The single-photon emitters (SPEs) are essential components of emerging quantum communication technology. It has been shown that the local defects in the wide-bandgap semiconductors can be sources of sharp zero-phonon lines (ZPLs) of emission, which is favorable for applications. Most of the reported SPEs emit photons in the visible light region, while fiber-based quantum communication applications require sources of near-infrared (NIR) emission. Therefore, the purpose of this paper is to search for defects which are promising candidates for SPEs in the NIR region by applying computational methods. To this end, based on an educated guess, 12 local doping substitutional defects in two-dimensional (2D) gallium sulfide were selected for consideration. The selected defects are $Y_X$, where $X=\mathrm{Ga}$ or S, and $Y=\mathrm{C}$, Si, Ge, N, P, As. Here, $X$ stands for a substituted element and $Y$ for a dopant. To evaluate the stability of the defects, their formation energy, binding energy of the dopants, and phonon spectra were calculated within the density functional theory (DFT) approximation. Three defects, ${\mathrm{Ge}}_{\mathrm{Ga}}, {\mathrm{N}}_{\mathrm{Ga}}$, and ${\mathrm{P}}_{\mathrm{Ga}}$, are found to have imaginary-frequency vibration modes, indicating their dynamical instability. They were removed from further consideration. A preliminary analysis of the DFT densities of electronic states (DOS) revealed six remaining defects, ${\mathrm{As}}_{\mathrm{Ga}}, {\mathrm{Ge}}_{\mathrm{S}}, {\mathrm{Si}}_{\mathrm{S}}, {\mathrm{C}}_{\mathrm{S}}, {\mathrm{N}}_{\mathrm{S}}$, and ${\mathrm{P}}_{\mathrm{S}}$, with DOS favorable for formation of local optical excitations in the NIR region. Next, for these six defects, the state-of-the-art linear response GW and the Bethe-Salpeter equation (BSE) methods were used to obtain their electronic structures, in terms of the independent quasiparticle (IQP) states, and optical excitation spectra, in terms of the frequency-dependent dielectric function and the dipole optical transition oscillator strength. The calculation results indicate that the ${\mathrm{C}}_{\mathrm{S}}, {\mathrm{N}}_{\mathrm{S}}$, and ${\mathrm{P}}_{\mathrm{S}}$ defects have intense sharp excitation peaks in the NIR region. The analysis of the contributions of the IQP states to the corresponding BSE eigenstates brings us to the conclusion that these excitations can result in emission with dominating narrow ZPL peaks. I thus propose here the ${\mathrm{C}}_{\mathrm{S}}, {\mathrm{N}}_{\mathrm{S}}$, and ${\mathrm{P}}_{\mathrm{S}}$ defects in 2D GaS as promising candidates for SPEs in the NIR emission region.

We investigated morphology evolution of 500-eV ${\mathrm{Ar}}^{+}$ sputtered Si surfaces at an incidence of ${67}^{\circ }$ using an unconventional method of substrate swinging by different azimuthal angles ($▵\varphi$ from $0^{\circ }\to {360}^{\circ }$) and speeds up to 16 rotations per minute (RPM). The samples displayed four different regimes when they were swung azimuthally by different angles at a speed of 1 RPM. Initially, a hierarchical structure (regime 1) comprised of ripples and triangles was obtained which gave way to only ripples (regime 2) at $▵\varphi ={80}^{\circ }$. A narrow third regime showed a completely flat surface at ${100}^{\circ }$. Above this angle, only disordered ripples devoid of any triangles were obtained up to ${360}^{\circ }$ (regime 4). This regime also demonstrated drastic changes in orientation of the ripple wave vector at certain angles of $▵\varphi$. The wavelengths and roughnesses decreased with higher azimuthal angles. Our observations were found to be highly reproducible. Upon swinging the samples for $▵\varphi ={70}^{\circ }$ at higher speeds resulted in disordered ripple structures with smoother surfaces. Ripples were found to be the most ordered for 1 RPM speed. In contrast to the above, samples rotated continuously for different durations, when compared with a static case, displayed isotropically roughened surfaces. Two-dimensional slope distributions of the morphologies demonstrated formation of asymmetric ripple structures on the surfaces. Our results were explained in the light of linear and nonlinear regimes of sputtering. The crucial role played by dispersive linear terms explained the formation of the hierarchical structures at small swing angles. Once the dispersive effects die down, the ripples change their orientation. The asymmetry in surface structures was explained by the near-surface mass transport phenomenon at oblique azimuthal angles. This study demonstrates the role of this unconventional technique to drive a system towards abrupt morphological transitions not observed otherwise.

