We describe and explain two competing regimes of ac magnetic response in current-carrying type-II superconducting wires which were observed experimentally. In the usual regime, voltage $V\left(t\right)$, induced by vortex motion across the wire, is “in phase” with the external magnetic field $H\left(t\right)\propto sin\left(\omega t\right)$. However, as frequency $\omega$ grows up or transport current $I$ decreases, an anomalous, “out-of-phase” peak in $V\left(t\right)$ appears. If these two regimes coexist, then two peaks in voltage are observed per half period of $H\left(t\right)$ in both experiment and numerical simulations. At certain combinations of $\omega , I$ and the amplitude of the external field, the out-of-phase mechanism even overwhelms the usual, in-phase one. It is shown that the out-of-phase maximum in $V\left(t\right)$ is due to the inhibition effect of zero-field (annihilation) lines on flux motion. Such lines, if present in the sample, significantly decelerate magnetic relaxation and dramatically affect the induced voltage. A phase diagram enabling one to distinguish between the in-phase and out-of-phase regimes is constructed.

Bound states in the continuum (BICs) enabling resonances with high quality ($Q$) factor allow for high-efficiency light-matter interactions and boost various research fields such as nonlinear photonics and optical detecting. However, further improvement of the $Q$ factor of the resonances is challenging since the accurate fabrication of the tiny symmetry defects of meta-atoms is limited by the nanofabrication technology, which is critical for transition from BICs to quasi-BICs in previous works. Here we propose a different approach to trigger the transition of quasi-BICs that does not need to change the space group symmetry of nanostructures. By modulating the periods of the nanostructures, our approach enlarges the unit cell and enriches the ideal BICs eigenstates, achieving multiband quasi-BICs in metasurfaces. By tuning the asymmetry parameters, the selective excitation of multiband quasi-BICs is demonstrated. Thanks to the multi-narrow-band resonances obtained by the unit cell perturbation, a multivariable sensing method is proposed to simultaneously obtain the birefringence property and thickness of a nanoscale layer. The proposed approach exciting multiband quasi-BICs offers a possible solution for ultra-high-$Q$ devices without complex fabrication techniques and may found realistic application in many fields such as harmonic generation, optical detecting, and compact low-threshold lasers.

Abelian Chern-Simons theory, characterized by the so-called $K$ matrix, has been quite successful in characterizing and classifying Abelian fractional quantum Hall effect as well as symmetry protected topological (SPT) phases, especially for bosonic SPT phases. However, there are still some puzzles in dealing with fermionic SPT (fSPT) phases. In this paper, we utilize the Abelian Chern-Simons theory to study the fSPT phases protected by arbitrary Abelian total symmetry $G_f$. Comparing to the bosonic SPT phases, fSPT phases with Abelian total symmetry $G_f$ have three new features: (1) they may support gapless Majorana fermion edge modes, (2) some nontrivial bosonic SPT phases may be trivialized if $G_f$ is a nontrivial extension of bosonic symmetry $G_b$ by ${\mathbb{Z}}_2^f$, and (3) certain intrinsic fSPT phases can only be realized in interacting fermionic system. We obtain edge theories for various fSPT phases, which can also be regarded as conformal field theories with proper symmetry anomaly. In particular, we discover the construction of Luttinger liquid edge theories with central charge $n-1$ for type-III bosonic SPT phases protected by ${\left({\mathbb{Z}}_n\right)}^3$ symmetry and the Luttinger liquid edge theories for intrinsically interacting fSPT protected by unitary Abelian symmetry. The ideas and methods used in these examples could be generalized to derive the edge theories of fSPT phases with arbitrary unitary Abelian total symmetry $G_f$.

