Momentum-matched type II van der Waals heterostructures (vdWHs) have been designed by assembling layered two-dimensional semiconductors (2DSs) with special band-structure combinations––that is, the valence band edge at the Γ point (the Brillouin-zone center) for one 2DS and the conduction-band edge at the Γ point for the other [Ubrig et al., Nat. Mater. 19, 299 (2020)]. However, the band offset sizes, band-alignment types, and whether momentum matched or not, all are affected by the interfacial effects between the component 2DSs, such as the quasichemical-bonding (QB) interaction between layers and the electrical dipole moment formed around the vdW interface. Here, based on density-functional theory calculations, first we probe the interfacial effects (including different QBs for valence and conduction bands, interface dipole, and the synergistic effects of these two aspects) on band-edge evolution in energy and valley (location in the Brillouin zone) and the resulting changes in band alignment and momentum matching for a typical vdWH of monolayer InSe and bilayer $\mathrm{W}{\mathrm{S}}_2$, in which the band edges of subsystems satisfy the special band-structure combination for a momentum-matched type II vdWH. Then, based on the conclusions of the studied interfacial effects, we propose a practical screening method for robust momentum-matched type II vdWHs. This practical screening method can also be applied to other band-alignment types. Our current study opens a way for practical screening and designing of vdWHs with robust momentum matching and band-alignment type.

Intrinsically gapless symmetry-protected topological phases (igSPT) are gapless systems with SPT edge states with properties that could not arise in a gapped system with the same symmetry and dimensionality. igSPT states arise from gapless systems in which an anomaly in the low-energy (infrared) symmetry group emerges from an extended anomaly-free microscopic (ultraviolet) symmetry. We construct a general framework for constructing lattice models for igSPT phases with emergent anomalies classified by group cohomology, and establish a direct connection between the emergent anomaly, group-extension, and topological edge states by gauging the extending symmetry. In many examples, the edge-state protection has a physically transparent mechanism: the extending UV symmetry operations pump lower-dimensional SPTs onto the igSPT edge, tuning the edge to a (multi)critical point between different SPTs protected by the IR symmetry. In two- and three-dimensional systems, an additional possibility is that the emergent anomaly can be satisfied by an anomalous symmetry-enriched topological order, which we call a quotient-symmetry-enriched topological order (QSET) that is sharply distinguished from the nonanomalous UV SETs by an edge phase transition. We construct exactly solvable lattice models with QSET order.

It is challenging to design electro-optic (EO) crystals with both a large bandgap and a high EO coefficient. We propose that some semiconductors synthesized under a high pressure environment can present both a large bandgap and a high linear EO coefficient because of the enhanced strength of the valence bond. The electronic band structures and linear EO coefficients of the $II\mathrm{Si}{\mathrm{N}}_2$ ($II=\mathrm{Mg}$, Sr, Ba) compounds are studied using first-principles calculation. Our calculated results predict that both $\mathrm{SrSi}{\mathrm{N}}_2$ and $\mathrm{BaSi}{\mathrm{N}}_2$, under a high-pressure condition, will crystalize into an orthorhombic structure with the same space group of $\mathrm{MgSi}{\mathrm{N}}_2$ under ambient pressure. The calculated bandgaps are 5.48, 4.41, and 3.57 eV, respectively, for $\mathrm{MgSi}{\mathrm{N}}_2, \mathrm{SrSi}{\mathrm{N}}_2$, and $\mathrm{BaSi}{\mathrm{N}}_2$, while the calculated linear EO coefficients are 1.55, 12.09, and 21.60 pm/V at a fiber communication wavelength of 1550 nm. The increasing linear EO coefficients can be interpreted by the enhanced bond strength and a reduced bandgap. This work indicates that it is feasible to enhance the linear EO coefficient with high pressure.

