(Ga,Fe)Sb is a promising ferromagnetic semiconductor for practical spintronic device applications because its Curie temperature $\left(T_{\mathrm{C}}\right)$ is above room temperature. However, the origin of ferromagnetism with high $T_{\mathrm{C}}$ remains to be elucidated. Here, we use soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) to investigate the valence-band (VB) structure of $\left(\mathrm{G}{\mathrm{a}}_{0.95},\mathrm{F}{\mathrm{e}}_{0.05}\right)\mathrm{Sb}$ including the $\mathrm{Fe}\text{−}3d$ impurity band (IB), to unveil the mechanism of ferromagnetism in (Ga,Fe)Sb. We find that the VB dispersion in $\left(\mathrm{G}{\mathrm{a}}_{0.95},\mathrm{F}{\mathrm{e}}_{0.05}\right)\mathrm{Sb}$ observed by SX-ARPES is similar to that of GaSb, indicating that the doped Fe atoms hardly affect the band dispersion. The $\mathrm{Fe}\text{−}3d$ resonant ARPES spectra demonstrate that the $\mathrm{Fe}\text{−}3d$ IB crosses the Fermi level $\left(E_{\mathrm{F}}\right)$ and hybridizes with the VB of GaSb. These observations indicate that the VB structure of $\left(\mathrm{G}{\mathrm{a}}_{0.95},\mathrm{F}{\mathrm{e}}_{0.05}\right)\mathrm{Sb}$ is consistent with that of the IB model which is based on double-exchange interaction between the localized $3d$ electrons of the magnetic impurities.

In time-reversal-breaking centrosymmetric systems, the appearance of Weyl points can be guaranteed by an odd number of even/odd-parity occupied bands at eight inversion-symmetry-invariant momenta. Here, based on symmetry analysis and first-principles calculations, we demonstrate that for time-reversal-invariant systems with $S_4$ symmetry, the Weyl semimetal phase can be characterized by the inequality between a well-defined invariant $\eta$ and an $S_4$ indicator $z_2$. By applying this criterion, we find that some candidates, previously predicted to be topological insulators, are actually Weyl semimetals in the noncentrosymmetric space group with $S_4$ symmetry. Our first-principles calculations show that four pairs of Weyl points are located in the $k_{x,y}=0$ planes, with each plane containing four same-chirality Weyl points. An effective model has been built and captures the nontrivial topology in these materials. Our strategy to find the Weyl points by using symmetry indicators and invariants opens a new route to search for Weyl semimetals in time-reversal-invariant systems.

We show that the charge density wave (CDW) ground state below the Peierls transition temperature $T_{CDW}$ of rare-earth tritellurides is not at its equilibrium value, but depends on the time where the system was kept at a fixed temperature below $T_{CDW}$. This ergodicity breaking is revealed by the increase of the threshold electric field for CDW sliding which depends exponentially on time. We tentatively explain this behavior by the reorganization of the oligomeric ${\left({\mathrm{Te}}_x\right)}^{2-}$ sequence forming the CDW modulation.

It is shown in a recent preprint (arXiv:2001.10008) that the central spin model with $XX$-type qubit-bath coupling is integrable for a central spin $s_0=1/2$. Two types of eigenstates, separable states (dark states) and entangled states (bright states) between the central spin and the bath spins, are manifested. In this work we show by using an operator product state approach that the $XX$ central spin model with central spin $s_0>1/2$ and inhomogeneous coupling is partially solvable. That is, a subset of the eigenstates are obtained by the operator product state ansatz. These are the separable states and those entangled states in the single-spin-excitation subspace with respect to the fully polarized reference state. Due to the high degeneracy of the separable states, the resulting Bethe ansatz equations are found to be nonunique. In the case of $s_0=1/2$ we show that all the separable and entangled states can be written in terms of the operator product states, recovering the results in arXiv:2001.10008. Moreover, we also apply our method to the case of homogeneous coupling and derive the corresponding Bethe ansatz equations.

In order to enable semiconductor-based quantum computing with many qubits, issues like residual interqubit coupling and constraints from scalable control hardware need to be tackled to retain the high gate fidelities demonstrated in current single-qubit devices. Here we focus on two exchange-coupled singlet-triplet spin qubits, considering realistic control hardware as well as Coulomb and exchange coupling that cannot be fully turned off. Using measured noise spectra, we optimize realistic control pulses and show that two-qubit (single-qubit) gate fidelities of 99.90% ($\ge 99.69\%$) can be reached in GaAs, while 99.99% ($\ge 99.95\%$) can be achieved in Si.

