The spin-liquid candidate $\kappa -{\left(\mathrm{ET}\right)}_2{\mathrm{Cu}}_2{\left(\mathrm{CN}\right)}_3$ [ET: bis(ethylenedithio)tetrathiafulvalene] does not exhibit magnetic ordering down to a very low temperature, but shows a mysterious anomaly at 6 K. The origin of the so-called 6-K anomaly is still under debate. We carried out nuclear quadrupole resonance (NQR) measurements on the copper sites of the insulating layers, which are sensitive to the charge dynamics unlike conventional spin-$1/2$ nuclear magnetic resonance (NMR). The main finding of this Rapid Communication is that the observation of a sharp peak behavior in the nuclear spin-lattice relaxation rate $T_1^{-1}$ of ${}^{63}\mathrm{Cu}$ NQR at 6 K while $T_1^{-1}$ of both ${}^{13}\mathrm{C}$ and ${}^1\mathrm{H}$ NMR show no clear anomaly. This behavior can be understood as a second-order phase transition related to charge disproportionation in the ET layers.

Localized excitons play a vital role in the optical response of monolayers of transition metal dichalcogenides and can be exploited as single photon sources for quantum information technology. While the optical properties of such localized excitons are vastly studied, the ultrafast capture process of delocalized excitons into localized potentials is largely unexplored. We perform quantum kinetic calculations of exciton capture via acoustic and optical phonons showing that efficient capture takes place on an ultrafast time scale. The polaron formation in the low-temperature limit leads to higher-energy excitons which can then be efficiently trapped. We demonstrate that the interplay of acoustic and optical phonons leads to an efficient broadening of energy-selection rules. Our studies provide a deep understanding of the carrier trapping from two-dimensional materials into zero-dimensional potentials.

In frustrated quantum magnetism, chiral spin liquids are a particularly intriguing subset of quantum spin liquids in which the fractionalized parton degrees of freedom form a Chern insulator. Here we study an exactly solvable spin-3/2 model, which harbors not only chiral spin liquids but also spin liquids with higher-order parton band topology—a trivial band insulator, a Chern insulator with gapless chiral edge modes, and a second-order topological insulator with gapless corner modes. With a focus on the thermodynamic precursors and thermal phase transitions associated with these distinct states, we employ numerically exact quantum Monte Carlo simulations to reveal a number of unconventional phenomena. This includes heightened thermal stability of the ground state phases, the emergence of a partial flux ordering of the associated ${\mathbb{Z}}_2$ lattice gauge field, and the formation of a thermal Majorana metal regime extending over a broad temperature range.

Extended single-domain growth of h-BN is observed on Pt(110), if the precursor molecules are deposited at sufficiently high temperatures. We examined the electronic structure of the h-BN/Pt(110) system by angle-resolved photoemission (ARPES), work-function measurements, and density-functional theory (DFT) calculations. van der Waals forces dominate the h-BN/Pt(110) interaction by far, although DFT analysis of the local density of states reveals the existence of a local covalent interaction of some N atoms with Pt surface atoms. The local bonding contributions cause the appearance of a $(1\times n)$ missing-row reconstruction $(n=5 \mathrm{or} 6)$ of the Pt (110) surface, if the system reverts to room temperature after h-BN adlayer formation at 1120 K. This unique phenomenon of the template adapting to the adlayer structure mitigates differences in the thermal-expansion coefficient upon cooling. The h-BN π bands hybridize with Pt $d$ bands. Nevertheless, the dispersion of π and σ bands as measured by ARPES is overall well represented by the free-standing monolayer band structure except for the appearance of replica bands induced by the Moiré structure. A comparison between the experimentally measured π bands and the band structure obtained from DFT slab calculations suggest the existence of significant correlation effects in photoemission from h-BN/Pt(110). The locally varying distribution of N-Pt hybrid states straddling the Fermi level indicates a corresponding spatial variation of the chemical reactivity.

Artificial lattices have served as a platform to study the physics of unconventional superconductivity. We study semiconductor artificial graphene—a honeycomb superlattice imposed on a semiconductor heterostructure—which hosts the Dirac physics of graphene but with a tunable periodic potential strength and lattice spacing, allowing control of the strength of the electron-electron interactions. We demonstrate a new mechanism for superconductivity due to repulsive interactions which requires a strong lattice potential and a minimum doping away from the Dirac points. The mechanism relies on the Berry phase of the emergent Dirac fermions, which causes oppositely moving electron pairs near the Dirac points to interfere destructively, reducing the Coulomb repulsion and thereby giving rise to an effective attraction. The attractive component of the interaction is enhanced by a novel antiscreening effect which, in turn, increases with doping; as a result, there is a minimum doping beyond which superconducting order generically ensues. The dominant superconducting state exhibits a spatially modulated gap with chiral $p$-wave symmetry. Microscopic calculations suggest that the possible critical temperatures are large relative to the low carrier densities, for a range of experimentally realistic parameters.

