We study the stroboscopic dynamics of hinge modes of a second-order topological material modeled by a tight-binding free fermion Hamiltonian on a cubic lattice in the intermediate drive frequency regime for both discrete (square pulse) and continuous (cosine) periodic drive protocols. We analyze the Floquet phases of this system and show that its quasienergy spectrum becomes almost gapless in the large drive amplitude regime at special drive frequencies. Away from these frequencies, the gapped quasienergy spectrum supports weakly dispersing Floquet hinge modes. Near them, these hinge modes penetrate into the bulk and eventually become indistinguishable from the bulk modes. We provide an analytic, albeit perturbative, expression for the Floquet Hamiltonian using Floquet perturbation theory (FPT), which explains this phenomenon and leads to analytic expressions of these special frequencies. We also show that in the large drive amplitude regime, the zero-energy hinge modes corresponding to the static tight-binding Hamiltonian display qualitatively different dynamics at these special frequencies. We discuss possible local density of state measurement using a scanning tunneling microscope, which can test our theory.

Traditionally, skyrmions are treated as continuum magnetic textures of classical spins with a well-defined topological skyrmion number. Owing to their topological protection, skyrmions have attracted great interest as building blocks in future memory technology. Smaller skyrmions offer greater memory density; however, quantum effects are not negligible for skyrmions with sizes of just a few lattice constants. In this paper we study quantum fluctuations around dense skyrmion crystal ground states, and focus on the utility of a discretized order parameter for quantum skyrmions. The order parameter is found to be a useful indicator of the existence of quantum skyrmions, and captures the quantum phase transition between two distinct skyrmion phases.

We report the experimental observation of net spin-orbit torques (SOT) in single conductive ferromagnets (FMs), e.g., ${\mathrm{Fe}}_{20}{\mathrm{Ni}}_{80}\left(\mathrm{Py}\right)$, Co, and Ni, despite the system symmetry constraint. Unlike the traditional FM/heavy metal (HM) bilayers, where only the in-plane (IP) damping-like SOT is presented, an unexpected out-of-plane (OOP) damping-like SOT is also unequivocally observed in the single FMs. Using the field angular-dependent, spin-torque ferromagnetic resonance technique, we quantify the IP and OOP SOT efficiencies of 0.018 ± 0.003 and 0.047 ± 0.002 in the pure Py, respectively. We argue that the IP SOT is primarily related to the Py bulk spin Hall effect, while the unconventional SOT is ascribed to the out-of-plane polarized spin current generated from the nonequilibrium spin swapping effect. Additionally, we confirm that such self-induced SOTs also generally exist in the most studied HM/FM multilayer systems, especially for the HMs with low conductivity. Hence, our findings provide a new perspective on understanding SOT control of magnetization reversal and dynamics by an in-plane current in thin, single FM and FM/HM bilayer systems.

We analyze theoretically two different setups for $s$-wave superconductivity proximitized $j=\frac{3}{2}$ particles in Luttinger materials that are able to host Majorana bound states (MBSs). First, we consider a one-dimensional superconductor (SC) wire with intrinsic bulk inversion asymmetry (BIA). In contrast to wires, modeled by a quadratic dispersion with Rashba spin-orbit coupling, there are two topological phase transitions in our systems at finite magnetic fields. Second, we analyze a two-dimensional Josephson junction on the Luttinger model finding a topological region even in the absence of BIA and Rashba spin-orbit couplings. This originates from the hybridization of the light and heavy hole bands of the $j=\frac{3}{2}$ states in combination with the SC pairing. As a consequence, both systems can be driven into a topological phase hosting MBSs. Hence, we predict that MBSs form in any SC proximitized Josephson junction on two-dimensional Luttinger materials by the application of magnetic field alone. This opens an avenue for the search of topological SC.

A single layer of the $1T^{\prime }$ phase of ${\mathrm{WTe}}_2$ provides a rich platform for exotic physical properties such as the nonlinear Hall effect and high-temperature quantum spin Hall transport. Utilizing a continuum model and the diagrammatic method, we calculate the second harmonic conductivity of monolayer $1T^{\prime }\text{−}{\mathrm{WTe}}_2$ modulated by an external vertical electric field and electron doping. We obtain a finite helicity and Faraday rotation for the second harmonic signal in response to linearly polarized incident light in the presence of time-reversal symmetry. The second harmonic signal's helicity is highly controllable by altering the bias potential and serves as an optical indicator of the nonlinear Hall current. Our study motivates future experimental investigation of the helicity spectroscopy of two-dimensional materials.

