We present a theoretical model to study the nonlinear optical response of materials with a low-energy spectrum characterized by strongly tilted, type-II Weyl cones in two dimensions, which we call 2D Weyl materials. Our findings reveal that the tilted nature of the Weyl cones is responsible for the appearance of even harmonics in the nonlinear signal, as well as its strong polarization dependence. We discuss how it is possible to control such a nonlinear response and envision how 2D Weyl materials can be used to realize different photonic devices for sensing applications.

We report our theoretical investigations on the magnetic field dependence of two-photon absorption (TPA) properties in ${\mathrm{MoS}}_2$. Based on the Landau levels obtained by the effective massive Dirac model, we developed TPA theories associated with both interband transitions and intraband transitions for transitional metal dichalcogenides (TMDCs) with a magnetic field under the second-order perturbation theory with respect to the electron-photon interaction. Our results reveal that the TPA coefficient is effectively enhanced by five orders of magnitude under the application of an external magnetic field, which results in the quantized horizontal Landau levels with the spin-orbit coupling (SOC) and valley Zeeman effects. The TPA peaks are located in the terahertz regime for intraband transitions and in the visible regime for interband transitions. The amplitude of TPA contributed by intraband transitions is much larger than that of interband transitions owing to the resonant transitions. Oscillatory behavior appeared in the interband transition-related TPA spectra. The separation of TPA for the transitions in the spin-up and spin-down states caused by the spin splitting of Landau levels resulting from the strong SOC is illustrated. We also analyzed the anisotropy of TPA associated with the direction of the incident light. Our results are of much importance for the application of TMDCs in optical devices.

High-order frequency mixing in graphene using a two-color radiation field consisting of the fundamental and the second harmonic fields of an ultrashort linearly polarized laser pulse is studied. It is shown that the harmonics originated from the interband transitions are efficiently generated in the case of the orthogonally polarized two-color field. In this case, the generated high harmonics are stronger than those obtained in the parallel polarization case by more than two orders of magnitude. This is in sharp contrast with the atomic and semiconductor systems, where the parallel polarization case is more preferable. The physical origin of this enhancement is also deduced from the three-step semiclassical electron-hole collision model, extended to graphene with pseudorelativistic energy dispersion. In particular, we discuss the influence of the many particle Coulomb interaction on the high-order harmonics (HHG) process within dynamical Hartree-Fock approximation. Our analysis shows that in all cases we have an overall enhancement of the HHG signal compared with the free-charged carrier model due to the electron-hole attractive interaction.

Terahertz (THz) technology is a cutting-edge scheme with various promising applications, such as next generation telecommunication, nondestructive evaluation, security check, and in-depth characterization, owing to its sensitivity to material geometric change and good transparency. Even though tremendous progress has been made during the past decade, exploration the mechanisms of THz-matter interaction microscopically is still in its infancy. In this work, we use thermodynamic theory to show how THz illumination deforms materials and use group-IV monochalcogenide compounds to illustrate it. According to our first-principles density functional theory calculations, THz light with intermediate intensity $\left(\sim {10}^9\mathrm{W}/\mathrm{c}{\mathrm{m}}^2\right)$ could yield elastic deformations on the order of $\sim 0.1\%$, depending on laser polarization direction. Large anisotropic optomechanical responses are also revealed. Finally, we show that such strain can be detected via measuring the layer-resolved shift current under a probe light irradiation.

The performance of persistent phosphors under given charging and working conditions is determined by the properties of the traps that are responsible for these unique properties. Traps are characterized by the height of their associated barrier for thermal detrapping, and a continuous distribution of trap depths is often found in real materials. Accurately determining trap depth distributions is hence of importance for the understanding and development of persistent phosphors. However, extracting the trap depth distribution is often hindered by the presence of a thermal barrier for charging as well, which causes a temperature-dependent filling of traps. For this case, we propose a method for extracting the trap depth distribution from a set of thermoluminescence glow curves obtained at different charging temperatures. The glow curves are first transformed into electron population functions via the Tikhonov regularization, assuming first-order kinetics. Subsequently, the occupation of the traps as a function of their depth, quantified by the so-called filling function, is obtained. Finally, the underlying trap depth distribution is reconstructed from the filling functions. The proposed method provides a substantial improvement in precision and resolution for the trap depth distribution compared with existing methods. This is hence a step forward in understanding the (de)trapping behavior of persistent and storage phosphors.