We design ultra-thin, entirely flat, dielectric lenses using crystal momentum transfer, so-called Umklapp processes, achieving the required wave control for a new mechanism of flat lensing; physically, these lenses take advantage of abrupt changes in the periodicity of a structured line array so there is an overlap between the first Brillouin zone of one medium with the second Brillouin zone of the other. At the interface between regions of different periodicity, surface, array guided waves hybridize into reversed propagating beams directed into the material exterior to the array. This control, and redirection, of waves then enables the device to emulate a Pendry-Veselago lens that is one unit cell in width, with no need for an explicit negative refractive index. Simulations using an array embedded in an idealized slab of silicon nitride (${\text{Si}}_3{\text{N}}_4$) in air, operating at visible wavelengths between $420\text{–}500\text{THz}$ demonstrate the effect.

We show that lattices with higher-order topology can support corner-localized bound states in the continuum (BICs). We propose a method for the direct identification of BICs in condensed matter settings and use it to demonstrate the existence of BICs in a concrete lattice model. Although the onset for these states is given by corner-induced filling anomalies in certain topological crystalline phases, additional symmetries are required to protect the BICs from hybridizing with their degenerate bulk states. We demonstrate the protection mechanism for BICs in this model and show how breaking this mechanism transforms the BICs into higher-order topological resonances. Our work shows that topological states arising from the bulk-boundary correspondence in topological phases are more robust than previously expected, expanding the search space for crystalline topological phases to include those with boundary-localized BICs or resonances.

We show that antiferromagnetic spin-density wave order in the two-dimensional Hubbard model yields a drop of the charge carrier density as observed in recent transport measurements for cuprate superconductors in high magnetic fields on entering the pseudogap regime. The amplitude and the (generally incommensurate) wave vector of the spin-density wave is obtained from dynamical mean-field theory (DMFT). An extrapolation of the finite temperature results to zero temperature yields an approximately linear doping dependence of the magnetic gap $\mathrm{\Delta }\left(p\right)\propto p^{*}-p$ in a broad doping range below the critical doping $p^{*}$. The magnetic order leads to a Fermi surface reconstruction with electron and hole pockets, where electron pockets exist only in a restricted doping range below $p^{*}$. The dc charge transport properties are computed by combining the renormalized band structure as obtained from the DMFT with a doping-independent phenomenological scattering rate. A pronounced drop of the longitudinal conductivity and the Hall number in a narrow doping range below $p^{*}$ is obtained.

Silicon carbide (SiC) hosts many interesting defects that can potentially serve as qubits for a range of advanced quantum technologies. Some of them have very interesting properties, making them potentially useful, e.g., as interfaces between stationary and flying qubits. Here we present a detailed overview of the relevant properties of the spins in silicon vacancies of the $6H$-SiC polytype. This includes the temperature-dependent photoluminescence, optically detected magnetic resonance, and the relaxation times of the longitudinal and transverse components of the spins during free precession as well as under the influence of different refocusing schemes.

The goal of this work is to build a dynamical theory of defects for quantum spin systems. This is done by explicitly giving an exhaustive case study of a one-dimensional spin chain with $\mathbf{Vec}(\mathbb{Z}/2\mathbb{Z})$ fusion rules, which can easily be extended to more general settings. A kinematic theory for an indefinite number of defects is first introduced exploiting distinguishable Fock space. Dynamics are then incorporated by allowing the defects to become mobile via a microscopic Hamiltonian. This construction is extended to topologically ordered systems by restricting to the ground state eigenspace of Hamiltonians generalizing the golden chain. Technically, this is done by employing generalized tube algebra techniques to model the defects in the chain. We illustrate this approach for the $\mathbf{Vec}(\mathbb{Z}/2\mathbb{Z})$ spin chain, in whose case the resulting dynamical defect model is equivalent to the critical transverse Ising model.

The magnetization reversal process in an interconnected kagome artificial spin ice has been investigated using high-resolution Kerr microscopy. The magnetization switching with two-dimensional system-size avalanches has been directly imaged for the applied field parallel to one of the sublattice branches, suggesting very low disorder in our sample. The discrete one-dimensional Dirac-string-like magnetization reversal is found for the field with a certain angle. The real-time imaging under the field suggests that the reversal of the third bars next to the reversed Dirac string can mediate the reversal of the neighboring Dirac string. The observed reversal behaviors can be understood by the ice rule, consistent with the micromagnetic simulation results. Our studies demonstrate that Kerr microscopy can be applied to study the magnetization reversal process in artificial spin ice structures in real space and real time under an external magnetic field.