As an intrinsically unbiased method, the quantum Monte Carlo (QMC) method is of unique importance in simulating interacting quantum systems. Although the QMC method often suffers from the notorious sign problem, the sign problem of quantum models may be mitigated by finding better choices of the simulation scheme. However, a general framework for identifying optimal QMC schemes has been lacking. Here, we propose a general framework using automatic differentiation to automatically search for the best QMC scheme within a given ansatz of the Hubbard-Stratonovich transformation, which we call “automatic differentiable sign optimization” (ADSO). We apply the ADSO framework to the honeycomb lattice Hubbard model with Rashba spin-orbit coupling and demonstrate that ADSO is remarkably effective in mitigating and even solving its sign problem. Specifically, ADSO finds a sign-free point in the model which was previously thought to be sign-problematic. For the sign-free model discovered by ADSO, its ground state is shown by sign-free QMC simulations to possess spiral magnetic ordering; we also obtained the critical exponents characterizing the magnetic quantum phase transition.

We investigate the characteristics of high harmonics (HHs) unique to the nonperturbative nonlinear regime. We show that the polarization state of HHs generated from GaAs changes drastically across the crossover from the weak-field perturbative regime to the strong-field extreme nonlinear regime, while the linearly polarized infrared excitation field $\left(E_{\mathrm{exc}}\right)$ is fixed to a particular crystal direction. The dependence on the $E_{\mathrm{exc}}$-field strength reveals that multiple emission processes with different nonlinear orders and temporal phases contribute to each order HH, and the interference among them plays a pivotal role. This interference manifests itself as a unique phenomenon: a large HH ellipticity emerges in the course of crossover, despite the fact that GaAs hosts no magnetization or linear birefringence. These results demonstrate that not only the material's symmetry but also the ultrafast nonlinear dynamics largely affects the HH polarization, and hence, HH polarization and its $E_{\mathrm{exc}}$-field dependence provide a useful experimental tool to probe ultrafast coherent dynamics in light-driven solid-state materials.

Ferroelectrics are technologically important, with wide application in micromechanical systems, nonlinear optics, and information storage. Recent discoveries of exotic polarization textures in these materials, which can strongly influence their properties, have brought to the forefront questions about the nature of their domain walls: long believed to be primarily Ising, with locally null polarization. Here, combining three complementary techniques—second-harmonic generation microscopy, piezoresponse force microscopy, and transmission electron microscopy—to cover all the relevant length scales, we reveal the Néel character (non-Ising polarization oriented perpendicular to the wall) of ${180}^{\circ }$ domain walls in $c$-phase tetragonal ferroelectric lead titanate epitaxial thin films, for both artificial and intrinsic domains at room temperature. Furthermore, we show that variations in the domain density—detected both optically and via local piezoresponse and then quantified by radial autocorrelation analysis—can give us insight into the underlying defect potential present in these materials.