Due to the atomic level thickness and novel properties, two-dimensional (2D) materials have received extensive attention on the research and application of future nanodevices. In this paper, the mechanical and electronic properties of $\alpha \text{−}{M}_2{X}_3$ ($M$ = Ga, In; $X$ = S, Se) monolayers are studied to explore their applications in 2D electronic devices. First-principles calculations based on density functional theory indicate that these four $\alpha \text{−}{M}_2{X}_3$ monolayers are all semiconductors and possess Young's modulus of less than 100 ${\mathrm{N}\mathrm{m}}^{-1}$ with a deformation range up to about 30%. In addition, the carrier mobility of the $\alpha \text{−}{M}_2{X}_3$ monolayer exceeds 600 ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ and remains high under strain. In particular, because the band edge shifts under compressive strain, the electron mobility of the $\alpha \text{−}{\mathrm{Ga}}_2{\mathrm{S}}_3$ monolayer increases to about 1800 ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ at $-3\%$ strain, which is approximately three times the value without strain. The excellent ductility and strain-promoted electronic properties make the 2D $\alpha \text{−}{M}_2{X}_3$ promising candidates for the application of flexible electronic devices.

Studies of the Fermi surface modification after $\mathit{in}\mathit{situ}$ covering NbP semimetal with heavy elements Pb and Nb ultrathin layers were performed by means of angle-resolved photoemission spectroscopy (ARPES). First, the electronic structure was investigated for pristine single crystals with two possible terminations (P and Nb) of the (0 0 1) surface. The nature of the electronic states of these two cleaving planes is different: the P-terminated surface shows spoon and bow tie-shaped surface states, whereas these shapes are not present in the Nb-terminated surface. ARPES studies show that even one monolayer (ML) of Pb causes topological Lifshitz transition (TLT) in P-terminated NbP where the surface Fermi arcs (SFAs) teleport to another pair of Weyl points connecting two adjacent Brillouin zones. Depositing one ML of Pb modifies the Fermi surface along with a shift in the Fermi energy. On the other hand, the deposition of approximately 0.8 ML of Nb modifies the electronic structure of P-terminated NbP, pushing the system on the verge of TLT but not yet fully transformed. Regardless of the dramatic surface evolution, SFAs remain connected to topologically protected Weyl points. Additionally, we studied the Nb-terminated NbP covered with 1.9 ML of Pb with only altered trivial surface states caused by an ordinary Lifshitz transition.

Semiconductor quantum dots are promising building blocks for quantum communication applications. Although deterministic, efficient, and coherent emission of entangled photons has been realized, implementing a practical quantum repeater remains outstanding. Here we explore the statistical limits for entanglement swapping with sources of polarization-entangled photons from the commonly used biexciton-exciton cascade. We stress the necessity of tuning the exciton fine structure, and explain why the often observed time evolution of photonic entanglement in quantum dots is not applicable for large quantum networks. We identify the critical, statistically distributed device parameters for entanglement swapping based on two sources. A numerical model for benchmarking the consequences of device fabrication, dynamic tuning techniques, and statistical effects is developed, in order to bring the realization of semiconductor-based quantum networks one step closer to reality.

Light-assisted micronanoscale temperature control in a complex nanoparticle network has attracted a lot of research interest. Many efforts have been put into the optical properties of nanoparticle networks, and only a few investigations have reported its light-induced thermal behavior. We consider a two-dimensional (2D) square-lattice nanoparticle ensemble made of typical metal Ag with a radius of 5 nm. The effect of complex multiple scattering and thermal accumulation on light-induced thermal behavior in plasmonic resonance frequency (around 383 nm) is analyzed through the Green's function approach. The regime borders of both multiple scattering and thermal accumulation effects on the photothermal behavior of the 2D square-lattice nanoparticle ensemble are figured out clearly and quantitatively. The dimensionless parameter $\phi$ is defined as the ratio of a full temperature increase to that without considering the multiple scattering or thermal accumulation to quantify the multiple scattering and thermal accumulation effects on photothermal behavior. The more compact the nanoparticle ensemble is, the stronger the multiple scattering effect on thermal behavior is. When the lattice spacing increases to dozens times of the radius, the multiple scattering becomes insignificant. When $\phi \approx 1$ (lattice spacing increases to hundreds times of the radius), the thermal accumulation effects are weak and can be neglected safely. The polarization-dependent distribution of the temperature increase of nanoparticles is observed only in the compact nanoparticle ensemble, while for a dilute ensemble, such a polarization-dependent temperature increase distribution can no longer be observed. This work may help with the understanding of the light-induced thermal transport in the 2D particle ensemble.

