Bound states in the continuum (BICs) exhibit significant electric field confinement capabilities and have recently been employed to enhance nonlinear optics response at the nanoscale. In this study, we achieve substantial enhancement of third-harmonic generation (THG) and degenerate four-wave mixing (dFWM) by implementing Brillouin zone folding-induced BICs (BZF-BICs) in an air-hole-type nonlinear metasurface. By introducing gap perturbations within the metasurface, guided modes below the light line can be folded into the light cone, resulting in three resonant modes: guided resonances, $\mathrm{\Gamma }$-BICs, and BZF-BICs. Through the eigenvalue analysis and multipole decompositions, we establish their excitation conditions. With their resonantly enhanced local field, we successfully boost both THG and dFWM under $x$ and $y$ polarizations within the same metasurfaces. The simulated results indicate that the BZF-BICs provide the most significant enhancement of third-order nonlinear optical responses, with the output power of THG to ${\mathrm{10}}^{-4}$ W and dFWM output power of ${\mathrm{10}}^{-2}$ W under a moderate input power density of 1 MW/${\mathrm{cm}}^2$. These findings demonstrate that the BZF-BICs can offer an effective pathway for chip-scale nonlinear optical applications.

In this work, we propose a method to control spontaneous emission in a double-V-type quantum emitter near a plasmonic nanostructure using optical excitation with vortex beams. By leveraging the spatial and phase variations of the vortex beams, a spatially dependent interference and coherence of the upper energy states is induced, leading to an altered emission spectrum. The study reveals phenomena such as emission peak narrowing and merging, along with pattern conversion and redistribution in the transverse plane. Importantly, this method allows for remote distance control of spontaneous emission by tuning the separation between the quantum emitter and the plasmonic nanostructure, offering a new way to manipulate emission properties in hybrid quantum-plasmonic systems. This approach could find applications in quantum information processing and tunable nanoscale photonic devices.

Two-dimensional $1T$ monolayer (ML) ${\mathrm{IrTe}}_2$ exhibits complex structural phase transitions under external stimuli like temperature or strain. While various mechanisms have been suggested as a major driving force for rich structural phases, their true origin is not comprehensively understood. In this study, we investigate the strain-induced phase transition in ML ${\mathrm{IrTe}}_2$ using first-principles calculations and constrained density-functional perturbation theory. We find that the soft phonons associated with structural instability are hardened by biaxial tensile strain and that the trigonal phase is stabilized. We show that sharp peaks in the bare electronic susceptibility due to the Fermi surface nesting persists even under the biaxial strain and that the renormalization of phonon self-energy is affected mostly by the interaction of phonons with electrons in low-energy bands crossing the Fermi level. We find that the off-diagonal matrix elements of the phonon self-energy is sensitive to the biaxial tensile strains, contributing to the stabilization of the soft phonon modes.

Recent experiments suggest quantum interference effects in the Coulomb drag of double-layer graphene systems. By accounting for correlated interlayer impurity scattering under a weak magnetic field, our theoretical results reveal drag resistivities resembling those in weak (anti)localization. It is established that the quantum interference effect is most significant when the chemical potentials of the two graphene sheets match. The theory clarifies the roles of intra- and interlayer Berry phases in Coulomb drag in double-layer graphene systems and helps delineate the intra- and intervalley contributions. These insights are valuable for designing graphene-based electronic devices exploiting quantum effects.

We report a magnetoconductivity tensor $\sigma$ for the intercalated graphite ${\mathrm{CaC}}_6$ in the ground state of a uniaxial charge density wave (CDW), under conditions of coherent magnetic breakdown due to a strong external magnetic field $\mathbf{B}$ perpendicular to the conducting plane. The uniaxial charge density wave reconstructs an initially closed Fermi surface into an open one, accompanied by the formation of a pseudogap in the electron density of states around the Fermi energy. The magnetoconductivity tensor is calculated within the quantum density matrix and the semiclassical magnetic breakdown approach, with the focus on modification of the main, so-called “classical” contribution to magnetoconductivity by magnetic breakdown, neglecting the higher-order corrections. In the presence of magnetic breakdown, in spite of an open Fermi surface configuration, all classical magnetoconductivity components—the one along the CDW apex ${\sigma }_{xx}\sim B^{-2}$, the one perpendicular to the CDW apex ${\sigma }_{yy}\sim \mathrm{const}$, as well as the Hall conductivity ${\sigma }_{xy}\sim B^{-1}$—undergo strong quantum oscillations versus an inverse magnetic field. Those oscillations do not appear as a mere additive correction, but rather they alter the classical result, becoming an inherent part of it, and making it essentially nonclassical.

It is well known that quantized topological charge pumping takes place in the half-filled Rice-Mele chain performing a closed cycle in parameter space. We extend previous studies to the case of charge and heat transport at arbitrary filling and temperature using the corresponding continuity equation with focus on the noninteracting case. The amount of charge or heat transported for any adiabatic time dependence of the parameters is given by a double integral of an analytical function. We find that quantized transport is lost except in trivial cases. In particular, for popular pumping circuits used which lead to quantized nontrivial charge transport at zero temperature, the heat transported in the cycle vanishes. For other pumping circuits, there is a heat transport among even and odd sites of the chain and the environment. As the temperature is increased, the transported charge and heat decrease and vanish at infinite temperature.