Population-transfer schemes are commonly used to convert information robustly stored in some quantum system for manipulation and memory into more macroscopic degrees of freedom for measurement. These schemes may include, e.g., spin-to-charge conversion for spins in quantum dots, detuning of charge qubits between a noise-insensitive operating point and a measurement point, spatial shuttling of qubits encoded in spins or ions, and parity-to-charge conversion schemes for qubits based on Majorana zero modes. A common strategy is to use a slow (adiabatic) conversion. However, in an adiabatic scheme, the adiabaticity conditions on the one hand, and accumulation of errors through dephasing, leakage, and energy relaxation processes, on the other hand, limit the fidelity that can be achieved. Here, we give explicit fast quasiadiabatic (fast-QUAD) conversion strategies (pulse shapes) beyond the adiabatic approximation that allow for optimal state conversion. In contrast with many other approaches, here we account for noise in combination with pulse shaping. Although we restrict to noise sources that can be modeled by a classical fluctuating parameter, we allow generally for anisotropic non-Gaussian noise that is nevertheless sufficiently weak to lead to a small error. Inspired by analytic methods that have been developed for dynamical decoupling theory, we provide a general framework for unique noise mitigation strategies that can be tailored to the system and environment of interest.

${\mathrm{CeRh}}_2{\mathrm{As}}_2$ is a multiphase superconductor with $T_{\text{c}}=0.26$ K. The two superconducting (SC) phases, SC1 and SC2, observed for a magnetic field $H$ parallel to the $c$ axis of the tetragonal unit cell, have been interpreted as even- and odd-parity SC states, separated by a phase boundary at ${\mu }_0{H}^{*}=4$ T. Such parity switching is possible due to a strong Rashba spin-orbit coupling at the Ce sites located in locally noncentrosymmetric environments of the globally centrosymmetric lattice. The existence of another ordered state (phase I) below a temperature $T_0\approx 0.4$ K suggests an alternative interpretation of the $H^{*}$ transition: It separates a mixed SC$+$I (SC1) and a pure SC (SC2) state. Here, we present a detailed study of higher-quality single crystals of ${\mathrm{CeRh}}_2{\mathrm{As}}_2$, showing much sharper signatures at $T_{\text{c}}=0.31$ K and $T_0=0.48$ K. We refine the $T-H$ phase diagram of ${\mathrm{CeRh}}_2{\mathrm{As}}_2$ and demonstrate that $T_0\left(H\right)$ and $T_{\text{c}}\left(H\right)$ lines meet at ${\mu }_{0}H\approx 6$ T, well above $H^{*}$, implying no influence of phase I on the SC phase switching. A basic analysis with the Ginzburg-Landau theory indicates a weak competition between the two orders.

In this work, we investigate the interplay between dissipation and symmetry-protected topological order. We considered the one-dimensional spin-1 Affleck-Kennedy-Lieb-Tasaki model interacting with an environment where the dissipative dynamics are described by the Lindladian master equation. The Markovian dynamics is solved by the implementation of a tensor network algorithm for mixed states in the thermodynamic limit. We observe that, for time-reversal symmetric dissipation, the resulting steady state has topological signatures even if being a mixed state. This is seen in finite string-order parameters as well as in the degeneracy pattern of singular values in the tensor network decomposition of the reduced density matrix. We also show that such features do not appear for nonsymmetric dissipation. Our work opens the way toward a generalized and more practical definition of symmetry-protected topological order for mixed states induced by dissipation.

We show that for the magic-angle twisted bilayer graphene (TBG) away from the charge neutrality point, although quantum Monte Carlo (QMC) simulations suffer from the sign problem, the computational complexity is at most polynomial at integer fillings of the flat-band limit. For even-integer fillings, the polynomial complexity survives even if an extra intervalley attractive interaction is introduced. This observation allows us to simulate magic-angle TBG and to obtain an accurate phase diagram and dynamical properties. At the chiral limit and filling $\nu =1$, the simulations reveal a thermodynamic transition separating the metallic state and a $C=1$ correlated Chern insulator—topological Mott insulator (TMI)—and the pseudogap spectrum slightly above the transition temperature. The ground state excitation spectra of the TMI exhibit a spin-valley U(4) Goldstone mode and a time-reversal restoring excitonic gap smaller than the single-particle gap. These results are qualitatively consistent with recent experimental findings at zero-field and $\nu =1$ filling in $h$-BN nonaligned TBG devices.