Topological superconductivity in one dimension requires time-reversal symmetry breaking, but at the same time, it is hindered by external magnetic fields. We offer a general prescription for inducing topological superconductivity in planar superconductor-normal-superconductor-normal-superconductor (SNSNS) Josephson junctions without applying any magnetic fields on the junctions. Our platform relies on two key ingredients: the three parallel superconductors form two superconductor-normal-superconductor junctions with phase winding, and the Fermi velocities for the two spin branches transverse to the junction must be different from one another. The two phase differences among the three superconductors define a parameter plane which includes large topological regions. We analytically derive the critical curves where the topological phase transitions occur and corroborate the result with a numerical calculation based on a tight-binding model. We further propose material platforms with unequal Fermi velocities, establishing the experimental feasibility of our approach.

A novel atomistic effective Hamiltonian scheme, incorporating an original and simple bilinear energetic coupling, is developed and used to investigate the temperature-dependent physical properties of the prototype antiferroelectric ${\mathrm{PbZrO}}_3$ (PZO) system. This scheme reproduces very well the known experimental hallmarks of the complex Pbam orthorhombic phase at low temperatures and the cubic paraelectric state of $\mathrm{Pm}\overline{3}\mathrm{m}$ symmetry at high temperatures. Unexpectedly, it further predicts a novel intermediate state also of Pbam symmetry, but in which antiphase oxygen octahedral tiltings have vanished with respect to the Pbam ground state. Interestingly, such a new state exhibits a large dielectric response and thermal expansion that remarkably agrees with previous experimental observations and the x-ray experiments we performed. We also conducted direct first-principles calculations at 0 K, which further support such a low-energy phase. Within this fresh framework, a reexamination of the properties of PZO is thus called for.

Unconventional Weyl phonons with higher topological charges in crystalline solids have attracted increasing attention. By symmetry analysis and a low-energy $\mathbf{k}·\mathbf{p}$ effective Hamiltonian, we propose the symmetry-enforced triple hourglass Weyl phonons (THWPs) with Chern number $\mathcal{C}=\pm 3$ protected by $6_3$ screw rotation symmetry in chiral space groups 173 $\left(P{6}_3\right)$ and 182 $\left(P{6}_{3}22\right)$. We take ${\mathrm{LiIO}}_3$ with space group 173 as a candidate and confirm that it possesses THWP with linear and quadratic dispersions along the $k_z$ direction and in the $k_x\text{−}{k}_y$ plane, respectively. Due to the constraints of crystal symmetry and topological charge conservation, six equivalent single Weyl phonons (SWPs) emerge and lie in the $k_z=0$ plane. Therefore, the unique phonon surface arcs connect the projections of two THWPs and six SWPs, leading to nontrivial sextuple-helicoid surface arcs on the (001) surface Brillouin zone. Our work proposes a class of topological phonons and realizes it in realistic materials, providing a perfect platform for experimental observation of THWPs. We expect our work to provide a new idea for detection of unconventional quasiparticles.

The unconventional Weyl point with nonlinear dispersion features higher topological charge $\left|\mathcal{C}\right|>1$ and multiple topologically protected Fermi arc states at its boundary. As a novel topological state, it has been attracting widespread attention. However, the unconventional Weyl point with $\left|\mathcal{C}\right|=3$ has not yet been reported in realistic materials, even though it has been theoretically proposed for more than a decade. In this work, based on first-principles calculations and theoretical analysis, we predict the existing material $\alpha \text{−}{\mathrm{LiIO}}_3$ as the first realistic example with this unconventional Weyl point. Specifically, in the phonon spectra of $\alpha \text{−}{\mathrm{LiIO}}_3$, two Weyl points with $\mathcal{C}=-3$, connected by time-reversal symmetry, appear at the neck crossing point of an hourglass-type band, leading to two hourglass charge-3 Weyl phonons. The symmetry protection and the associated novel triple- and sextuple-helicoid surface arc states of the hourglass charge-3 Weyl phonons are revealed. Our results uncover a hidden topological character of $\alpha \text{−}{\mathrm{LiIO}}_3$ and also show that the phonon spectra are a great platform for exploring unconventional topological states.