We study the evolution and persistence of quantum and classical correlations between spatially separated sites in the disordered XXZ spin chain in its ergodic and many-body-localized phases using exact diagonalization and matrix product state simulations. We show that in the many-body-localized phase, quantum entanglement survives in long-time unitary dynamics of the system, even though it is more fragile than the classical correlations. We study the dependence of the residual entanglement and correlations on the system size and on the disorder strength. Finally, we demonstrate the robustness of entanglement and correlations with respect to local dissipation in the bulk of the system.

We study transport in twisted bilayer graphene and show that electrostatic barriers can act as valley splitters, where electrons from the $K$ ($K^{\prime }$) valley are transmitted only to, e.g., the top (bottom) layer, leading to valley-layer locked currents. We show that such a valley splitter is obtained when the barrier varies slowly on the moiré scale and induces a Lifshitz transition across the junction, i.e., a change in the Fermi-surface topology. Furthermore, we show that for a given valley the reflected and transmitted current are transversely deflected, as time-reversal symmetry is effectively broken in each valley separately, resulting in valley-selective transverse focusing at zero magnetic field.

Motivated by the recent discovery of superconductivity in doped ${\mathrm{NdNiO}}_2$, we study the magnetic exchange interaction $J$ in layered $d^9$ nickelates from first principles. The mother compounds of the high-$T_{\mathrm{c}}$ cuprates belong to the charge-transfer regime in the Zaanen-Sawatzky-Allen diagram and have $J$ larger than 100 meV. While this feature makes the cuprates very different from other transition metal oxides, it is of great interest whether layered $d^9$ nickelates can also have such a large $J$. However, one complexity is that ${\mathrm{NdNiO}}_2$ is not a Mott insulator due to carrier doping from the block layer. To compare the cuprates and $d^9$ nickelates on an equal basis, we study ${\mathrm{RbCa}}_2{\mathrm{NiO}}_3$ and $A_2{\mathrm{NiO}}_2{\mathrm{Br}}_2$ ($A$ denotes a cation with a valence of $2.5+$), which were recently designed theoretically by block-layer engineering. These nickelates are free from the self-doping effect and belong to the Mott-Hubbard regime. We show that these nickelates share a common thread with the high-$T_{\mathrm{c}}$ cuprates in that they also have a significant exchange interaction $J$ as large as about 100 meV.

The fast diffusion of alkali ions and resulting high ionic conductivity is a defining feature of alkali borate glasses. Here we report impedance spectroscopy and atomic-scale x-ray photon correlation spectroscopy measurements on rubidium borate systems ${\left({\mathrm{Rb}}_2\mathrm{O}\right)}_x{\left({\mathrm{B}}_2{\mathrm{O}}_3\right)}_{100-x}$ with particular focus on $x=30\text{mol}$ %. We find that the coherently scattered intensity does not show temporal fluctuations on timescales corresponding to ionic diffusion. We conclude that the spatial configuration of alkali sites as stepping stones for ionic diffusion evolves on the same timescales as the borate backbone, and that the defect density is low, i.e., at a given instant, nearly all alkali sites are singly occupied.

We develop a theory of edge states based on the Hermiticity of Hamiltonian operators for tight-binding models defined on lattices with boundaries. We describe Hamiltonians using shift operators which serve as differential operators in continuum theories. It turns out that such Hamiltonian operators are not necessarily Hermitian on lattices with boundaries, which is due to the boundary terms associated with the summation by parts. The Hermiticity of Hamiltonian operators leads to natural boundary conditions, and for models with nearest-neighbor (NN) hoppings only, there are reference states that satisfy the Hermiticity and boundary conditions simultaneously. Based on such reference states, we develop a Bloch-type theory for edge states of NN models on a half-plane. This enables us to extract Hamiltonians describing edge states at one end, which are separated from the bulk contributions. It follows that we can describe edge states at the left and right ends separately by distinct Hamiltonians for systems of cylindrical geometry. We show various examples of such edge-state Hamiltonians, including Hofstadter model, graphene model, and higher-order topological insulators.