We present a dynamical mean-field study of antiferromagnetic magnons in a one-, two-, and three-orbital Hubbard model of square and bcc cubic lattice at intermediate coupling strength. We investigate the effect of anisotropy introduced by an external magnetic field or single-ion anisotropy. For the latter we tune continuously between the easy-axis and easy-plane models. We also analyze a model with spin-orbit coupling in cubic site-symmetry setting. The ordered states as well as the magnetic excitations are sensitive to even a small breaking of $SU\left(2\right)$ symmetry of the model and follow the expectations of spin-wave theory as well as general symmetry considerations.

We present an ab initio derivation method for effective low-energy Hamiltonians of material with strong spin-orbit interactions. The effective Hamiltonian is described in terms of the Wannier function in the spinor form, and effective interactions are derived with the constrained random phase approximation (cRPA) method. Based on this formalism and the developed code, we derive an effective Hamiltonian of a strong spin-orbit interaction material ${\mathrm{Ca}}_5{\mathrm{Ir}}_3{\mathrm{O}}_{12}$. This system consists of three edge-shared ${\mathrm{IrO}}_6$ octahedral chains arranged along the $c$ axis, and the three Ir atoms in the $ab$ plane compose a triangular lattice. For such a complicated structure, we need to set up the Wannier spinor function under the local coordinate system. We found that a density-functional band structure near the Fermi level is formed by local $d_{xy}$ and $d_{yz}$ orbitals. Then, we constructed the ab initio $d_{xy}/d_{yz}$ model. The estimated nearest-neighbor transfer $t$ is close to 0.2 eV, and the cRPA on-site $U$ and neighboring $V$ electronic interactions are found to be 2.4–2.5 eV and 1 eV, respectively. The resulting characteristic correlation strength defined by $(U-V)/t$ is above 7, and thus this material is classified as a strongly correlated electron system. The on-site transfer integral involved in the spin-orbit interaction is 0.2 eV, which is comparable to the on-site exchange integrals $\sim 0.2$ eV, indicating that the spin-orbit-interaction physics would compete with the Hund physics. Based on these calculated results, we discuss possible rich ground-state low-energy electronic structures of spin, charge, and orbitals with competing Hund, spin-orbit, and strong-correlation physics.

Electronic correlations could have significant impact on the material properties. They are typically pronounced for localized orbitals and enhanced in low-dimensional systems, so two-dimensional (2D) transition metal compounds could be a good platform to study their effects. Recently, a new class of 2D transition metal compounds, the ${\mathrm{MoSi}}_2{\mathrm{N}}_4$-family materials, has been discovered, and some of them exhibit intrinsic magnetism. Here, taking monolayer ${\mathrm{VSi}}_2{\mathrm{P}}_4$ as an example from the family, we investigate the impact of correlation effects on its physical properties, based on the first-principles calculations with the $\mathrm{DFT}+U$ approach. We find that different correlation strengths can drive the system into a variety of interesting ground states, with rich magnetic, topological, and valley features. With increasing correlation strength, while the system favors a ferromagnetic semiconductor state for most cases, the magnetic anisotropy and the band gap type undergo multiple transitions, and in the process, the band edges can form single, two, or three valleys for electrons or holes. Remarkably, there is a quantum anomalous Hall (QAH) insulator phase, which has a unit Chern number and has its chiral edge states polarized in one of the valleys. The boundary of the QAH phase corresponds to the half-valley semimetal state with fully valley polarized bulk carriers. We further show that for phases with the out-of-plane magnetic anisotropy, the interplay between spin-orbit coupling and orbital character of valleys enables an intrinsic valley polarization for electrons but not for holes. This electron valley polarization can be switched by reversing the magnetization direction, providing a new route of magnetic control of valleytronics. Our result sheds light on the possible role of correlation effects in the 2D transition metal compounds, and it will open new perspectives for spintronic, valleytronic, and topological nanoelectronic applications based on these materials.