Magnetic hysteresis is a direct manifestation of nonequilibrium physics that has to be understood if a system is to be used for information storage and processing. The dilanthanide endofullerene ${\mathrm{Tb}}_2\mathrm{ScN}@{\mathrm{C}}_{80}$ is shown to be a single-molecule magnet with a remanence time on the order of 100 s at 400 mK. Three different temperature-dependent relaxation barriers are discerned. The lowest 1 K barrier is assigned to intermolecular dipole-dipole interaction, the 10 K barrier to intramolecular exchange and dipolar coupling, and the 50 K barrier to molecular vibrations as was observed for ${\mathrm{Dy}}_2\mathrm{ScN}@{\mathrm{C}}_{80}$. The 4 orders of magnitude difference in the prefactor between the Tb and the Dy compound in the decay process across the 10 K barrier is assigned to the lack of Kramers protection in ${\mathrm{Tb}}^{3+}$. The sub-Kelvin hysteresis follows changes in the magnetization at level crossings of the four possible ${\mathrm{Tb}}_2$ ground-state configurations. Comparison to a hysteresis model, with magnetic relaxation at level crossings only, reveals cooperative action between nearby molecules.

We report achievement of a depairing current limit along the $c$ axis in ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{1-x}{\mathrm{Se}}_x$ single crystals. A series of crystals with $T_{\mathrm{c}}$ ranging from 8.6 K to 13.7 K (different amounts of excess Fe, $y$) were fabricated into $c$-axis bridges with a square-micrometer cross-section. The critical current density, $J_{\mathrm{c}}$, was directly estimated from the transport current-voltage measurements. The transport $J_{\mathrm{c}}$ reaches a very large value, which is about one order of magnitude larger than the depinning $J_{\mathrm{c}}$, but comparable to the calculated depairing $J_{\mathrm{c}}\sim 2\times {10}^6$ $\mathrm{A}/{\mathrm{cm}}^2$ at 0 K, based on the Ginzburg-Landau (GL) theory. The temperature dependence of the depairing $J_{\mathrm{c}}$ follows the GL theory [$\propto$(1-$T/T_{\mathrm{c}}{)}^{3/2}$] down to $\sim 0.83T_{\mathrm{c}}$, then increases with a reduced slope at low temperatures, which can be qualitatively described by the Kupriyanov-Lukichev theory. Our study provides a route to understand the behavior of depairing $J_{\mathrm{c}}$ in iron-based superconductors in a wide temperature range.

Microscopic details such as interactions and Fermiology determine the structure of superconducting pairing beyond the spatial symmetry classification along irreducible point group representations. From the effective pairing vertex, the pairing wave function related to superconducting order unfolds in its orbital-resolved Fourier profile which we call the harmonic fingerprint (HFP). The HFP allows us to formulate a concise connection between microsopic parameter changes and their impact on superconductivity. From a random phase approximation analysis of twisted bilayer graphene (TBG) involving $d+id$-, $s_{\pm }$-, and $f$-wave order, we find that nonlocal interactions, which unavoidably enter the low-energy electronic description of TBG, not only increase the weight of higher lattice harmonics but also have a significant effect on the orbital structure of these pairing states. For gapped unconventional superconducting order such as $s_{\pm }$ and $d+id$, a change in HPF induces enhanced gap anisotropies. Experimental implications to distinguish the different gaps and HPFs are also discussed.

Based on density-functional first-principles calculations and the Wannier interpolation technique, we have investigated the electron-phonon coupling (EPC) in hole-doped monolayer hexagonal boron nitride (h-BN). We align the Fermi level to a Van Hove singularity by 0.4 holes/cell doping, the largest hole concentration that can be realized in monolayer h-BN. Under this doping level, the EPC for free-standing monolayer h-BN is weak. However, biaxial tensile strain can enhance the EPC significantly. We predict that monolayer h-BN can become a phonon-mediated superconductor by the synergic effects of hole doping and biaxial tensile strain, with transition temperature exceeding 40 K.