Strontium ruthenates have a lot of similarities with copper oxide superconductors and are a very interesting object for investigation of mechanisms and conditions which lead to high-temperature superconductivity. We report here on multiple experimental indications of superconductivity with the onset above $40\mathrm{K}$ in strontium ruthenate doped by rhenium and selenium with chlorine used as a flux. The main experimental evidence comes from FTIR spectroscopy of this material followed by the ac and dc magnetization, as well as its heat capacity and magnetoresistance. Structural and morphological studies reveal the heterophase nature of this polycrystalline material as well as changes of lattice parameters relative to the original phases. Experimental data provide evidence of higher critical temperature on the surface compared to that in the bulk of the sample.

We explore the magnetic-field-driven motion of domain walls with different chiralities in thin ferromagnetic films made of Pt/Co/Pt, Au/Co/Pt, and Pt/Co/Au. From the analysis of domain wall dynamics, we extract parameters characterizing the interaction between domain walls and weak pinning disorder of the films. The variations of domain wall structure, controlled by an in-plane field, are found to modify the characteristic length scale of pinning in strong correlation with the domain wall width, whatever its chirality and the interaction strength between domain walls and pinning defects. These findings should be also relevant for a wide variety of elastic interfaces moving in weak pinning disordered media.

We theoretically study the nonlinear response of interacting neutral bosonic gas in a synthetically driven one-dimensional optical lattice. In particular, we examine the bosonic analog of electronic higher harmonic generation in a strong time-dependent synthetic vector potential manifesting itself as the synthetic electric field. We show that the vector potential can generate reasonably high harmonics in the insulating regime, while the superfluid regime exhibits only a few harmonics. In the insulating regime, the number of harmonics increases with the increase in the strength of the vector potential. This originates primarily due to the field-driven resonant and nonresonant excitations in the neutral Mott state and their recombination with the ground state. If the repulsive interaction between two atoms $\left(U\right)$ is close to the strength of the gauge potential $\left(A_0\right)$, the resonant quasiparticle-quasihole pairs on nearest-neighbor sites, namely, dipole states, are found to a play a dominant role in generating higher harmonics. However, in the strong-field limit $A_0\gg U$, the nonresonant states where quasiparticle-quasihole pairs are not on nearest-neighbor sites give rise to higher harmonics.

In some kind of three-dimensional higher-order topological insulators, bulk and surface electronic states are gapped, while there appear gapless hinge states protected by spatial symmetry. Here we show by ab initio calculations that the La apatite electride is a higher-order topological crystalline insulator. It is a one-dimensional electride, in which the one-dimensional interstitial hollows along the $c$ axis support anionic electrons, and the electronic states in these one-dimensional channels are well approximated by the one-dimensional Su-Schrieffer-Heeger model. When the crystal is cleaved into a hexagonal prism, the ${120}^{\circ }$ hinges support gapless hinge states, with their filling quantized to be 2/3. This quantization of the filling comes from a topological origin. We find that the quantized value of the filling depends on the fundamental blocks that constitute the crystal. The apatite consists of the triangular blocks, which is crucial for giving nontrivial fractional charge at the hinge.

The structural and electronic properties of twisted bilayer graphene are investigated from first-principles and tight-binding approach as a function of the twist angle (ranging from the first “magic” angle $\theta =1.{08}^{\circ }$ to $\theta =13.{17}^{\circ }$, with the former corresponding to the largest unit cell, comprising 11,164 carbon atoms). By properly taking into account the long-range van der Waals (vdW) interaction, we provide the patterns for the atomic displacements (with respect to the ideal twisted bilayer). The out-of-plane relaxation shows an oscillating (“buckling”) behavior, very evident for the smallest angles, with the atoms around the AA stacking regions interested by the largest displacements. The out-of-plane displacements are accompanied by a significant in-plane relaxation, showing a vortexlike pattern, where the vorticity (intended as curl of the displacement field) is reverted when moving from the top to the bottom plane and vice versa. Overall, the atomic relaxation results in the shrinking of the AA stacking regions in favor of the more energetically favorable AB/BA stacking domains. The measured flat bands emerging at the first magic angle can be accurately described only if the atomic relaxations are taken into account. Quite importantly, the experimental gaps separating the flat-band manifold from the higher and lower energy bands are intimately related to out-of-plane relaxations. The stability of the relaxed bilayer at the first magic angle is estimated to be of the order of 0.5–0.9 meV per atom (or 7–10 K). Our calculations shed light on the importance of an accurate description of the vdW interaction and of the resulting atomic relaxation to envisage the electronic structure of this really peculiar kind of vdW bilayers.

