Tailoring optical nonlinear effects (e.g., harmonic generation, sum-frequency mixing, etc.) in the recently emerging all-dielectric platform is important for both fundamental science and industrial development of high-efficiency, ultrafast, and miniaturized photonic devices. In this paper, we propose a method for the dynamically switchable high-harmonic generation in silicon nanodimer metasurfaces by exploiting the polarization-controlled dual bound states in the continuum (BIC). Owing to the high-quality factor of BIC resonances, efficient harmonic signals including the third-harmonic generation and fifth-harmonic generation from a direct process as well as a cascaded process by degenerate four-wave mixing are obtained. Moreover, the BIC and their resonantly enhanced harmonics can be switched on or off with high selectivity with respect to the fundamental pump polarization. Compared with previous reports, our paper provides a simple but effective tuning strategy by fully exploring the structural symmetry and polarization degree of freedom rather than resorting to additional external stimuli, which would possess great advantages in designing tunable and switchable nonlinear light sources for chip-scale applications.

We study the effect of disorder on the space-time supersymmetry that is proposed to emerge at the quantum critical point of pair density wave transition in $(2+1)$-dimensional (D) Dirac semimetals and $(3+1)$D Weyl semimetals. In the $(2+1)$D Dirac semimetal, we consider three types of disorder, including random scalar potential, random vector potential and random mass potential, whereas the random mass disorder is absent in the $(3+1)$D Weyl semimetal. Via a systematic renormalization-group analysis, we find that any type of weak random disorder is irrelevant due to the couplings between the disorder potential and the Yukawa vertex. The emergent supersymmetry is, thus, stable against weak random potentials. Our paper will pave the way for exploration supersymmetry in realistic condensed-matter systems.

Lattice gauge theories are fundamental to such distinct fields as particle physics, condensed matter, and quantum information science. Their local symmetries enforce the charge conservation observed in the laws of physics. Impressive experimental progress has demonstrated that they can be engineered in table-top experiments using synthetic quantum systems. However, the challenges posed by the scalability of such lattice gauge simulators are pressing, thereby making the exploration of different experimental setups desirable. Here, we realize a U(1) lattice gauge theory with five matter sites and four gauge links in classical electric circuits employing nonlinear elements connecting LC oscillators. This allows for probing previously inaccessible spectral and transport properties in a multisite system. We directly observe Gauss's law, known from electrodynamics, and the emergence of long-range interactions between massive particles in full agreement with theoretical predictions. Our paper paves the way for investigations of increasingly complex gauge theories on table-top classical setups, and demonstrates the precise control of nonlinear effects within metamaterial devices.

We study an interplay between the orbital degeneracy and the spin-orbit coupling (SOC) giving rise to spin-orbital entangled states in concentrated systems (cooperative Jahn-Teller [JT] effect). As a specific example, we analyze the interaction of electrons occupying triply degenerate single-ion $t_{2g}$ levels with trigonal vibrations (the $t\otimes T$ problem). A more general problem of the electron–lattice interaction involving both tetragonal and trigonal vibrations is also considered. It is shown that the result of such interaction crucially depends on the occupation of $t_{2g}$ levels leading to either the suppression or the enhancement of the JT effect by the SOC.

The dynamical correlations of a strongly correlated system are an essential ingredient to describe its nonequilibrium properties. We present a general method to calculate exactly the dynamical correlations of hard-core anyons in one-dimensional lattices, valid for any type of confining potential and any temperature. We obtain exact explicit expressions of the Green's function, the spectral function, and the out-of-time-ordered correlators (OTOCs). We find that the anyonic spectral function displays three main singularity lines which can be explained as a double spectrum in analogy to the Lieb-Liniger gas. The dispersion relations of these lines can be given explicitly and they cross at a hot point $(q_m,{\omega }_m)$, which induces a peak in the momentum distribution function at $q_m$ and a power-law singularity in the local spectral function at ${\omega }_m$. We also find that the anyonic statistics can induce spatial asymmetry in the Green's function, its spectrum, and the OTOC. Moreover, the information spreading characterized by the OTOCs shows light-cone dynamics, asymmetric for general statistics and low temperatures, but symmetric at infinite temperature. Our results pave the way toward studying the nonequilibrium dynamics of hard-core anyons and experimentally probing anyonic statistics through spectral functions.