We show that systematic full configuration-interaction (FCI) calculations enable prediction of the energy spectra and the intrinsic spatial and spin structures of the many-body wave functions as a function of the detuning parameter for the case of three-electron hybrid qubits based on GaAs asymmetric double quantum dots. Specifically, in comparison with the case of weak interactions and treating the entire three-electron double-dot hybrid qubit as an integral unit, it is shown that the predicted spectroscopic patterns, originating from strong electron correlations, manifest the formation of Wigner molecules (WMs). Signatures of WM formation include: (1) a strong suppression of the energy gaps relative to the noninteracting-electron modeling, and (2) the appearance of a pair of avoided crossings arising between states associated with two-electron occupancies in the left and right wells. The Wigner molecule is a physical entity associated with electron localization within each well, and it cannot be captured by the previously employed independent-particle or two-site-Hubbard theoretical modeling of the hybrid qubits. The emergence of strong WMs is investigated in depth through the concerted use of FCI-adapted diagnostic tools like charge and spin densities, as well as conditional probability distributions. Furthermore, the energy spectrum as a function of the strength of the Coulomb repulsion (at constant detuning) is calculated in order to complement the thorough analysis of the factors contributing to WM emergence. We report remarkable agreement with recent experimental measurements. The present FCI methodology for multiwell quantum dots can be straightforwardly extended to treat valleytronic two-band Si/SiGe hybrid qubits, where the central role of the WMs was confirmed recently. Such valleytronic FCI could be also adapted and employed in simulations of Si-based two-qubit logical gates made of two interacting double-quantum-dot hybrid qubits confining a total of $N\le 6$ electrons.

An exotic coexistence of polar states is known to occur in $\text{ferroelectric}|\text{dielectric} {\mathrm{PbTiO}}_3|{\mathrm{SrTiO}}_3$ ($\mathrm{PTO}|\mathrm{STO}$) heterostructures. In the PTO layers with a thickness of 10–20 unit cells, in-plane-polarized regions order alongside the so-called vortex phase with a whirl-like arrangement of electric dipoles. We investigate the optical signature of these polar phases noninvasively using optical second harmonic generation (SHG) on $\mathrm{PTO}|\mathrm{STO}$-based heterostructures. We identify the phase coexistence down to a single $\mathrm{PTO}|\mathrm{STO}$ bilayer. By comparing the SHG yield in dependence of the number of PTO layers, we further find that interlayer coupling plays an important role in setting the final polarization state. Our nonlinear optical experiments demonstrate the potential of this noninvasive approach for the identification and understanding of complex noncollinear ordering of dipole moments in oxide heterostructures and lays the groundwork for their operando investigation.

In this work, we study the spectral properties of the adjacency matrix of critical Erdös-Rényi (ER) graphs, i.e., when the average degree is of order $lnN$. In a series of recent inspiring papers, Alt, Ducatez, and Knowles have rigorously shown that these systems exhibit a “semilocalized” phase in the tails of the spectrum where the eigenvectors are exponentially localized on a subextensive set of nodes with anomalously large degree. We propose two approximate analytical strategies to analyze this regime based respectively on the simple “rules of thumb” for localization and ergodicity and on an approximate treatment of the self-consistent cavity equation for the resolvent. Both approaches suggest the existence of two different regimes: a fully Anderson localized phase at the spectral edges, in which the eigenvectors are localized around a unique center, and an intermediate partially delocalized but nonergodic phase, where the eigenvectors spread over many resonant localization centers. In this phase, the exponential decay of the effective tunneling amplitudes between the localization centers is counterbalanced by the large number of nodes towards which tunneling can occur, and the system exhibits minibands in the local spectrum over which the Wigner-Dyson statistics establishes. We complement these results by a detailed numerical study of the finite-size scaling behavior of several observables that supports the theoretical predictions and allows us to determine the critical properties of the two transitions. Critical ER graphs provide a pictorial representation of the Hilbert space of a generic many-body Hamiltonian with short-range interactions. In particular, we argue that their phase diagram can be mapped onto the out-of-equilibrium phase diagram of the quantum random energy model.

Local structural symmetries play a key role in the functionalities of ferroelectric materials and are often found different from average symmetry. Here, we study the real space nanoscale structure in Pb-free ${\mathrm{BaZr}}_x{\mathrm{Ti}}_{1-x}{\mathrm{O}}_3$ ($x\le 0.10$) by pair distribution function measurements, complemented by transmission electron microscopy and x-ray diffraction. Our observations show existence of the rhombohedrally distorted unit cells; however, at intermediate length scales, at least up to 5 nm, there exist nanoscale correlated regions of monoclinic symmetry. This is complemented by the observation of curved frustrated nanodomains. Further, the average structure is found to have coexisting monoclinic and rhombohedral symmetries. Our observation of a two-phase ferroelectric state is in contrast to interferroelectric instabilities of conventional polymorphic phase boundaries reported for doped ${\mathrm{BaTiO}}_3$.