We present a theoretical study of second-harmonic generation from a singular metasurface. The singular metasurfaces strongly interact with the incident light, where the large field enhancement forms an intense surface polarization that generates the second-harmonic field. By using transformation optics, the calculation of nonlinear optical response is converted from the metasurface frame to that of a simple slab geometry, largely reducing the complexity of the problem. In addition, the singular metasurface exhibits a weak dependence on the incident angle of light, which can be potentially used as an all-angle device for harmonic generations. Finally, we study the symmetry dependence of second-harmonic generation in the far field for the singular metasurface and show how to enhance the conversion efficiency under normal incidence by breaking the surface inversion symmetry.

Systematic variation of the low-temperature heat transport properties of the quasi-two-dimensional quantum spin liquid (QSL) candidate ${\beta }^{\prime }\text{−}{\mathrm{EtMe}}_3\mathrm{Sb}\left[\mathrm{Pd}\right({\mathrm{dmit})}_2{]}_2$ and its analog compounds showing antiferromagnetic (AFM) ordering is discussed using thermal conductivity $\kappa$ measurements. All compounds with the QSL ground state show a monotonic decrease in $\kappa$ with decreasing temperature, which is a typical feature of glass or amorphous materials, caused by their low-energy phonon structures. In contrast, the AFM compounds exhibit a peak structure of $\kappa$ around 10 K, which is the characteristic temperature dependence of normal crystals with coherent phonons. Our results indicate that the anomalous glassy behavior in ${\beta }^{\prime }\text{−}{\mathrm{EtMe}}_3\mathrm{Sb}\left[\mathrm{Pd}\right({\mathrm{dmit})}_2{]}_2$ is an intrinsic and universal feature of the organic QSL phase and is not caused by extrinsic factors, such as the quality of the samples and the microcracks occurring during the cooling process. We propose a scenario for the origin of the glassy behavior of QSL compounds in which the charge fluctuation within the Pd(${\mathrm{dmit})}_2$ dimers changes the phonon properties from crystalline to glasslike.

${\mathrm{Er}}^{3+}\text{:}{\mathrm{Y}}_2{\mathrm{SiO}}_5$ is a material of particular interest due to its suitability for telecom-band quantum memories and quantum transducers interfacing optical communication with quantum computers working in the microwave regime. Extending the coherence lifetimes of the electron spins and the nuclear spins is essential for implementing efficient quantum information processing based on such hybrid electron-nuclear spin systems. The electron spin coherence time of ${\mathrm{Er}}^{3+}\text{:}{\mathrm{Y}}_2{\mathrm{SiO}}_5$ is so far limited to several microseconds, and there are significant challenges in optimizing coherence lifetimes simultaneously for both the electron and nuclear spins. Here we perform a pulsed-electron-nuclear-double-resonance investigation for an ${\mathrm{Er}}^{3+}$-doped material at subkelvin temperatures. At the lowest working temperature, the electron spin coherence time reaches 290 $\pm$ 17 $\mathrm{\mu }\mathrm{s}$, which has been enhanced by 40 times compared with the previous results. In the subkelvin regime, a rapid increase in the nuclear spin coherence time is observed, and the longest coherence time of 738 $\pm$ 6 $\mathrm{\mu }\mathrm{s}$ is obtained. These extended coherence lifetimes could be valuable resources for further applications of ${\mathrm{Er}}^{3+}\text{:}{\mathrm{Y}}_2{\mathrm{SiO}}_5$ in fiber-based quantum networks.

Variational and diffusion quantum Monte Carlo methods are employed to investigate the zero-temperature phase diagram of the three-dimensional homogeneous electron gas at very low density. Fermi fluid and body-centered cubic Wigner crystal ground-state energies are determined using Slater-Jastrow-backflow and Slater-Jastrow many-body wave functions at different densities and spin polarizations in finite simulation cells. Finite-size errors are removed using twist-averaged boundary conditions and extrapolation of the energy per particle to the thermodynamic limit of infinite system size. Unlike previous studies, our results show that the electron gas undergoes a first-order quantum phase transition directly from a paramagnetic fluid to a body-centered cubic crystal at density parameter $r_{\text{s}}=86.6\left(7\right)$, with no region of stability for an itinerant ferromagnetic fluid.