Designing high-performance piezoelectric materials based on atomic-scale calculations is highly desired in recent years, following the understanding of the structure-property relationship of state-of-the-art piezoelectric materials. Previous mesoscale simulations showed that local structural heterogeneity plays an important role in the piezoelectric property of ferroelectrics; that is, larger structural heterogeneity leads to higher piezoelectricity. In this Rapid Communication, by combining first-principles calculations and experimental characterizations, we explored the atomic-scale origin of the high piezoelectricity for samarium-doped $\mathrm{Pb}\left(\mathrm{M}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}\right){\mathrm{O}}_3\text{−}\mathrm{PbTi}{\mathrm{O}}_3$ (PMN-PT) ceramics, which possesses the highest piezoelectric $d_{33}$ of $\sim 1500\mathrm{pC}{\mathrm{N}}^{-1}$ among all known piezoelectric ceramics. The impacts of various dopants on local structure and piezoelectric properties of PMN-PT ceramics were investigated in terms of the effective ionic radius and cation valence. Our results show that A-site dopants with a valence of 3+ are more effective to produce local structural heterogeneity in PMN-PT when compared with the A-site dopants with a valence of 2+, and a smaller dopant size leads to a larger variation of local structure. According to this study, the outstanding piezoelectricity in Sm-doped PMN-PT ceramics is attributed to the fact that $\mathrm{S}{\mathrm{m}}^{3+}$ is the smallest ions that can entirely go to the A site of PMN-PT rather than the B site. The present work may benefit the design of high-performance piezoelectric materials based on the concept of local structural engineering.

The successful preparation of the freestanding perovskite materials down to the monolayer limit [Ji et al., Nature (London) 570, 87 (2019)] provided the opportunity to make the two-dimensional (2D) oxide and heterostructure, which could be significantly distinctive from the conventional oxide superlattices and other 2D van der Waals heterostructures. By stacking one unit-cell $\mathrm{BaTi}{\mathrm{O}}_3$ (BTO) and one unit-cell $\mathrm{SrTi}{\mathrm{O}}_3$ (STO) on top of each other, we constructed two isolated bilayers of the 2D heterostructure systems. From our density functional theory simulation, their ground states exhibit an in-plane ferroelectricity in both BTO layer and STO layer, while the antiferrodistortive mode of the STO layer is totally suppressed. These two systems show band gaps in the range of 2–2.5 eV (by using HSE06), which are smaller than their monolayer and bulk phases. The layer arrangement strongly influences their electronic properties. We reveal that they adopt the type-II electronic band alignment. The tensile biaxial strain can strongly promote the ferroelectricity and increase the band gaps of these systems. Our results will contribute to the further understanding of layered materials based on the transition metal oxide perovskites and developing relevant experimental devices.

Macroscale experiments for structural response of materials often do not capture deformation features at the atomistic scale. This limitation becomes more pronounced in materials subjected to shock loading, especially in covalently bonded ceramics such as boron carbide, which exhibit a deleterious mechanism called pressure-induced amorphization. Perhaps because ceramics have high thermal resistance, temperature was never considered a prime factor influencing deformation mechanisms, leaving such phenomena unexplained for more than two decades. In this article, we probe fundamental material behavior using molecular dynamics (MD) to link nanoscale thermodynamics to amorphization during shock loading. We have generated Rayleigh lines via MD shock simulations, subsequently constructing the shock Hugoniot, which aligns well with experimental data. We estimate shock-induced temperature in ceramics to be well above their melting point, resolving a plethora of interconnected scientific issues pertaining to anomalous behavior of boron carbide.

We have shown that ferromagnetic double perovskite ${\mathrm{Gd}}_2{\mathrm{CoMnO}}_6$ exhibits the characteristics of clustered phase, which are quite different from that of Griffiths phase observed in several perovskite compounds. The Yang-Lee theory of phase transition predicts that the essential singularity of the Griffiths phase leads to very unusual critical phenomena, including a discontinuity in the magnetization isotherm at $T=T_C$, which is reflected through the large value of critical exponent $\delta$. However, the critical exponent $\delta$ for ${\mathrm{Gd}}_2{\mathrm{CoMnO}}_6$, determined from magnetization scaling analysis, comes out to be very small ($\delta =1.55\pm 0.03$). The small value of $\delta$ suggests that the continuous ordering is slow, indicating non-Griffiths-like cluster formation in the studied system, which is further supported by the evolution of susceptibility with temperature and magnetic field. Also, the observed values of all the three exponents $\beta$ ($=1.18\pm 0.06$), $\gamma$ ($=0.65\pm 0.01$), and $\delta$ are far from any existing universality class and they deviate from the mean-field values in the opposite direction to that for the conventional universality.