Optical drives at terahertz and midinfrared frequencies in quantum materials are increasingly used to reveal the nonlinear dynamics of collective modes in correlated many-body systems and their interplay with electromagnetic waves. Recent experiments demonstrated several surprising optical properties of transient states induced by driving, including the appearance of photo-induced edges in the reflectivity in cuprate superconductors (SCs), observed both below and above the equilibrium transition temperature. Furthermore, in other driven materials, reflection coefficients larger than unity have been observed. In this paper we demonstrate that unusual optical properties of photoexcited systems can be understood from the perspective of a Floquet system, a system with periodically modulated parameters originating from pump-induced oscillations of a collective mode. These oscillations lead to an effective Floquet system with periodically modulated parameters. We present a general phenomenological model of reflectivity from Floquet materials, which takes into account parametric generation of excitation pairs. We find a universal phase diagram of drive-induced features in reflectivity which evidence a competition between driving and dissipation. To illustrate our general analysis, we apply our formalism to two concrete examples motivated by recent experiments: A single plasmon band, which describes Josephson plasmons (JPs) in layered SCs, and a phonon-polariton system, which describes upper and lower polaritons in materials such as insulating SiC. Finally, we demonstrate that our model can be used to provide an accurate fit to results of phonon-pump–terahertz-probe experiments in the high-temperature SC ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.5}$. Our model explains the appearance of a pump-induced edge, which is higher in energy than the equilibrium JP edge, even if the interlayer Josephson coupling is suppressed by the pump pulse.

Phonons are the main heat carriers in semiconductor devices. In small devices, heat is not driven by a local temperature gradient, but by local points of heat input and removal. This complicates theoretical modeling. Study of the propagation of vibrational energy from an initial localized pulse provides insight into nonlocal phonon heat transport. We report simulations of pulse propagation in one dimension. The 1d case has tricky anomalies, but provides the simplest pictures of the evolution from initially ballistic toward longer time diffusive propagation. Our results show surprising details, such as diverse results from different definitions of atomistic local energy, and failure to exhibit pure diffusion at long times. Boltzmann phonon gas theory, including external energy insertion, is applied to this inherently time-dependent and nonlocal problem. The solution, using relaxation time approximation for impurity scattering, does not closely agree with the simulated results.

In recent years antiferromagnets (AFMs) have become very promising for nanoscale spintronic applications due to their unique properties, such as THz dynamics and the absence of stray fields. Manipulating antiferromagnetic textures is currently, however, limited to very few exceptional material symmetry classes allowing for staggered torques on the magnetic sublattices. In this work, we predict for kagome AFMs with broken mirror symmetry a new coupling mechanism between antiferromagnetic domain walls (DWs) and spin currents, produced by the relativistic Dzyaloshinskii-Moriya interaction (DMI). We microscopically derive the DMI's free-energy contribution for the kagome AFMs. Unlike ferromagnets and collinear AFMs, the DMI does not lead to terms linear in the spatial derivatives, but instead renormalizes the spin-wave stiffness and anisotropy energies. Importantly, we show that the DMI induces a highly nontrivial, twisted DW profile that is controllable via two linearly independent components of the spin accumulation. This texture manipulation mechanism goes beyond the concept of staggered torques and implies a higher degree of tunability for the current-driven DW motion compared to conventional ferromagnets and collinear AFMs.

Using a combination of time-domain terahertz (THz) spectroscopy (TDTS) and mutual inductance measurements, we have investigated the low-energy electrodynamic response of overdoped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ films that have been exposed to ion irradiation. Films went through three rounds of irradiation (2, 4, and $6\times {10}^{13}\mathrm{ions}/{\mathrm{cm}}^2$), and mutual inductance and TDTS experiments were performed between each step. Together with the as-grown film, this gives four different levels of disorder. The transport scattering rate that is measured directly in the THz experiments is an approximately linear function of the radiation dose at all temperatures. This is consistent with a proportionate increase in elastic scattering. In the superconducting state we find that the relation between $T_c$, the superfluid density, and the scattering rates is quantitatively at odds with the predictions based on the extant theory of Abrikosov-Gorkov-like pair breaking in a dirty $d$-wave superconductor. Increasing disorder causes only a small change in the superconducting transition temperature for the overdoped films, but the changes in the $\omega \sim 0$ superfluid density are much larger.