Motivated by recent observations of symmetry-broken phases in lightly doped multilayer graphene, we investigate magnetic phase transitions in a generalized electron gas model with four-component electron spin. This model simplifies the problem with a parabolic dispersion band, abstracting away the details of the graphene band structure to focus solely on the effects of the Coulomb interaction. We report four findings: (i) In the Hartree-Fock approximation, we observe that the paramagnetic state undergoes a sequence of density-driven, first-order phase transitions, progressively depopulating electrons from each spin component until achieving complete polarization within a very a narrow density window where $1.2<r_s<2$ ($r_s$ being the electron gas parameter). (ii) Further incorporating the correlation energy via the Bohm-Pines random-phase approximation shows that the cascade of transitions obtained within the Hartree-Fock approximation is replaced by a single ferromagnetic phase transition at $r_s=6.12$. (iii) The disappearance of cascade is due to the correlation energy difference between the four-component paramagnetic state and symmetry-broken phases, which is nearly an order of magnitude more negative than the corresponding Hartree-Fock energy difference for $1.2<r_s<2$. (iv) The transition from the paramagnetic state to the fully polarized state at $r_s=6.12$ is governed by the balance between exchange and correlation energies, a competition that cannot be captured by mean-field approximations to models featuring effective (density-dependent) $\delta$-function interactions, such as the Stoner model. We use the insights from our model to comment on the phase diagram of multilayer graphene electron gas.

We explore the near adiabatic dynamics in a non-Hermitian quantum many-body system by investigating a finite-time ramp of the imaginary vector potential in the interacting Hatano-Nelson model. The excess energy, the Loschmidt echo, and the density imbalance are analyzed using bosonization and exact diagonalization. The energy becomes complex valued, despite the instantaneous Hamiltonian having the same real spectrum throughout. The adiabatic limit is approached very slowly through damped oscillations. The decay scales with ${\tau }^{-1}$ with $\tau$ the ramp duration, while the oscillation period is $2L/v$ with $v$ the Fermi velocity and $L$ the system length. Yet, without the need for auxiliary controls, a shortcut to adiabaticity is found for ramp times commensurate with the period. Our work highlights the intricate interplay of adiabaticity and non-Hermitian many-body physics.

In this paper, we present a microscopic quantum theory that elucidates the nonlinear and nonperturbative optical response of biased bilayer graphene subjected to bichromatic strong laser fields. This response is analyzed using a four-band Hamiltonian derived from ab initio calculations. For the laser-stimulated dynamics, we employ structure gauge-invariant evolutionary equations to accurately describe the evolution of the single-particle density matrix across the entire Brillouin zone. The resonant generation of electron-hole pairs by the high-frequency component of the field, combined with the induction of high-order harmonic generation and high-order wave mixing by the strong low-frequency field component, leads to significant alterations in the resulting spectra. These changes are driven by the effects of Berry curvature and the shift vector, which modify the relative contributions of interband and intraband channels, thereby fundamentally reshaping the radiation spectra at high-order frequency multiplication. The numerical results are further supported by approximate analytical calculations, demonstrating that high-order wave mixing can be modeled using the classical trajectory analysis of electron-hole pairs, with Berry curvature and the shift vector significantly influencing the saddle-point equations.

Loop arrangements and their quantum superpositions describe several interesting many-particle states. We propose that they also describe bonding in a class of transition metal dichalcogenides. We present an effective quantum loop model for monolayers with a $1T$ structure and a $d^2$ valence electron configuration: materials of the form $M{X}_2$ $(M=\text{Mo}, \mathrm{W} \text{and} X=\text{S}, \mathrm{Se}, \mathrm{Te})$ and $A{M}^{\prime }{Y}_2$ $(A=\text{Li}, \mathrm{Na}; M^{\prime }=\text{V}, \mathrm{Nb}; \text{and} Y=\text{O}, \mathrm{S}, \mathrm{Se})$. Their $t_{2g}$ orbitals exhibit strongly directional overlaps between neighboring atoms, favoring the formation of valence bonds. A transition metal atom forms two valence bonds, each with one of its neighbors. When connected, these bonds form loops that cover the triangular lattice. We construct a minimal Rokhsar-Kivelson-like model with resonance processes that cut and reconnect loops that run in proximity. The resulting dynamics is more constrained than in traditional quantum dimer models, with a “bending” constraint that arises from orbital structure. In the resulting phase diagram, we find phases that resemble distorted phases seen in materials, viz., the $1T^{\prime }$ and trimerized phases. As a testable prediction, we propose that a single $d^1$ or $d^3$ impurity will terminate a loop and give rise to a long-ranged texture. For example, a Ti/Cr defect in ${\mathrm{LiVO}}_2$ will produce one or more domain walls that propagate outward from the impurity. We discuss the possibility of a loop-liquid phase that can emerge in these materials.