The hyperpolarization of nuclear spin ensembles through their interaction with optically active color centers has emerged as a promising technique with applications ranging from sensing and imaging to quantum computation. Here, we investigate the efficiency of this approach for the hyperpolarization of the nuclear-spin bath in a monolayer of hexagonal boron nitride via the optically active boron vacancy centers (${\mathrm{V}}_{\mathrm{B}}$). To that end we design a polarization sequence based on the suitable combination of optical polarization and microwave driving of the ${\mathrm{V}}_{\mathrm{B}}$, which we optimize numerically. To extend our study to realistic systems with a large number of interacting spins, we employ an approximate method based on the Holstein-Primakoff transformation, whose validity we benchmark against exact numerics for small system sizes. Our results suggest that a high-degree of polarization in the boron and nitrogen nuclear-spin lattices is achievable also at room temperature. Our work provides the first step toward the realization of a two-dimensional quantum simulator based on natural nuclear spins and it can prove useful for extending the coherence time of the ${\mathrm{V}}_{\mathrm{B}}$ centers.

The fragmentation description is used to analyze calculated neutron scattering intensities from kagome ice and spin ice systems. The longitudinal, transverse, and harmonic fragments produce independent contributions to the neutron scattering intensity. This framework is used to analyze the ordering due to quantum fluctuations in the topologically constrained phase of kagome ice and the monopole crystal phase of spin ice. Here, quantum fluctuations are restricted to the transverse fragment, and they drive the system into a double-$q$ structure in which longitudinal and transverse fragments have a different ordering wave vector. The intensity reduction of the Bragg peaks for the transverse fragments, compared with known classical limits, can be used as a diagnostic tool for quantum fluctuations. Published quantum Monte Carlo data for spin ice in a [111] field are consistent with the proposed protocol.

Scrutiny of an established monoclinic magnetic space group for ${\mathrm{NdFeO}}_3$ reveals hitherto unknown properties of the orthoferrite. Future experiments using neutron and x-ray diffraction techniques can verify them. Neodymium ions possess Dirac multipoles, both time odd (magnetic) and parity odd (polar), that come with unique diffraction conditions. Nonmagnetic polar Nd multipoles are permitted even though the monoclinic space group is centrosymmetric. Dirac multipoles are forbidden by symmetry at sites occupied by ferric ions. Available diffraction patterns have not been analyzed for Dirac multipoles, nor all permitted components of the axial dipoles and quadrupoles. In the case of neutron diffraction, magnetic quadrupoles are correlations between anapole and orbital degrees of freedom. We give conditions for the observation of Templeton-Templeton scattering of x rays, created by angular anisotropy in the electronic charge distribution. Axial multipoles are the sole providers of dichroic signals.

Here we present a detailed investigation of the Co and Ir local electronic structures in ${\mathrm{La}}_{1.5}{A}_{0.5}{\mathrm{CoIrO}}_6$ ($A=\mathrm{Ba}$, Ca) compounds in order to unravel the orbital hybridization mechanism in these CoIr-based double perovskites. Our results of x-ray powder diffraction, ac and dc magnetization, Co and Ir $L_{2,3}$-edge and Co $K$-edge x-ray absorption spectroscopy and x-ray magnetic circular dichroism suggest a competition between magnetic interactions. A dominant antiferromagnetic coupling is found to be responsible for the ferrimagnetic behavior observed for $A=\mathrm{Ca}$ below $\sim 96\mathrm{K}$, the competing magnetic phases, and the cationic disorder in this compound giving rise to a spin-glass state at low temperatures. For the $A=\mathrm{Ba}$, on the other hand, there is no evidence of long-range order down to its spin-glass transition temperature. The remarkably different magnetic properties observed between these two compounds are discussed in terms of the structural distortion that alters the strength of the Co-Ir couplings, with a relevant role played by the Co $3d{e}_g–\mathrm{Ir} 5d{j}_{\mathrm{eff}}=1/2$ hybridization.

