capable of handling high-level noises and out-of-distribution data and providing uncertainty estimation. A distinct feature lies in the unsupervised learning framework inherent in deep generative models, providing our approach with unparalleled flexibility across diverse experimental setups. This flexibility is exemplified through the successful application of our method to measured optical spectra.