The recent discovery of two-dimensional (2D) van der Waals magnets is a crucial turning point in the quantum magnet research field, since quantum fluctuations and experimental difficulties often elude stable magnetic orders in two dimensions. This opens new doors to delve for novel quantum and topological spin configurations, which may or may not have direct analogs in bulk counterparts. Here we study a twisted bilayer geometry of 2D magnets in which long-range spin-spin interactions naturally commence along the interlayer Heisenberg ($J_{\perp }$) and dipole-dipole ($J_D$) channels. The $J_{\perp }\text{−}{J}_D$ parameter space unveils a hierarchy of distinct skyrmion phases, including point-, rod-, and ring-shaped topological charge distributions. Furthermore, we predict a topological antiferroelectric phase, where oppositely charged antiskyrmion pairs are formed, and the corresponding topological dipole moments become ordered in a Néel-like state—hence dubbed the topological antiferroelectric state. The results indicate that the twisted magnetic layer provides a versatile setting to engineer and tune a plethora of skyrmion phases and their dynamics.

Domain wall dynamics and spatial fluctuations are closely related to each other and to universal features of disordered systems. Experimentally measured roughness exponents characterizing spatial fluctuations have been reported for magnetic thin films, with values generally different from those predicted by the equilibrium, depinning, and thermal reference states. Here we study the roughness of domain walls in GdFeCo thin films over a large range of magnetic field and temperature. Our analysis is performed in the framework of a model considering length-scale crossovers between the reference states, which is shown to bridge the differences between experimental results and theoretical predictions. We also quantify the size of the depinning avalanches below the depinning field at finite temperatures.

Photoelectron spectroscopy experiments in ionic solutions reveal important electronic structure information, in which the interaction between hydrated ions and water solvent can be inferred. Based on many-body perturbation theory with GW approximation, we theoretically compute the quasiparticle electronic structure of chloride anion solution, which is modeled by path-integral $abinitio$ molecular dynamics simulation by taking account the nuclear quantum effects (NQEs). The electronic levels of hydrated anion as well as water are determined and compared with the recent experimental photoelectron spectra. It is found that NQEs improve the agreement between theoretical prediction and experiment because NQEs effectively weaken the hybridization of the between the ${\mathrm{Cl}}^{-}$ anion and water. Our study indicates that NQEs plays a small but non-negligible role in predicting the electronic structure of the aqueous solvation of ions of the Hofmeister series.

Mappings between fermions and qubits are valuable constructions in physics. To date only a handful exist. In addition to revealing dualities between fermionic and spin systems, such mappings are indispensable in any quantum simulation of fermionic physics on quantum computers. The number of qubits required per fermionic mode, and the locality of mapped fermionic operators strongly impact the cost of such simulations. We present a fermion to qubit mapping that outperforms all previous local mappings in both the qubit to mode ratio and the locality of mapped operators. In addition to these practically useful features, the mapping bears an elegant relationship to the toric code, which we discuss. Finally, we consider the error mitigating properties of the mapping—which encodes fermionic states into the code space of a stabilizer code. Although there is an implicit tradeoff between low weight representations of local fermionic operators, and high distance code spaces, we argue that fermionic encodings with low-weight representations of local fermionic operators can still exhibit error mitigating properties which can serve a similar role to that played by high code distances. In particular, when undetectable errors correspond to “natural” fermionic noise. We illustrate this point explicitly both for this encoding and the Verstraete-Cirac encoding.

We present a numerical modeling workflow based on machine learning which reproduces the total energies produced by Kohn-Sham density functional theory (DFT) at finite electronic temperature to within chemical accuracy at negligible computational cost. Based on deep neural networks, our workflow yields the local density of states (LDOS) for a given atomic configuration. From the LDOS, spatially resolved, energy-resolved, and integrated quantities can be calculated, including the DFT total free energy, which serves as the Born-Oppenheimer potential energy surface for the atoms. We demonstrate the efficacy of this approach for both solid and liquid metals and compare results between independent and unified machine-learning models for solid and liquid aluminum. Our machine-learning density functional theory framework opens up the path towards multiscale materials modeling for matter under ambient and extreme conditions at a computational scale and cost that is unattainable with current algorithms.