Excitonic insulator is a macroscopic quantum system whose ground state consists of spontaneously formed and condensed excitons. While various excitonic insulators have been explored, they are almost exclusively characterized by the condensation of dark (optically inactive) excitons. Our first-principles $GW$-BSE calculations show that a transverse electric field will drive an organometallic manganese-cyclopentadienyl wire into a unique “bright” excitonic insulator phase, as the combined consequence of giant Stark effect and electric-field-induced symmetry breaking. Such bright excitonic insulators can be directly identified by experiment due to their coherent radiation. Our results are not only of scientific interest in electric-field control of exciton condensation, but may also open an opportunity to create a coherent light source based on the excitonic insulator.

We present measurements of the polarization dependencies of the x-ray signal photons generated by the effect of parametric down-conversion of x rays into ultraviolet radiation. The results exhibit pronounced discrepancies with the classical model for the nonlinearity but qualitatively agree with a recently developed quantum mechanical theory for the nonlinear interaction. Our work shows that the reconstruction of the atomic scale charge distribution of valence electrons in crystals by using nonlinear interaction between x rays and longer wavelength radiation, as was suggested in previous works, requires the knowledge of polarization of the generated x-ray signal beam. The results presented in this work indicate a methodology for the study of properties of the Wannier functions in crystals.

We perform ab initio density functional theory calculations to study the lattice vibrations of graphene quantum dots (GQDs) with triangular, quadrate, and hexagonal shapes in lateral confinement of 1 to 3 nm and terminated in both zigzag and armchair orientations. We see the vibrational properties of GQD transform from molecular type into bulk-like type with increasing dot size and the features of the vibrational density of state also highly depend on the symmetry of the GQD. We find that the out-of-plane vibrated standing waves induced by lateral confinement instead of the in-plane vibrations dominate the lattice vibrations of small GQDs. By projecting the vibrational eigenvectors of GQDs on those of single-layer graphene, we see the mixture of the out-of-plane and in-plane vibrational characters in GQDs as the result of the strong lateral confinement. We identify coherent acoustic phonon modes in GQDs and find that the size dependence of coherent acoustic phonon frequency increases with increasing the isotropy of nanostructures. Moreover, different confinement effects on lattice vibrations along zigzag and armchair edge orientations are identified from ab initio calculations. We describe the Raman intensity of GQDs and observe a blue shift of the G-mode position in GQDs comparing to that of the single-layer graphene. Combining the bond length distribution at the edge of GQDs with the positive Grüneisen parameters, we link such blue shift to the surface effect of GQDs.

Two-photon spectroscopy is an important tool for detection of states that are dark in single-photon spectra. We calculate two-photon absorption spectra for monolayer transition-metal dichalcogenides including excitonic effects based on a Bethe-Salpeter equation approach. Both spin-orbit coupling and trigonal warping are included. Our results reveal that otherwise dark $p$ states dominate the two-photon response. However, weakly allowed two-photon transitions coinciding with single-photon $s$-state resonances appear due to trigonal warping. The angular momentum character of participating excitons is analyzed and we study the sensitivity to screening by dielectric surroundings and discuss the free-carrier limit. Biexciton contributions to the high-energy two-photon response are estimated using a simplified Wannier approach. Finally, a brief comparison with recent experiments is given.

The quasiparticle band-gap renormalization induced by the doped carriers is an important and well-known feature in two-dimensional semiconductors, including transition-metal dichalcogenides (TMDs), and it is of both theoretical and practical interest. To get a quantitative understanding of this effect, here we calculate the quasiparticle band-gap renormalization of the electron-doped monolayer ${\mathrm{MoS}}_2$, a prototypical member of TMDs. The many-body electron-electron interaction induced renormalization of the self-energy is found within the random phase approximation and to account for the quasi-2D character of the Coulomb interaction in this system a Keldysh-type interaction with a nonlocal dielectric constant is used. Considering the renormalization of both the valence and the conduction bands, our calculations reveal a large and nonlinear band-gap renormalization upon adding free carriers to the conduction band. We find a 410 meV reduction of the band gap for the monolayer ${\mathrm{MoS}}_2$ on ${\mathrm{SiO}}_2$ substrate at the free carrier density $n=4.9\times {10}^{12}{\mathrm{cm}}^{-2}$ which is in excellent agreement with available experimental results. We also discuss the role of exchange and correlation parts of the self-energy on the overall band-gap renormalization of the system. The strong dependence of the band-gap renormalization on the surrounding dielectric environment is also demonstrated in this work, and a much larger shrinkage of the band gap is predicted for the freestanding monolayer ${\mathrm{MoS}}_2$.