Quantum spin systems can break time-reversal symmetry by developing spontaneous magnetization or spin chirality. However, collinear magnets and chiral spin states are invariant under different symmetries, implying that the order parameter of one phase vanishes in the other. We show how to construct one-dimensional anisotropic spin models that exhibit a “Landau-forbidden” continuous phase transition between such states. As a concrete example, we focus on a zigzag chain with bond-dependent exchange and six-spin interactions. Using a combination of exact solutions, effective field theories, and numerical simulations, we show that the transition between the chiral and magnetic phases has an emergent $U\left(1\right)$ symmetry. The excitations governing the transition from the chiral phase can be pictured as mobile defects in a ${\mathbb{Z}}_2$ flux configuration which bind fermionic modes. We briefly discuss extensions to two dimensions and analogies with deconfined quantum criticality. Our results suggest new prospects for unconventional phase transitions involving chiral spin states.

Motivated by recent nuclear magnetic resonance experiments, we calculated the spin susceptibility, Knight shift, and spin-lattice relaxation rate ($1/T_{1}T$) of the single-component molecular conductor $\left[\mathrm{Ni}{\left(\mathrm{dmdt}\right)}_2\right]$ using the random phase approximation in a multi-orbital Hubbard model describing the Dirac nodal line electronic system in this compound. This Hubbard model is composed of three fragment orbitals and on-site repulsive interactions obtained using ab initio many-body perturbation theory calculations. We found fragment-orbital-dependent spin fluctuations with the momentum $\mathbf{q}=\mathbf{0}$ and an incommensurate value of the wave number $\mathbf{q}=\mathbf{Q}$ at which a diagonal element of the spin susceptibility is maximum. The $\mathbf{q}=\mathbf{0}$ and $\mathbf{Q}$ responses become dominant at low and high temperatures, respectively, with the Fermi-pocket energy scale as the boundary. We show that $1/T_{1}T$ decreases with decreasing temperature, but starts to increase at low temperature owing to the $\mathbf{q}=\mathbf{0}$ spin fluctuations, while the Knight shift keeps monotonically decreasing. These properties are due to the intramolecular antiferromagnetic fluctuations caused by the characteristic wave functions of this Dirac nodal line system, which is described by an $n$-band ($n\ge 3$) model. We show that the fragment orbitals play important roles in the magnetic properties of $\left[\mathrm{Ni}{\left(\mathrm{dmdt}\right)}_2\right]$.

Recent systematic measurements of the quantum well width dependence of the excitation gaps of fractional quantum Hall states in high mobility samples [Villegas Rosales et al., Phys. Rev. Lett. 127, 056801 (2021)] open the possibility of a better quantitative understanding of this important issue. We present what we believe to be accurate theoretical gaps including the effects of finite width and Landau level (LL) mixing. While theory captures the width dependence, there still remains a deviation between the calculated and the measured gaps, presumably caused by disorder. It is customary to model the experimental gaps of the $n/(2n\pm 1)$ states as ${\mathrm{\Delta }}_{n/(2n\pm 1)}=C{e}^2/\left[(2n\pm 1)\varepsilon l\right]-\mathrm{\Gamma }$, where $\varepsilon$ is the dielectric constant of the background semiconductor and $l$ is the magnetic length; the first term is interpreted as the cyclotron energy of composite fermions and $\mathrm{\Gamma }$ as a disorder-induced broadening of composite-fermion LLs. Fitting the gaps for various fractional quantum Hall states, we find that $\mathrm{\Gamma }$ can be nonzero even in the absence of disorder.

The Holstein model with dispersionless Einstein phonons is one of the simplest models describing electron-phonon interactions in condensed matter. A naive extrapolation of perturbation theory in powers of the relevant dimensionless electron-phonon coupling ${\lambda }_0$ suggests that at zero temperature the model exhibits a Pomeranchuk instability characterized by a divergent uniform compressibility at a critical value of ${\lambda }_0$ of order unity. In this work, we re-examine this problem using modern functional renormalization group (RG) methods. For dimensions $d>3$ we find that the RG flow of the Holstein model indeed exhibits a tricritical fixed point associated with a Pomeranchuk instability. This non-Gaussian fixed point is ultraviolet stable and is closely related to the well-known ultraviolet stable fixed point of ${\varphi }^3$-theory above six dimensions. To realize the Pomeranchuk critical point in the Holstein model at fixed density both the electron-phonon coupling ${\lambda }_0$ and the adiabatic ratio ${\omega }_0/{\epsilon }_F$ have to be fine-tuned to assume critical values of order unity, where ${\omega }_0$ is the phonon frequency and ${\epsilon }_F$ is the Fermi energy. However, for dimensions $d\le 3$ we find that the RG flow of the Holstein model does not have any critical fixed points. This rules out a quantum critical point associated with a Pomeranchuk instability in $d\le 3$.