Results are presented for Floquet systems in two spatial dimensions where the Floquet driving breaks an effective time-reversal symmetry. The driving protocol also induces flat bands that correspond to anomalous Floquet phases where the Chern number is zero and yet chiral edge modes exist. Analytic expressions for the edge modes, Berry curvature, and orbital magnetization are derived for the flat bands. Results are also presented for the static Haldane model for parameters when the bands are flat. Floquet driving of the same model is shown to give rise to Chern insulators as well as anomalous Floquet phases. The orbital magnetization for these different topological phases are presented and are found to be enhanced at half filling by the broken particle-hole symmetry of the Haldane model.

We have obtained and analyzed optical spectra of a semiconductor slab, containing a quantum well which serves as a source of magnetoexcitons in the presence of an external dc magnetic field. Our setup corresponds to the optical technique based on the breaking of the total internal reflection. Specifically, the semiconductor slab is sandwiched between two external leads/prisms separated from the slab by two gap layers of a dielectric or metal. The reflectivity, transmissivity, and absorption spectra for such a system in a quantizing magnetic field were revealed to display a resonant structure originating from the excitation of electromagnetic modes localized on the semiconductor slab which are strongly coupled with the confined magnetoexcitons. It is surprising that the Rabi splitting for the setup with dielectric gap layers (weak guiding) turns out to be greater than that for metallic and even semiconductor microcavities (strong guiding). Our results were verified by comparing the calculated optical spectra with the dispersion curves for localized modes derived analytically.

We use finite-temperature strong coupling expansions to calculate thermodynamic properties of the honeycomb-lattice SU(4) Hubbard model. We present numerical results for various properties including chemical potential, compressibility, entropy and specific heat as a function of temperature and density at several $U/t$ values. We study the onset of charge incompressibility and Mott gaps as the temperature is lowered at integer densities. In the incompressible Mott regime, the expansions are recast into a high-temperature expansion for a generalized spin model with SU(4) symmetry, which is then used to study the convergence of strong coupling expansions in $t/U$. We discuss lessons that can be drawn from high-temperature properties of a simple Hubbard model regarding twisted bilayer graphene and other magic-angle flat-band systems.

Many magnetic equilibrium states and phase transitions are characterized by fluctuations. Such magnetic fluctuation can, in principle, be detected with scattering-based x-ray photon correlation spectroscopy (XPCS). However, in the established approach of XPCS, the magnetic scattering signal is quadratic in the magnetic scattering cross section, which results not only in often prohibitively small signals but also in a fundamental inability to detect negative correlations (anticorrelations). Here, we propose to exploit the possibility of heterodyne mixing of the magnetic signal with static charge scattering to reconstruct the first-order (linear) magnetic correlation function. We show that the first-order magnetic scattering signal reconstructed from heterodyne scattering now directly represents the underlying magnetization texture. Moreover, we suggest a practical implementation based on an absorption mask rigidly connected to the sample, which not only produces a static charge scattering signal but also eliminates the problem of drift-induced artificial decay of the correlation functions. Our method thereby significantly broadens the range of scientific questions accessible by magnetic x-ray photon correlation spectroscopy.

The quasi-two-dimensional antiferromagnet ${\mathrm{Ni}}_{\text{2}}{\mathrm{P}}_{\text{2}}{\mathrm{S}}_{\text{6}}$ belongs to the family of magnetic van der Waals compounds that provides a rich material base for the realization of fundamental models of quantum magnetism in low dimensions. Here, we report high-frequency/high-magnetic field electron spin resonance measurements on single crystals of ${\mathrm{Ni}}_{\text{2}}{\mathrm{P}}_{\text{2}}{\mathrm{S}}_{\text{6}}$. The results enable one to reliably determine the positive, “easy”-plane type of the single ion anisotropy of the ${\mathrm{Ni}}^{2+}$ ions with the upper limit of its magnitude $\lesssim 0.7\mathrm{meV}$. The resonance response reveals strongly anisotropic spin fluctuations setting in shortly above the Néel temperature $T_{\mathrm{N}}=158\mathrm{K}$ and extending in the antiferromagnetically ordered state down to low temperatures. There, a low-energy magnon excitation gapped from the ground state by only $\sim 1\mathrm{meV}$ was found. This magnon mode may explain unusual low temperature relaxation processes observed in the latest nuclear magnetic resonance experiments and together with the estimate of the single ion anisotropy they should enable setting up a realistic spin model for an accurate description of magnetic properties of ${\mathrm{Ni}}_{\text{2}}{\mathrm{P}}_{\text{2}}{\mathrm{S}}_{\text{6}}$.