Many-body phenomena far from equilibrium present challenges beyond reach by classical computational resources. Digital quantum computers provide a possible way forward but noise limits their use in the near term. We propose a scheme to simulate and characterize many-body Floquet systems hosting a rich variety of phases that operates with a shallow circuit, defined as a quantum circuit that does not scale with system sizes. Starting from a periodic circuit that simulates the dynamical evolution of a Floquet system, we introduce quasiperiodicity to the circuit parameters to prevent thermalization by introducing many-body localization. By inspecting the time-averaged properties of the many-body integrals of motion, the phase structure can then be probed using random measurements. This approach avoids the need to compute the ground state and operates at finite energy density. We numerically demonstrate this scheme with a simulation of the Floquet Ising model of time crystals and present results clearly distinguishing different Floquet phases that are protected by many-body localization. Our results pave the way for mapping out phase diagrams of exotic systems on near-term quantum devices.

Quantum many-body scar (QMBS) and quantum integrability (QI) have been recognized as two distinct mechanisms for the breakdown of eigenstate thermalization hypothesis (ETH) in an isolated system. In this paper, we reveal a smooth route to connect these two ETH-breaking mechanisms in the Ising chain with transverse and longitudinal fields. Specifically, starting from an initial Ising antiferromagnetic state, we find that the dynamical system undergoes a smooth nonthermal crossover from QMBS to QI by changing the Ising coupling ($J$) and longitudinal field ($h$) simultaneously while keeping their ratio fixed, which corresponds to the Rydberg Hamiltonian with an arbitrary nearest-neighbor repulsion. Deviating from this ratio, we further identify a continuous thermalization trajectory in the ($h,J$) plane that is exactly given by the Ising transition line, signifying an intimate relation between thermalization and quantum critical point. Finally, we map out a completely different dynamical phase diagram starting from an initial ferromagnetic state, where the thermalization is shown to be equally facilitated by the resonant spin flip at special ratios of $J$ and $h$. By bridging QMBS and QI in Ising chains, our results demonstrate the breakdown of ETH in much broader physical settings, which also suggest an alternative way to characterize quantum phase transition via thermalization in nonequilibrium dynamics.

We theoretically study the generation of spin current due to a surface acoustic wave (SAW) in a superconductor. We model an $s$-wave superconductor as the mean-field Hamiltonian and calculate spin current generated via spin-vorticity coupling based on quantum kinetic theory. The results suggest that the spin current can be driven in a single-superconductor layer, and our estimation suggests that the detectable magnitude of the spin current can be generated in aluminum. Our proposal may contribute to the advancement of spin transport in superconductors from application and fundamental physics aspects.

We report the synthesis, crystal structure, and magnetic properties of ${\mathrm{Co}}^{2+}$ kagome magnets ${\mathrm{Co}}_3{\mathrm{V}}_2{\mathrm{O}}_7{\left(\mathrm{OH}\right)}_2·2{\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{BaCo}}_3$(${\mathrm{VO}}_4{)}_2{\left(\mathrm{OH}\right)}_2$, which can be recognized as Co analogs of the intensively researched quantum kagome magnet volborthite ${\mathrm{Cu}}_3{\mathrm{V}}_2{\mathrm{O}}_7{\left(\mathrm{OH}\right)}_2·2{\mathrm{H}}_2\mathrm{O}$ and vesignieite ${\mathrm{BaCu}}_3$(${\mathrm{VO}}_4{)}_2{\left(\mathrm{OH}\right)}_2$. For each compound, the ground state is seemingly $A$-type antiferromagnetic order. At low temperatures, applying a magnetic field causes a metamagneticlike transition described by the transition in which antiferromagnetically aligned canted moments change to ferromagnetically aligned ones. These ground and field-induced states include a canted ferromagnetic component perpendicular to the kagome planes favored by Dzyaloshinskii-Moriya interactions. These magnetic properties are well characterized by the $J_{\mathrm{eff}}=\frac{1}{2}$ physics. Our findings will be a step toward clarifying the $J_{\mathrm{eff}}=\frac{1}{2}$ kagome physics, which has not been thoroughly studied experimentally or theoretically.