We theoretically investigated the magnetoresistance (MR) and planar Hall effect (PHE) in Weyl semimetals based on the semiclassical Boltzmann theory, focusing on the fine structure of the band dispersion. We identified that the negative longitudinal MR and sign change in the PHE occur because of the negative off-diagonal effective mass with no topological effects or chiral anomaly physics. Our results highlight the important role of the off-diagonal effective mass, which can cause anomalous galvanomagnetic effects. We propose that the PHE produces a dip in the temperature dependence, which enables the experimental detection of the singularity of effective mass, i.e., the Weyl point.

Magnetotransport measurements have been performed on ultrathin Bi films grown on GaAs(110). While large positive magnetoresistance is observed at low temperatures, the Hall resistance is found to be extremely small. This is explained by the cancellation of contributions of electrons and holes, which are estimated to have close density and mobility values. By analogy with graphene near the charge neutral point, magnetotransport properties are discussed in relation to the formation of Dirac cones.

Coherent control and optical readout of the electron spin of the ${}^{29}{\mathrm{SiV}}^{-}$ center in diamond have been demonstrated in the literature, with exciting prospects for implementations as memory nodes and spin qubits. Nuclear spins may be even better suited for many applications in quantum information processing due to their long coherence times. Control of the ${}^{29}{\mathrm{SiV}}^{-}$ nuclear spin using conventional NMR techniques is feasible, albeit at slow kilohertz rates due to the nuclear spin's low gyromagnetic ratio. In this work we theoretically demonstrate how indirect control using the electron spin-orbit effect can be employed for high-speed, megahertz control of the ${}^{29}\mathrm{Si}$ nuclear spin. We discuss the impact of the nuclear spin precession frequency on gate times and the exciting possibility of all-optical nuclear spin control.

We theoretically investigate the effect of Rashba field and magnetic exchange interaction on the topological properties of monolayer transition-metal dichalcogenides (TMDs) based on a tight-binding model. The tight-binding Hamiltonian incorporates magnetic exchange interaction and the Rashba field which can be induced by proximity to a ferromagnetic (FM) substrate, such as europium oxide (EuO) or similar materials. Through tuning the Rashba field and magnetic exchange interaction, we find a quantum state of matter—the valley-resolved quantum anomalous Hall state—in monolayer ${\mathrm{MoTe}}_2$. The most amazing property of this topological quantum state is that the edge states of this phase are all located only at valley $-K$ (or $+K$), i.e., they are completely valley resolved. Additionally, the calculated Chern number $\mathcal{C}=-2$ indicates that there are two pairs of valley-resolved chiral edge modes at the boundaries of the monolayer ${\mathrm{MoTe}}_2$ nanoribbon. The electronic transmission demonstrates the robustness of the edge states of the valley-resolved quantum anomalous Hall effect against defects. This newfound valley-resolved quantum anomalous Hall effect provides an idea for designing dissipationless valleytronics utilizing TMD/FM heterojunctions.

The origin of the two-dimensional electron gas (2DEG) in the interface between $\gamma \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$ (GAO) and ${\mathrm{SrTiO}}_3$ (STO) (GAO/STO) as well as the reason for the high mobility of the 2DEG is still in debate. In this paper, the electronic structures of [001]-oriented GAO/STO heterostructures with and without oxygen vacancies are investigated by first-principles calculations based on density functional theory. The calculation results show that the necessary condition for the formation of 2DEG is that the GAO/STO heterostructure have an interface composed of Al and ${\mathrm{TiO}}_2$ layers. For the heterostructure without oxygen vacancies on the GAO side, the 2DEG originates from the polar discontinuity near the interface, and there is a critical thickness for the GAO film, below which the 2DEG would not present and the heterostructure exhibits insulator characteristics. For the case that only the GAO film contains oxygen vacancies, the polar discontinuity near the interface disappears, but the 2DEG still exists. In this situation, the critical thickness of the GAO film for 2DEG formation does not exist either. When the GAO film and STO substrate both contain oxygen vacancies, it is found that the 2DEG is retained as long as the oxygen vacancies on the STO side are not very close to the interface. The low-temperature mobilities of the 2DEGs in these GAO/STO heterostructures are considered to be governed by the ionized impurity scattering, and $\sim 3$ to $\sim 11$ times as large as that in ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ heterojunction. The high mobility of the 2DEG is mainly due to the small electron effective mass in GAO/STO heterostructures.

