We study how transition radiation is modified by the presence of a generic magnetoelectric medium with a special focus on topological insulators. To this end, we use the Green's function for the electromagnetic field in presence of a plane interface between two topological insulators with different topological parameters, permittivities, and permeabilities. We employ the far-field approximation together with the steepest descent method to obtain approximate analytical expressions for the electromagnetic field. Through this method, we find that the electric field is a superposition of spherical waves and lateral waves. Contributions of both kinds can be attributed to a purely topological origin. After computing the angular distribution of the radiation, we find that in a region far from the interface the main contribution to the radiation comes from the spherical waves. We present typical radiation patterns for the topological insulator ${\mathrm{TlBiSe}}_2$ and the magnetoelectric ${\mathrm{TbPO}}_4$. In the ultrarelativistic case, the additional contributions from the magnetoelectric coupling appreciably enhance the global maximum of the angular distribution. We also present an analytic expression for the frequency distribution of the radiation for this case. We find that in the limit where the permittivities are equal, there still exists transition radiation of the order of the square of the topological parameter with a pure topological origin.

While periodically driven phases offer a unique insight into nonequilibrium topology that is richer than its static counterpart, their experimental realization is often hindered by ubiquitous decoherence effects. Recently, we have proposed a decoherence-free approach of realizing these Floquet phases. The central insight is that the reflection matrix, being unitary for a bulk insulator, plays the role of a Floquet time-evolution operator. We have shown that reflection processes off the boundaries of systems supporting higher-order topological phases (HOTPs) simulate nontrivial Floquet phases. So far, this method was shown to work for one-dimensional Floquet topological phases protected by local symmetries. Here, we extend the range of applicability by studying reflection off three-dimensional HOTPs with corner and hinge modes. We show that the reflection processes can simulate both first-order and second-order Floquet phases, protected by a combination of local and spatial symmetries. For every phase, we discuss appropriate topological invariants calculated with the nested scattering matrix method.

Ideal Chern insulating phases arise in two-dimensional systems with broken time-reversal symmetry. They are characterized by having nearly flat bands, and a uniform quantum geometry—which combines the Berry curvature and quantum metric—and by being incompressible. In this work, we analyze the role of the quantum geometry in ideal Chern insulators aiming to describe transport in presence of external out-of-plane magnetic and electric fields. We first show that in the absence of external perturbations, novel Berry connections appear in ideal Chern insulating phases. Second, we provide a detailed analysis of the deformation of the quantum geometry once weak out-of-plane magnetic fields are switched on. The perturbed Berry curvature and quantum metric provide an effective quantum geometry, which is analyzed in the insulating regime and provides an application of our novel connections. The conditions under which the Girvin-MacDonald-Platzman algebra is realized in this situation are discussed. Furthermore, an investigation of electrical transport due to the new effective quantum geometry is presented once an electric field is added. Restricting to the case of two bands in the metallic regime the quantum metric appears as a measurable quantum-mechanical correction in the Hall response. Our findings can be applied, for instance, to rhombohedral trilayer graphene at low energies.

We report electronic band structure calculations for ${\mathrm{Sr}}_4{\mathrm{Ru}}_3{\mathrm{O}}_{10}$ which display both ferromagnetic and metamagnetic behavior. The density functional calculations find the ground state to be ferromagnetic in agreement with experiment and we show that the resulting spin polarization has dramatic consequences for the electronic properties. The minority-spin bands are mainly empty and disperse steeply upward at the Fermi energy, whereas the majority-spin bands are full or nearly fully occupied and form narrow bands near the Fermi energy, which could be the electronic origin of the metamagnetism. Inclusion of a Hubbard interaction $U$ applied to the Ru $4d$ states has major effects on the narrow bands, which reveal the role of Coulomb interactions and correlated many-body physics. The results are in qualitative agreement with recent angle-resolved photoemission spectroscopy (ARPES) experiments and show the need for a combined theoretical study and experimental ARPES investigation with better energy resolution to reveal the nature of the narrow bands close to the Fermi level, which is critical for understanding the exotic magnetic properties observed in this material.

We put forward a relation between the static charge fluctuations and the conductance of correlated many-fermion systems at zero temperature, avoiding the use of time-dependent fluctuations as in the fluctuation-dissipation theorem. Static charge fluctuations can efficiently be computed for low-dimensional systems using tensor network approaches, while the conductance is often significantly more difficult to obtain, requiring a challenging low-frequency linear response computation or an explicit time evolution. We put this relation to the test for quantum dot and quantum point contact setups, where in limiting cases exact results are known. Our study includes systems in which the one-dimensional reservoirs are interacting.

Recent experiments have revealed the tantalizing possibility of fabricating lattice electronic systems strongly coupled to quantum fluctuations of electromagnetic fields, e.g., by means of geometry confinement from a cavity or artificial gauge fields in quantum simulators. In this work, we develop a high-frequency expansion to construct the effective models for lattice electrons strongly coupled to a continuum of off-resonant photon modes with arbitrary dispersion. The theory is nonperturbative in the light-matter coupling strength and is therefore particularly suitable for the ultrastrong light-matter coupling regime. Using the effective models, we demonstrate how the dispersion and topology of the electronic energy bands can be tuned by the cavity. In particular, quasi-one-dimensional physics can emerge in a two-dimensional square lattice due to a spatially anisotropic band renormalization, and a topologically nontrivial anomalous quantum Hall state can be induced in a honeycomb lattice when the cavity setup breaks time-reversal symmetry. We also demonstrate that the photon-mediated interaction induces an unconventional superconducting paired phase distinct from the pair-density-wave state discussed in models with truncated light-matter coupling. Finally, we study a realistic setup of a Fabry-Pérot cavity. Our work provides a systematic framework to explore the emergent phenomena due to strong light-matter coupling and points out alternative directions of engineering orders and topological states in solids.