In this Letter I apply Kohn's theorem to quantum oscillations in cuprates. I show that when combined with Gell-Mann and Low theorem, there is rigorous justification of quantum oscillations as in a Fermi liquid for cuprates. For simplicity, understanding the lack of interaction effects in quantum oscillations can be addressed in the language of a fully translationally invariant system. The Gell-Mann and Low theorem is not perturbative, and applies more generally as long as the adiabatic evolution from an isolated nondegenerate state is the case. Oscillation frequencies are identical to the noninteracting problem. It is the cyclotron gap that justifies Fermi-liquid-based theory of quantum oscillations. This is not to imply that other properties of the cuprates that do not have a gap in the spectrum could not exhibit non-Fermi-liquid behavior, for example, angle-resolved photoemission spectroscopy.

Spin qubits in gate-defined quantum dots are emerging as a leading technology due to their scalability and long coherence times. However, maintaining these qubits at ultralow temperatures typically requires complex cryogenic systems. This paper proposes a gate-defined double quantum dot (DQD) cooling system, where the DQDs act as refrigerants to reduce the local phonon environment around computational qubits. The cooling process occurs in two distinct stages: the first step involves microwave-induced state depopulation combined with fast cyclic detuning to transfer the DQD's population to the ground state, effectively lowering the DQD's temperature. In the second step, the cooled DQD interacts with and absorbs phonons resonant with the DQD spin energy, thereby filtering out these phonons that contribute to spin-lattice relaxation in the surrounding environment. This study focuses on the first step, presenting detailed calculations and numerical results that demonstrate the feasibility of achieving local DQD temperatures below $10\mathrm{mK}$ at a bath temperature of $1\mathrm{K}$. The sensitivity of the cooling performance to detuning energy, magnetic field strength, spin-orbit couplings, and diabatic return time is analyzed, while the phonon filtering in the second step will require further investigation.

In this paper, we study a stochastically driven nonequilibrium quantum system where the driving protocols consist of hopping and waiting processes. The waiting times between two hopping processes satisfy a heavy-tailed distribution. By calculating the squared width of the wave packets, our findings demonstrate the emergence of various anomalous transport phenomena when the system remains unchanged within the heavy-tailed regime, including superdiffusive, subdiffusive, and standard diffusive motion. Only subdiffusion occurs when the system has evolved during the waiting process. All these transport behaviors are accompanied by a breakdown of ergodicity, highlighting the complex dynamics induced by the stochastic driving mechanism.

In frustrated spin systems, creation and annihilation of topological defects are processes that are hard to characterize and information about them is often extracted by measuring averaged quantities without direct visualization of their dynamics. Here we show that, in a quenched colloidal ice, an external force can be used to generate a plasma state of topological defects which are continuously created and destroyed with bias-dependent rates. By developing a general tracking algorithm which can be extended to other artificial spin systems, we fully characterize the statistical properties and the kinetics of the generation-recombination process at the steady state, both in the plasma regime and far from it. Further, we determine the defects' waiting and survival time distributions which unveil a maximum lifetime in the plasma regime where the string tension is minimized.

Kinetics of order-disorder transitions including melting in low-dimensional systems is theoretically predicted to be heterogeneous that start by nucleation in defect sites at lower than homogeneous bulk-melting temperature requiring substantially lower energy barrier. We probed kinetics of such transition in thin multilayered films using x-ray scattering and in situ atomic force microscopy (AFM) studies. X-ray results show linear increase with time in in-plane separation of ordered domains while in situ AFM measurements show a linear increase in height of disordered domains with time. The structure factor of the diffraction peak reduces linearly with square of time as the material disorder.

Nonlinear transport effects in response to external magnetic fields, i.e., electrical magnetochiral anisotropy (eMChA), have attracted much attention for their importance in studying quantum and spin-related phenomena. Indeed, they have permitted the exploration of topological surface states and charge to spin conversion processes in low-symmetry systems. Nevertheless, despite the inherent correlation between the symmetry of the material under examination and its nonlinear transport characteristics, there is a lack of experimental demonstration to delve into this relationship and to unveil their microscopic mechanisms. Here, we study eMChA in chiral elemental tellurium (Te) along different crystallographic directions, establishing the connection between the different eMChA components and the crystal symmetry of Te. We observed different longitudinal eMChA components with collinear current and magnetic field, demonstrating experimentally the radial angular momentum texture of Te. We also measured a transverse nonlinear resistance which, as the longitudinal counterpart, scales bilinearly with current and magnetic fields, illustrating that they are different manifestations of the same effect. Finally, we study the scaling law of the eMChA, evidencing that extrinsic scattering is the dominant microscopic mechanism. These findings underscore the efficacy of symmetry-based investigations in understanding and predicting nonlinear transport phenomena, with potential applications in spintronics and energy harvesting.