Candidate materials for the Kitaev spin liquid generically have residual interactions beyond the Kitaev coupling. It therefore becomes necessary to understand how signatures of the quantum spin liquid, e.g., fractionalization of the spin excitations, are affected by the presence of these interactions. Recently it was shown that phonon dynamics is an indirect but effective probe to study fractionalized excitations in the Kitaev spin liquid. Ultrasound experiments can measure sound attenuation, which should show characteristic temperature and angular dependence of the sound-attenuation coefficient if the scattering of phonons happens predominantly on Majorana fermions. So far the computation of the sound attenuation was only done in the pure spin-phonon coupled Kitaev model, without taking into account residual interactions. In order to understand experimental signatures, here we present a mean-field study of the sound attenuation in the generalized $J\text{−}K\text{−}\mathrm{\Gamma }$ model, which is relevant to candidate materials. Our findings show that as long as the system is in the spin liquid phase, characteristic features of the sound attenuation remain observable even in the presence of residual interactions.

Excess quasiparticles play a crucial role in superconducting quantum devices ranging from qubits to quantum sensors. In this work we analyze their dynamics for phase-biased finite-length weak links with several Andreev subgap states, where the coupling to a microwave resonator allows for parity state (even or odd) readout. Our theory shows that almost perfect dynamical polarization in a given parity sector is achievable by applying a microwave pulse matching a transition in the opposite parity sector. Our results qualitatively explain key features of recent experiments on hybrid semiconducting nanowire Josephson junctions and provide theoretical guidelines for efficiently controlling the parity state of Andreev qubits.

The quantum-classical correspondence between local minima on the classical energy landscape and excited eigenstates in the energy spectrum is studied within the context of many-body quantum spin systems. In mean-field approximations of a quantum problem, one usually focuses on attaining the global minimum of the resulting energy function, while other minimum solutions are usually ignored. For frustrated systems, a strict distinction between global and local minimum is often not tenable since first-order-type transitions can interchange the roles played by two different minima. This begs the question of whether there is any physical interpretation for the local minima encountered in mean-field approximations of quantum systems. We look at the problem from the perspective of quantum spin systems. Two models are studied, a frustrated model with quenched disorder and a pure system without frustration. Accurate classical energies of the minima are compared with the full spectrum of energy levels, allowing us to search for signs of correspondence between them. It is found that the local minima can generally be interpreted as excited energy eigenstates. Instances of spurious minima are also reported.

Reduced graphene oxide (rGO) is a bulk-processable quasiamorphous two-dimensional material with broad spectral coverage and fast electronic response. rGO sheets are suspended in a polymer matrix and sequentially photoreduced to measure the optical absorption and ultrafast hot-electron cooling dynamics. Photoreduced graphene oxide (GO) yields absorption spectra that fit the same Fano line shape parameters as monolayer (ml) graphene. With increasing photoreduction time, rGO transient absorption kinetics reach an optimal point that matches the hot-electron cooling dynamics simultaneously measured in ml-graphene. After each stepwise photoreduction, the rGO ultrafast kinetics are simulated with the hot-electron cooling model of graphene that is mediated by disorder-assisted supercollisions. The hot-electron cooling rate of moderately photoreduced GO is 0.31 ${\mathrm{ps}}^{-1}$ and closely matches the ml-graphene result. Subsequent photoreduction increases the disorder parameter associated with the hot-election cooling, consistent with an order of magnitude smaller mean-free scattering length from photoionized point defects. GO photoreduction yields increasing photoluminescence from localized graphene quantum dot (GQDs) and decreasing 2.3 eV emission from oxygenated defect-edge states. By pumping localized GQD states and probing graphene, a 0.17 ps delayed rise-time emerges that accelerates with photoreduction, suggesting an energy transfer process.