Motivated by recent transport experiments, we theoretically study the quantum Hall effect in topological semimetal films. Owing to the confinement effect, the bulk subbands originating from the chiral Landau levels establish energy gaps that have quantized Hall conductance and can be observed in relatively thick films. We find that the quantum Hall state is strongly anisotropic for different confinement directions not only due to the presence of the surface states but also because of the bulk chiral Landau levels. As a result, we re-examine the quantum Hall effect from the surface Fermi arcs and chiral modes in Weyl semimetals and give a more general view into this problem. Also, we find that when a topological Dirac semimetal is confined in its rotational symmetry axis, it hosts both quantum Hall and quantum spin Hall states, in which the helical edge states are protected by the conservation of the spin-$z$ component.

The past decade has witnessed a booming development of topological photonics, which revolutionizes the methodology for controlling the behavior of light. A gigantic achievement is to engineer robust confined modes localized at interfaces between topologically distinct regions, where the optical context can trigger exotic topological phenomena exclusive to photons. Here, we provide an experimentally flexible approach to engineering topologically induced interface states in the visible regime via a unique design of complex superlattice formed by connecting two component superlattices of distinguished topological phases. Assisted by the intrinsic pseudospin degree due to the splitting between TM and TE polarized modes, we attain a precise manipulation of the spin-dependent topological interface states that can manifest themselves straightforwardly through transmission spectra. More specifically, since these topological localized modes stem from the hybridization of artificial photonic orbitals that are of topological origin as well, they are deemed a topological effect and thus named as the secondary topological interface states. Our work develops an innovative and productive strategy to tune topologically protected localized modes, based on which various applications such as selective local enhancement can be exploited.

By using symmetry analysis and first-principles calculations, we propose that the simple cubic structure of copper chloride (CuCl) in high-pressure phase can be worked as a three-dimensional (3D) topological phononic material, hosting the so-called topological phononic nodal links (TPNLs) in phonon spectra, which are intersected around the center of the Brillouin zone (BZ). More interestingly, another class of nodal-line phonons, i.e., the topological phononic nodal hexahedron net (TPNHN), exists at all the boundaries of 3D BZ and is composed of interconnected quadruply degenerate straight nodal lines. Their topologically nontrivial natures are confirmed by the Berry phase calculations. The TPNHN-induced Dirac cones and the TPNLs-induced drumheadlike surface states are observed in the surface BZ. In addition, the appearance of large surface arcs strongly supports the robustness of nodal-link phonons. Our theoretical results not only pave a route to explore new nodal-line phonons but also reveal that the semiconductor CuCl in high-pressure phase is an ideal material candidate to realize TPNHN and TPNLs.

The proximity effect from a spin-triplet $p_x$-wave superconductor to a dirty normal-metal has been shown to result in various unusual electromagnetic properties, reflecting a cooperative relation between topologically protected zero-energy quasiparticles and odd-frequency Cooper pairs. However, because of a lack of candidate materials for spin-triplet $p_x$-wave superconductors, observing this effect has been difficult. In this paper, we demonstrate that the anomalous proximity effect, which is essentially equivalent to that of a spin-triplet $p_x$-wave superconductor, can occur in a semiconductor/high-$T_c$ cuprate superconductor hybrid device in which two potentials coexist: A spin-singlet $d$-wave pair potential and a spin-orbit coupling potential sustaining the persistent spin-helix state. As a result, we propose an alternative and promising route to observe the anomalous proximity effect related to the profound nature of topologically protected quasiparticles and odd-frequency Cooper pairs.

Hole spins localized in semiconductor structures, such as quantum dots or defects, serve in the realization of efficient gate-tunable solid-state quantum bits. Here, we study two electrically driven spin-$3/2$ holes coupled to the electromagnetic field of a microwave cavity. We show that the interplay between the non-Abelian Berry phases generated by local time-dependent electrical fields and the shared cavity photons allows for fast manipulation, detection, and long-range entanglement of the hole-spin qubits in the absence of any external magnetic field. Owing to its geometrical structure, such a scheme is more robust against external noises than conventional hole-spin qubit implementations. These results suggest that hole spins are favorable qubits for scalable quantum computing by purely electrical means.