van der Waals heterostructures are promising candidates for bringing the materials-on-demand paradigm into reality [F. Capasso, Science 235, 172 (1987)], since their electrical properties can be engineered by playing on the several available degrees of freedom, as the number of layers, the materials, and the order in which they are stacked. In the present work we present ab initio and transport simulations of five different homo- and heterostructures based on two-dimensional materials, using a multiscale computational platform to compute the out-of-plane transmission coefficients. This information can be relevant for a wide range of applications, from nanoscale devices to inkjet-printed circuits based on two-dimensional materials. Each structure has been investigated considering several parameters as the stacking sequence and orientation, as well as the different overlappings between the flakes. We have found that, while transmission across the junction is not significantly influenced by the degree of overlap between the flakes, the different stacking orientation plays a more relevant role.

We have studied magnetotransport of a degenerate two-dimensional electron gas in a Hall sample in the Knudsen regime, when the mean free paths of electrons with respect to their collisions with each other and with impurities are much larger than the width of the sample. In contrast to the usually considered symmetric sample, whose both edges reflect electrons diffusely, we considered an asymmetric sample, one edge of which reflects them diffusely, while the other specularly. It is shown that in such a structure in low magnetic fields the Hall coefficient is parametrically large in comparison with its standard value. Also the situation is discussed when all types of scattering can be neglected except for scattering at the edges of the sample.

The notion of Thouless energy plays a central role in the theory of Anderson localization. We investigate and compare the scaling of Thouless energy across the many-body localization (MBL) transition in a Floquet model. We use a combination of methods that are reliable on the ergodic side of the transition (e.g., spectral form factor) and methods that work on the MBL side (e.g., typical matrix elements of local operators) to obtain a complete picture of the Thouless energy behavior across the transition. On the ergodic side, Thouless energy decreases slowly with the system size, while at the transition it becomes comparable to the level spacing. Different probes yield consistent estimates of Thouless energy in their overlapping regime of applicability, giving the location of the transition point nearly free of finite-size drift. This work establishes a connection between different definitions of Thouless energy in a many-body setting and yields insights into the MBL transition in Floquet systems.

Chalcogenides have attracted wide attention on the structural characterizations and physicochemical properties due to their potential practicability and scientific interests, such as the intriguing stoichiometries and involved fascinating morphologies of sulfur under compression. In this work, our ab initio structure prediction simulations for the Ca-S system predicted unprecedented tetragonal $I4/mcm\text{−}\mathrm{Ca}{\mathrm{S}}_2$ and $P\text{−}{42}_{1}m\text{−}\mathrm{Ca}{\mathrm{S}}_3$ containing the isolated dimeric ${{\mathrm{S}}_2}^{2–}$ and V-shape trimeric ${{\mathrm{S}}_3}^{2–}$ units, respectively, under high pressure. Motived by these theoretical results, remarkably, we have successfully synthesized $\mathrm{Ca}{\mathrm{S}}_2$ and $\mathrm{Ca}{\mathrm{S}}_3$ as unambiguously identified through in situ x-ray diffraction and Raman spectra measurements. The present results have unraveled the high-pressure structure behavior of Ca-S system and further provided key insights for exploring other interesting polysulfides at extreme conditions.