The inverse Faraday effect (IFE), where a static magnetization is induced by circularly polarized light, offers a promising route to ultrafast control of spin states. Here we study the IFE in Mott insulators using the Floquet theory. We find two distinct IFE behavior governed by the inversion symmetry. In the Mott insulators with inversion symmetry, we find that the effective magnetic field induced by the IFE couples ferromagnetically to the neighboring spins. While for the Mott insulators without inversion symmetry, the effective magnetic field due to IFE couples antiferromagnetically to the neighboring spins. We apply the theory to the spin-orbit coupled single- and multiorbital Hubbard model that is relevant for the Kitaev quantum spin liquid material and demonstrate that the magnetic interactions can be tuned by light.

The generation of a nonlinear high-frequency response in solids from powerful optical pumps has gained momentum over the past decade. High-harmonic generation (HHG) in solids can be obtained from strong-field laser excitation, usually restricted to optical frequencies and limited both in amplitude and in harmonic order. Here, we demonstrate that high-harmonic emission can be achieved by exploiting conventional spin pumping, without the need for optical excitation. Considering a noncentrosymmetric (ferro- or antiferro-)magnet excited at a frequency $\omega$, we demonstrate the emergence of HHG in two main regimes: (i) In the perturbative regime, where a weak spin-orbit interaction is considered, the carrier pumping features a number of harmonics with a cutoff order $n_{\max }<10$. (ii) When the spin-orbit coupling strength is close to, or higher than, the $s\text{−}d$ exchange energy, a strongly nonlinear regime resulting from resonantlike spin-flip scattering occurs leading to the emission of a large number of harmonics. This is in sharp contrast to conventional pumping, where the corresponding time-dependent currents simply oscillate with the frequency of the magnetic drive $\omega$. Our proposal enables the enhancement of both spin and charge dynamics by orders of magnitude. This effect could be used to trigger high-frequency emission deep in the terahertz regime.

Fracton phases feature elementary excitations with fractionalized mobility and are exciting interest from multiple areas of theoretical physics. However, the most exotic type-II fracton phases, like the Haah codes, currently have no known experimental diagnostics. Here, we explain how type-II fracton phases with gapless gauge modes, such as the $\text{U}\left(1\right)$ Haah code, may be identified experimentally. Our analysis makes use of the multipole gauge theory description of type-II fracton phases, which exhibits ultraviolet-infrared (UV-IR) mixing. We show that neutron scattering experiments on gapless type-II fracton phases should generically exhibit exotic pinch points in the structure factor, with distinctive anisotropic contours as a direct consequence of UV-IR mixing. This characteristic pinch point structure provides a clean diagnostic of type-II fracton phases. We also identify distinctive signatures of the ($3+1$)-D $\text{U}\left(1\right)$ Haah code in the low-temperature specific heat.

After the discovery of near room-temperature superconductivity in superhydrides at extremely high pressure close to 300 GPa, there is increasing interest in finding superconducting systems to maintain the similar superconductivity but at pressures below megabar. To examine such a possibility in metal-free hydrides, we investigate the thermodynamical stability and dynamical stability, electronic structures, and electron-phonon interactions of ${\mathrm{H}}_6\mathrm{S}X$ ($X=$ Cl and Br) from the theoretical viewpoint. The results show that ${\mathrm{H}}_6\mathrm{SCl}$ and ${\mathrm{H}}_6\mathrm{SBr}$ are potential superconductors with the transition temperatures of 155.4 K at 90 GPa and 136 K at 140 GPa, respectively. Remarkably, ${\mathrm{H}}_6\mathrm{SCl}$ can be stabilized at the pressure above 82.5 GPa but maintain the superconducting transition above 150 K. Compared with ${\mathrm{H}}_3\mathrm{S}$, the substitution of S by Cl with lower electronic energy states leads to the enhancement of Cl-H covalent bonding. As a result, the stable pressure of ${\mathrm{H}}_3\mathrm{S}$-like superconductors is substantially reduced below 100 GPa but the transition temperature can be maintained as high as 150 K.