In topological materials, Dirac fermions can split into two Weyl fermions with opposite chiralities due to the breaking of space-inversion symmetry, while in noncentrosymmetric superconductors, novel superconducting electron pairing mechanisms arise because of the antisymmetric spin-orbit coupling. In this work, we report the pressure-introduced superconductivity in a typical noncentrosymmetric Weyl semimetal $\mathrm{LaAl}X$ ($X=\mathrm{Si}$ and Ge). Superconductivity was observed at around 65 GPa without structural phase transition. A typical dome-shape phase diagram is obtained with the maximum $T_{\mathrm{c}}$ of 2.5 K (2.1 K) for LaAlSi (LaAlGe). Furthermore, the application of pressure does not destroy the nontrivial band topology of LaAlSi up to 80.4 GPa, making such materials potential candidates for realizing topological superconductivity. Our report of superconductivity in $\mathrm{LaAl}X$ ($X=\mathrm{Si}$ and Ge) will provide critical insight in noncentrosymmetric superconductors and stimulate further study on superconductivity in Weyl semimetals.

Chemical doping or elemental substitution and applying pressure are considered to be two effective tools to create superconductivity from a parent compound. The latter is more powerful than the former for contracting the lattice but does not bring about impurities, defects, or disorders compared with the former, thus serving as a good platform to examine or test any theoretical models for the born superconductivity. Among the various theories proposed for superconductivity, the electron-phonon interactions (or phonon mediated), spin fluctuations (or spin mediated), and valence fluctuations (or valence mediated) are the three major mechanisms. The first two have received extensive support from many elements and compounds over the last century, and only a few examples are believed to be a result of valence fluctuations. Lead telluride (PbTe), a Group IV-VI compound, is being examined as a promising candidate whose superconductivity is mediated by the valence fluctuations. The superconductivity has been reported in doped PbTe with the rocksalt NaCl structure at ambient pressure or parent PbTe with the body-centered cubic CsCl structure at high pressure. The pressure-driven intermediate orthorhombic phase in between has been known as a semiconductor with a large band gap and no expectation for the appearance of superconductivity. By doping I in the Te site to adding more carriers and partially substituting Pb by In with the expected valence change, however, we show that the co-doped PbTe exhibits superconductivity upon compression in the orthorhombic phase besides the expected one with the CsCl structure. Through the systematic studies of the structural, vibrational, electrical, and superconducting properties, we suggest that the observed superconductivity in the orthorhombic phase is mediated by the valence fluctuations associated with mixed valency on the In sites. This work thus not only offers an example for valence-mediated superconductivity but also fills the gap for the superconducting phase of PbTe in between the NaCl and CsCl structure.

The doped topological insulator $A_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$, with $A=\{\mathrm{Cu},\mathrm{Sr},\mathrm{Nb}\}$, becomes a nematic superconductor below $T_c\approx 3$–4 K. The associated electronic nematic director is described by an angle $\alpha$ and is experimentally manifested in the elliptical shape of the in-plane critical magnetic field $H_{c2}$. Because of the threefold rotational symmetry of the lattice, $\alpha$ is expected to align with one of three high-symmetry directions corresponding to the in-plane nearest-neighbor bonds, consistent with a $Z_3$-Potts nematic transition. Here, we show that the nematic coupling to the acoustic phonons, which makes the nematic correlation length tend to diverge along certain directions only, can fundamentally alter this phenomenology in trigonal lattices. Compared to hexagonal lattices, the former possesses a sixth independent elastic constant $c_{14}$ due to the fact that the in-plane shear strain doublet $({\epsilon }_{xx}-{\epsilon }_{yy},-2{\epsilon }_{xy})$ and the out-of-plane shear strain doublet $(2{\epsilon }_{yz},-2{\epsilon }_{zx})$ transform as the same irreducible representation. We find that, when $c_{14}$ overcomes a threshold value, which is expected to be the case in doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, the nematic director $\alpha$ unlocks from the high-symmetry directions due to the competition between the quadratic phonon-mediated interaction and the cubic nematic anharmonicity. This implies the breaking of the residual in-plane twofold rotational symmetry ($C_{2x}$), resulting in a triclinic phase. We discuss the implications of these findings for the structure of nematic domains, for the shape of the in-plane $H_{c2}$ in $A_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$, and for the presence of nodes inside the superconducting state.