Entanglement properties of bipartite unitary operators are studied via their local invariants, namely the entangling power $e_p$ and a complementary quantity, the gate typicality $g_t$. We characterize the boundaries of the set $K_2$ representing all two-qubit gates projected onto the plane $(e_p,g_t)$ showing that the fractional powers of the swap operator form a parabolic boundary of $K_2$, while the other bounds are formed by two straight lines. In this way, a family of gates with extreme properties is identified and analyzed. We also show that the parabolic curve representing powers of swap persists in the set $K_N$ for gates of higher dimensions ($N>2$). Furthermore, we study entanglement of bipartite quantum gates applied sequentially $n$ times, and we analyze the influence of interlacing local unitary operations, which model generic Hamiltonian dynamics. An explicit formula for the entangling power of a gate applied $n$ times averaged over random local unitary dynamics is derived for an arbitrary dimension of each subsystem. This quantity shows an exponential saturation to the value predicted by the random matrix theory, indicating “thermalization” in the entanglement properties of sequentially applied quantum gates that can have arbitrarily small, but nonzero, entanglement to begin with. The thermalization is further characterized by the spectral properties of the reshuffled and partially transposed unitary matrices.

A IV-VI compound SnTe is proposed to be a topological crystalline insulator (TCI), and the band structures have been studied by using angle-resolved photoemission spectroscopy (ARPES). However, the topological surface states (TSSs) that hallmark the nontrivial topology disperse mostly in the unoccupied side where access via ARPES is limited. Here we investigate the (111) face of a SnTe film by using pump-probe ARPES. We find distinct energy sections in the unoccupied side that behave differently in terms of the excitation and recovery dynamics. From these different behaviors, the boundaries of these sections are attributed to the edges of the conduction and valence bands. High statistics data reveal that the TSSs are traversing the band gap, which evidences that SnTe belongs to a TCI. We also find that the bulk bands near the gap show splitting, which is attributed to the Rashba effect occurring in the dipolar surface region.

We study what kind of topological crystalline insulator phase emerges from nodal line semimetal phases in the face centered cubic system, which is not indicated by topological indices when spin-orbit coupling (SOC) is introduced. We construct an effective model, which hosts two different nodal lines phases, and calculated mirror Chern numbers in it by introducing SOC. As a result, we find that the two nodal line phases with different nodal line configurations are mapped to different topological crystalline insulator phases. This result shows that turning off SOC and checking the nodal line configuration can distinguish the two topological crystalline insulator phases, which have not been distinguished by previous methods.

We apply a microscopic theory of polarization and magnetization to crystalline insulators at zero temperature and consider the orbital electronic contribution of the linear response to spatially varying, time-dependent electromagnetic fields. The charge and current density expectation values generally depend on both the microscopic polarization and magnetization fields, and on the microscopic free charge and current densities. But contributions from the latter vanish in linear response for the class of insulators we consider. Thus we need only consider the former, which can be decomposed into “site” polarization and magnetization fields, from which “site multipole moments” can be constructed. Macroscopic polarization and magnetization fields follow, and we identify the relevant contributions to them; for electromagnetic fields varying little over a lattice constant these are the electric and magnetic dipole moments per unit volume, and the electric quadrupole moment per unit volume. A description of optical activity and related magneto-optical phenomena follows from the response of these macroscopic quantities to the electromagnetic field and, while in this paper we work within the independent particle and frozen-ion approximations, both optical rotary dispersion and circular dichroism can be described with this strategy. Earlier expressions describing the magnetoelectric effect are recovered as the zero frequency limit of our more general equations. Since our site quantities are introduced with the use of Wannier functions, the site multipole moments and their macroscopic analogs are generally gauge dependent. However, the resulting macroscopic charge and current densities, together with the optical effects to which they lead, are gauge invariant, as would be physically expected.

Using a semianalytic guided-mode expansion technique, we present theory and analysis of intrinsic propagation losses for topological photonic crystal slab waveguide structures with modified honeycomb lattices of circular or triangular holes. Although conventional photonic crystal waveguide structures, such as the W1 waveguide, have been designed to have lossless propagation modes, they are prone to disorder-induced losses and backscattering. Topological structures have been proposed to help mitigate this effect as their photonic edge states may allow for topological protection. However, the intrinsic propagation losses of these structures are not well understood and the concept of the light line can become blurred. For four example topological edge state structures, photonic band diagrams, loss parameters, and electromagnetic fields of the guided modes are computed. Two of these structures, based on armchair edge states, are found to have significant intrinsic losses for modes inside the photonic bandgap, more than 100 dB/cm, which is comparable to or larger than typical disorder-induced losses using slow-light modes in conventional photonic crystal waveguides, while the other two structures, using the valley Hall effect and inversion symmetry, are found to have a good bandwidth for exploiting lossless propagation modes below the light line (at least in the absence of disorder).