The equiatomic face-centered-cubic (fcc) high-entropy alloy CrMnFeCoNi has a valence electron concentration of 8 electrons per atom, equivalent to that of Fe. Its ground-state properties are similar to that of fcc-Fe and exhibits the anti-Invar effect at finite temperatures. It additionally exhibits various magnetic interactions at low temperatures which are not fully understood. To resolve this, we prepare and investigate the magnetic and structural properties of off-equiatomic ${\mathrm{Cr}}_{20+x}{\mathrm{Mn}}_{20}{\mathrm{Fe}}_{20}{\mathrm{Co}}_{20}{\mathrm{Ni}}_{20-x}$ high-entropy alloys with $0\le x\le 20$ at.% by making use of the composition-dependent transformation from the fcc to the $\sigma$ phase with increasing $x$. We employ x-ray diffraction, scanning electron microscopy, magnetization, and microhardness studies. The results suggest that these alloys order antiferromagnetically around 75–90 K before entering into a spin-frozen state below 40 K.

X-ray magnetic circular dichroism (XMCD), which by virtue of the sum rules provides element-specific spin and orbital moments, is obtained from the difference between two polarized spectra by reversing the direction of either the light helicity or the applied magnetic field. Usually, it is tacitly assumed that these two spectra are obtained using the same absolute degree of light and magnetic polarization. This is, however, not always possible and depends on circumstances that can be beyond control. First, we recapitulate the conventional XMCD sum rule method to obtain the values of the moments and emphasize some of the complications in the case of the rare-earth $M_{4,5}$ edges, such as the presence of strong core-hole $jj$ overlap, linear dichroism, and magnetic dipole term $\langle T_z\rangle$. Instead, we propose an alternative method. Using the individual polarized x-ray absorption spectra obtained at the Ho and Dy $M_5$ edges, where each of the $\mathrm{\Delta }J=-1,0$, and $+1$ transitions are separated by $\sim 2$ eV in photon energy, we are able to determine independently the degree of circular dichroism in a single spectrum. Since light is a transverse wave, we need to include, apart from the circular dichroism, also a linear dichroism contribution in order to fit the circularly polarized spectra. In the measurements on paramagnetic rare-earth dopants it was found that reversing the field produces the same degree of circular dichroism, while reversing the helicity yields a $\sim$20% difference in the degree of circular dichroism.

Superconductivity in binary ruthenium pnictides occurs proximal to and upon suppression of a nonmagnetic ground state, preceded by a pseudogap phase associated with Fermi surface instability, and its critical temperature, $T_c$, is maximized around the pseudogap quantum critical point. By analogy with isoelectronic iron-based counterparts, antiferromagnetic fluctuations became “usual suspects” as putative mediators of superconducting pairing. Here we report on a high-temperature local symmetry breaking in RuP, the nonsuperconducting parent of the maximum-$T_c$ branch of these novel superconductors, revealed by combined nanostructure-sensitive powder and single-crystal x-ray total scattering analyses. Large local distortions of Ru chains associated with orbital-charge trimerization, further assembled into hexamers, exist above the two-stage electronic transition in RuP. In the pseudogap regime the precursors order with distortions retaining their strength, whereas they acquire spin-singlet characteristics which dramatically enhances the distortion as the nonmagnetic ground state establishes. The precursors enable the nonmagnetic ground state and presumed complex oligomerization, and the relevance of pseudogap fluctuations for superconductivity emerges as a distinct prospect. As a transition metal system in which partial d-manifold filling combined with high crystal symmetry promotes electronic instabilities, this represents a further example of local electronic precursors underpinning the macroscopic collective behavior of quantum materials.