Hexagonal boron nitride (h-BN) is a promising platform for quantum information processing due to its potential to host optically active defects with attractive optical and spin properties. Recent studies suggest that carbon trimers might be the defect responsible for single-photon emission in the visible spectral range in h-BN. In this theoretical study, we combine group theory together with density-functional theory (DFT) calculations to predict the properties of the neutral ${\mathrm{C}}_2{\mathrm{C}}_{\mathrm{N}}$ carbon trimer defect. We find the multi-electron states of this defect along with possible radiative and nonradiative transitions assisted by the spin-orbit and the spin-spin interactions. We also investigate the Hamiltonian for external magnetic-field and ground-state hyperfine interactions. Lastly, we use the results of our investigation in a Lindblad master-equation model to predict an optically detected magnetic resonance signal and the $g^2\left(\tau \right)$ correlation function. Our findings can have important outcomes in quantum information applications such as quantum repeaters used in quantum networks and quantum sensing.

The rigid rotor is a classic problem in quantum mechanics, describing the dynamics of a rigid body with its center of mass held fixed. It is well known to describe the rotational spectra of molecules. The configuration space of this problem is SO(3), the space of all rotations in three dimensions. This is a topological space with two types of closed loops: trivial loops that can be adiabatically shrunk to a point and nontrivial loops that cannot. In the traditional formulation of the problem, stationary states are periodic over both types of closed loops. However, periodicity conditions may change if Berry phases are introduced. We argue that time-reversal symmetry allows for only one new possibility—a Berry phase of $\pi$ attached to all nontrivial loops. We derive the corresponding stationary states by exploiting the connection between SO(3) and SU(2) spaces. The solutions are antiperiodic over any nontrivial loop; i.e., stationary states reverse sign under a $2\pi$ rotation about any axis. Remarkably, this framework is realized in the low-energy physics of certain quantum magnets. The magnets must satisfy the following conditions: (a) the classical ground states are unpolarized, carrying no net magnetization, (b) the set of classical ground states is indexed by SO(3), and (c) the product $N\times S$ is a half-integer, where $N$ is the number of spins and $S$ is the spin quantum number. We demonstrate this result in a family of Heisenberg antiferromagnets defined on polygons with an odd number of vertices. At each vertex, we have a spin-$S$ moment that is coupled to its nearest neighbors. In the classical limit, these magnets have coplanar ground states. Their quantum spectra, at low energies, correspond to “spherical top” and “symmetric top” rigid rotors. For integer values of $S$, we recover traditional rigid-rotor spectra. With half-integer $S$, we obtain rotor spectra with a Berry phase of $\pi$.

The competition between external drives and interparticle interactions are important for the static and dynamic properties of topological quasiparticles in confined geometries. Here, we study the dynamics of particlelike skyrmions under a current-induced compression, where skyrmions are stabilized in a confined space and form a skyrmion solid. We find that a lattice structural transition of nanoscale skyrmions could be induced by the compression, which depends on the initial lattice configuration and the compression level. The compression effect in the skyrmion solid with strong skyrmion-skyrmion repulsion could produce a wave front showing an abrupt change of the skyrmion density, which may propagate through the skyrmion solid depending on the compression level. Besides, a chain of skyrmions on the surface of the compressed skyrmion solid could show a fast and stable motion along the surface as long as it remains within the driving region. Some skyrmions may collapse during the compression due to the destructive skyrmion-skyrmion collision, which mainly happens in the area around the skyrmion injection boundary between the driving and compressive regions. Our results are useful for understanding the compression physics of quasiparticles carrying nontrivial topology.

Controlled creation of localized magnetic textures beyond conventional $\pi$-skyrmions is an important problem in the field of magnetism. Here, by means of spin dynamics simulations, Monte Carlo simulations, and harmonic transition state theory we demonstrate that an elementary chiral magnetic soliton with zero topological charge—the chiral droplet—can be created by thermal fluctuations in the presence of the tilted magnetic field. The proposed protocol relies on an unusual kinetics combining the effects of the entropic stabilization and low-energy barrier for the nucleation of a topologically trivial state. Following this protocol by varying temperature and the tilt of the external magnetic field, one can selectively generate chiral droplets or $\pi$-skyrmions in a single system. The coexistence of two distinct magnetic solitons establishes a basis for a rich magnetization dynamics and opens up the possibility for the construction of more complex magnetic textures such as skyrmion bags and skyrmions with chiral kinks.