Anisotropic magnetization and anomalous electrical transport properties of flux-grown $\mathrm{Pr}{\mathrm{Mn}}_2{\mathrm{Ge}}_2$ single crystals were investigated. The grown crystals exhibit strong magnetic anisotropy with easy magnetization along the $c$ axis. Linear magnetoresistance (MR) and a conventional anomalous Hall effect (AHE) are observed for $H\text{//}c$ and $I\text{//}b$ in contrast to a nonlinear positive MR and a large planar topological Hall effect (PTHE) observed for $H\text{//}a$ and $I\text{//}b$. The anisotropy of the magnetic and electrical transport properties is increased as temperature decreases because magnetic ordering of Pr atoms brings about an increase of the saturation magnetic field of nonlinear MR and the critical field for the maximum values of PTHE. A scaling analysis using the Tian-Ye-Jin model suggests that the AHE in this compound has a larger intrinsic contribution compared to the extrinsic contribution. The nonlinear positive MRs are attributed to the field-induced nontrivial spin textures. The PTHE is related to the real-space skyrmionic bubbles and the noncoplanar spin texture with nonzero spin chirality. These results demonstrate a magnetic field induced anisotropic magnetoresistance and Hall resistivity and its tunability by the ferromagnetic ordering of rare earth atoms in conical-ordered magnets.

${\mathrm{PbCuTe}}_2{\mathrm{O}}_6$ is considered to be one of the rare candidate materials for a three-dimensional quantum spin liquid (QSL). This assessment was based on the results of various magnetic experiments, performed mainly on polycrystalline material. More recent measurements on single crystals revealed an even more exotic behavior, yielding ferroelectric order below $T_{\text{FE}}\approx 1\text{K}$, accompanied by distinct lattice distortions, and a somewhat modified magnetic response which is still consistent with a QSL. Here we report on low-temperature measurements of various thermodynamic, magnetic, and dielectric properties of single-crystalline ${\mathrm{PbCuTe}}_2{\mathrm{O}}_6$ in magnetic fields $B\le 14.5\text{T}$. The combination of these various probes allows us to construct a detailed $B\text{−}T$ phase diagram including a ferroelectric phase for $B\le 8$ T and a $B$-induced magnetic phase at $B\ge 11$ T. These phases are preceded by or coincide with a structural transition from a cubic high-temperature phase into a distorted noncubic low-temperature state. The phase diagram discloses a ferroelectric quantum critical point at $B_{c1}=7.9\text{T}$, where the second-order phase transition line associated with ferroelectric order is suppressed to zero. In addition, a magnetic quantum phase transition is revealed at $B_{c2}=11\text{T}$. The corresponding phase transition to a field-induced magnetic order at $B>B_{c2}$ is likely to be of first order. Field-induced lattice distortions, observed in the state at $T>1$ K and which are assigned to the effect of spin-orbit interaction of the ${\mathrm{Cu}}^{2+}$ ions, are considered as the key mechanism by which the magnetic field couples to the dielectric degrees of freedom in this material.

We study the electronic origin of parasitic ferroic quadrupole moments in antiferroic quadrupole orders by extending a model studied in G. Chen et al., Phys. Rev. B 82, 174440 (2010) with the effective angular momentum $J_{\mathrm{eff}}=3/2$ quartet ground states. Taking into account the first crystalline-electric-field (CEF) excited doublet, cubic anisotropy in the quadrupole moments emerges, which leads to the induced ferroic quadrupole moments in the antiferroic quadrupolar phases. The hybridization with the CEF excited quartet states also causes finite magnetic moments compatible to the observed size of the effective moment in typical $J_{\mathrm{eff}}=3/2$ systems, as opposed to the naive expectation of vanishing moments in the $J_{\mathrm{eff}}=3/2$ systems. These results suggest the importance of the corrections arising from the high-energy CEF excited states in the $J_{\mathrm{eff}}=3/2$ systems.

We present a computational approach to simulate local contrast observed in field ion microscopy (FIM). It is based on density-functional theory utilizing the Tersoff-Hamann approach as done in scanning tunneling microscopy (STM). A key requirement is the highly accurate computation of the surface states' wave-function tails. To refine the Kohn-Sham states from standard iterative global solvers we introduce and discuss the EXtrapolation of Tails via Reverse integration Algorithm (EXTRA). The decaying tails are obtained by reverse integration (from outside in) using a Numerov-like algorithm. The starting conditions are then iteratively adapted to match the values of plane-wave Kohn-Sham wave functions close to the surface. We demonstrate the performance of the proposed algorithm by analyzing and showing the chemical contrast for Ta, W, and Re at Ni surface.