Polar chiral structures have recently attracted much interest within the scientific community, as they pave the way towards innovative device concepts similar to the developments achieved in nanomagnetism. Despite the growing interest, many fundamental questions related to the mechanisms controlling the appearance and stability of ferroelectric topological structures remain open. In this context, ferroelectric nanoparticles provide a flexible playground for such investigations. Here, we present a theoretical study of ferroelectric polar textures in a cylindrical core-shell nanoparticle. The calculations reveal a chiral polarization structure containing two oppositely oriented diffuse axial domains located near the cylinder ends, separated by a region with a zero-axial polarization. We name this polarization configuration “flexon” to underline the flexoelectric nature of its axial polarization. Analytical calculations and numerical simulation results show that the flexon's chirality can be switched by reversing the sign of the flexoelectric coefficient. Furthermore, the anisotropy of the flexoelectric coupling is found to critically influence the polarization texture and domain morphology. The flexon rounded shape, combined with its distinct chiral properties and its localization near the surface, are reminiscent of chiral bobber structures in magnetism. In the azimuthal plane, the flexon displays the polarization state of a vortex with an axially polarized core region, i.e., a meron. The flexoelectric effect, which couples the electric polarization and elastic strain gradients, plays a determining role in the stabilization of these chiral states. We discuss similarities between this interaction and the recently predicted ferroelectric Dzyaloshinskii-Moriya interaction leading to chiral polarization states.

Charge transport in organic semiconductors is affected by the complex interaction between charge carriers and molecular vibrations. A common way to treat the molecular vibrations in hopping approaches is by condensing them into a single analytical parameter, the reorganization energy. In contrast, here we present a nonadiabatic hopping transport approach that avoids this approximation by dividing the vibrational spectrum of organic molecules into three distinct analytical classes, namely the quasistatic, low-frequency dynamic, and high-frequency dynamic modes. The quasistatic and dynamic regimes are separated time consistently with respect to the timescale of the hopping events, which results in charge transfer events driven by a set of strongly coupling driving modes. Using these time-consistent hopping rates, we compute the charge carrier mobilities for a set of hopping transport materials and a control set of band-transport materials and compare them to experimental values. The resulting mobilities are consistent for both sets by showing similar values for the hopping transport materials and lower values for the control set of band-transport materials due to the absence of coherent transport contributions. We further study other popular hopping approaches such as the Marcus theory and the Levich-Jortner theory and observe substantial inconsistencies for them.

Motion signatures of nonstationary electromagnetic bodies are imprinted in their scattering spectrum. While the Doppler frequency shift holds information about the velocity of its center of mass, internal degrees of freedom in a nonrigid body, such as rotation and vibration, introduce nontrivial spectral distortions, termed micro-Doppler signatures. Contemporary analytic characterization of such signatures typically neglects subwavelength electromagnetic coupling, which can dominate the scattering signatures of motion. To address this overlooked scattering regime, a theory of moving coupled dipoles is used to model a moving nonrigid body. The method is verified experimentally in the microwave regime, demonstrating remote sensing of subwavelength information. The method can be useful for analyzing and characterizing effects that frequently emerge in radar science, healthcare monitoring, optical manipulation of particles, and many other applications, where remote sensing and classification of motion are important.

The interaction of intense single-cycle ultrashort (0.1 to 9 fs) light pulses with diamond crystal and thin film is simulated, combining the dependent Kohn-Sham equation with the Maxwell equations. Distinct features are observed depending on the duration of the pulse. In the diamond crystal, maximum energy transfer from light pulse is observed with a pulse duration 0.3 fs. In this case, the phase of current density $J\left(t\right)$ coincides with that of the electric field $E\left(t\right)$. For the incident pulse of duration 0.1 fs, most of the light will transmit on passing the thin film. But for the pulse of duration 0.5 fs, there is more reflection than transmission. For light pulses of durations 7 and 9 fs in diamond crystal, traditional nonlinear behavior of energy transfer are observed. Interestingly, for the attosecond pulse, there is a linear scaling behavior with the pulse intensity below ${10}^{16}\mathrm{W}/{\mathrm{cm}}^2$ on the one hand and an unusual linear dynamic interference response behavior with the pulse width, determined by the interference of the different quantum pathways, on the other hand.