New surprises continue to be revealed about ${\mathrm{La}}_2{\mathrm{CuO}}_4$, the parent compound of the original cuprate superconductor. Here we present neutron scattering evidence that the structural symmetry is lower than commonly assumed. The static distortion results in anisotropic Cu-O bonds within the ${\mathrm{CuO}}_2$ planes; such anisotropy is relevant to pinning charge stripes in hole-doped samples. Associated with the extra structural modulation is a soft phonon mode. If this phonon were to soften completely, the resulting change in ${\mathrm{CuO}}_6$ octahedral tilts would lead to weak ferromagnetism. Hence, we suggest that this mode may be the “chiral” phonon inferred from recent studies of the thermal Hall effect. We also note the absence of interaction between the antiferromagnetic spin waves and low-energy optical phonons, in contrast to what is observed in hole-doped samples.

We report ${}^{77}\mathrm{Se}$ NMR data in the normal and superconducting states of a single crystal of FeSe for several different field orientations. The Knight shift is suppressed in the superconducting state for in-plane fields, but does not vanish at zero temperature. For fields oriented out of the plane, little or no reduction is observed below $T_c$. These results reflect spin-singlet pairing emerging from a nematic state with large orbital susceptibility and spin-orbit coupling. The spectra and spin-relaxation rate data reveal electronic inhomogeneity that is enhanced in the superconducting state, possibly arising from enhanced density of states in the vortex cores. Despite the spin polarization of these states, there is no evidence for antiferromagnetic fluctuations.

An ab initio study of $\beta \text{−}{\mathrm{As}}_2{\mathrm{Te}}_3$ ($R\overline{3}m$ symmetry) at hydrostatic pressures shows that this compound is a trivial small band-gap semiconductor at room pressure that undergoes a quantum topological phase transition to a 3D topological Dirac semimetal around 2 GPa. At higher pressures, the band gap reopens and again decreases above 4 GPa. Our calculations predict an insulator-metal transition above 6 GPa due to the closing of the band gap, with strong topological features persisting between 2 and 10 GPa with ${\mathrm{Z}}_4=3$ topological index. By investigating the lattice thermal conductivity $\left({\kappa }_{\text{L}}\right)$, we observe that close to room conditions ${\kappa }_{\text{L}}$ is very low, either for the in-plane and the out-of-plane axis, with 0.098 and $0.023{\mathrm{Wm}}^{-1}{\mathrm{K}}^{-1}$, respectively. This effect occurs due to the presence of two low-frequency optical modes, namely ${\mathrm{E}}_u$ and ${\mathrm{E}}_g$, which increase the phonon-phonon scattering rate. Therefore, our results suggest that ultralow lattice thermal conductivities, which enable highly efficient thermoelectric materials, can be engineered in systems that are close to a structural instability derived from phonon Kohn anomalies. At higher pressures, the values of the in- and out-of-plane thermal conductivities not only increase in magnitude, but also approximate in value as the layered character of the compound decreases.

Scaling aspects of Gaussian topological phase transitions in quantum spin chains are investigated using the prototypical one-dimensional spin-1 XXZ Heisenberg model with uniaxial single-ion anisotropy $D$. This model presents a critical line separating the gapped Haldane and large-$D$ phases, with the relevant energy gap closing at the transition point. We show that a proper tangential finite-size scaling analysis is able to accurately locate the Gaussian critical line and to probe the continuously varying set of correlation length critical exponents. The specific features of the tangential scaling are highlighted, in contrast to the standard scaling holding in the Ising-like transition between the gapless antiferromagnetic Néel and gapped Haldane phases. Our results are compared with field-theoretic predictions and available high-accuracy data for specific points along the Gaussian line.

Flat bands—dispersionless bands—have been actively studied thanks to their sensitivity to perturbations, which makes them natural candidates for hosting novel and exotic states of matter. In parallel, non-Hermitian systems have attracted much attention in recent years as a simplified description of open system with gain and loss, motivated by potential applications. In particular, non-Hermitian system with dispersionless energy bands in their spectrum have been studied theoretically and experimentally. In general, flat bands require fine tuning of the Hamiltonian or protection by a symmetry. A number of methods was suggested to construct non-Hermitian flat bands relying either on the presence of symmetries, or specific frustrated geometries, often inspired by Hermitian models featuring flat bands. We introduce a systematic method to construct non-Hermitian flat bands using one-dimensional two-band tight-binding networks as an example, extending the methods used to construct systematically Hermitian flat bands. We show that the non-Hermitian case admits fine-tuned, nonsymmetry protected flat bands and provides more types of flat bands than the Hermitian case.