Within the Floquet theory of periodically driven quantum systems, we demonstrate that a high-frequency electromagnetic field can be used as an effective tool to control excitonic properties of semiconductor quantum dots (QDs). It is shown, particularly, that the field both decreases the exciton binding energy and dynamically stabilizes the exciton, increasing its radiative lifetime. The developed theory can serve as a basis for the ultrafast method to tune spectral characteristics of the QD-based photon emitters by a high-frequency field.

Understanding the magnetic properties of atomic chains on superconductors is an essential cornerstone on the road towards controlling and constructing topological electronic matter. Yet, even in simple models, the magnetic ground states remain debated. Ferromagnetic (FM), antiferromagnetic (AFM), and spin spiral configurations have been suggested and experimentally detected, while noncoplanar phases and complex collinear phases have been additionally conjectured. Here, we resolve parts of the controversy by determining the magnetic ground states of chains of magnetic atoms in proximity to a superconductor with Monte Carlo methods. We confirm the existence of FM, AFM, and spin spiral ground states, exclude noncoplanar phases in the model, and clarify the parametric region of a $\uparrow \uparrow \downarrow \downarrow$ phase. We further identify a number of complex collinear spin configurations, including the periodic spin configurations $\uparrow \uparrow \uparrow \downarrow$ and $\uparrow \uparrow \uparrow \downarrow \uparrow \downarrow \downarrow \downarrow \uparrow \downarrow$, which are in some cases combined with harmonic and anharmonic spirals to form the ground state. We topologically classify the electronic structures, investigate their stability against increasing the superconducting order parameter, and explain the complex collinear order by an effective Heisenberg model with dominant four-spin interactions.

Density functional theory has been employed to study graphene on the (111), (100), and (110) surfaces of bare silicon (Si) substrates, which provide three different densities of surface atoms. There are several interesting findings. First, carbon atoms in graphene can form covalent bonds with Si atoms, when placed close enough on Si (111) and (100) surfaces, but not on the (110) surface. The Si (111) surface shifts the Fermi level of graphene into its conduction band, resulting in an increase of the electron density by three orders of magnitude. The work function of graphene is increased by 0.29 eV on the (111) surface, likely due to the surface dipole from the redistribution of $\pi$ orbitals. The change in the number of available states below the Fermi level of graphene due to its interaction with the Si surface, is the main cause for the unconventional doping reported in this paper. The electron density can also be increased by eighty times on a Si (100) substrate without the shift of Fermi level, which is another clear example of the proposed doping mechanism. These striking effects that different orientations of a silicon substrate can have on the properties of graphene are related to the surface atom density of the substrate. These results provide valuable guidance to the growth of graphene on Si for various purposes for electronic devices.

Temperature-dependent Brillouin and Raman spectroscopic investigations are carried out on energetic material 4,10-Dinitro-2,6,8,12-tetraoxa-4,10-diazatetracyclo $\left[{5.5.0.0}^{5,9}{.0}^{3,11}\right]$ -dodecane (TEX; ${\mathrm{C}}_6{\mathrm{H}}_6{\mathrm{N}}_4{\mathrm{O}}_8$) from −196 to 180 °C, close to its sublimation point, to study its elastic and phonon mode behaviors. Ambient values of experimental shear ($G$) and bulk ($K$) moduli of TEX are obtained as $G=5.6\mathrm{GPa}$ and $K=17.7\mathrm{GPa}$. We have computed the elastic tensor and obtained the averaged bulk and shear moduli to compare with the experimentally obtained polycrystalline values. The known lower sensitivity of TEX compared with similar caged secondary explosives as CL-20, Royal Demolition eXplosive, and β-High Melting eXplosive is reckoned as due to its inherently high bulk modulus. We report direct experimental evidence of rapid reduction in elastic constant of TEX from 20 to 80 °C, much before its melting point. Though there are no phase transitions in this temperature span, all Raman bands exhibit softening behavior around this temperature. We observe two distinct anomalies. Some bending modes such as bending of CNC $\left(356\mathrm{c}{\mathrm{m}}^{-1}\right)$, in-plane bending of NNO $\left(552\mathrm{c}{\mathrm{m}}^{-1}\right)$, bending of NCO $\left(610\mathrm{c}{\mathrm{m}}^{-1}\right)$, in-plane bending of NNO (635 $\mathrm{c}{\mathrm{m}}^{-1}$), and in-plane bending of ONO and OCO (712 $\mathrm{c}{\mathrm{m}}^{-1}$) exhibit slight hardening before and after the elastic anomaly. However, the asymmetric stretching modes at 1574 $\mathrm{c}{\mathrm{m}}^{-1}$ (asymmetric stretch of NO) and 1590 $\mathrm{c}{\mathrm{m}}^{-1}$ (asymmetric stretch of ${\mathrm{NO}}_2$) exhibit hardening below 20 °C but soften after 80 °C. This hardening switching over to softening above the anomaly indicates a release of stiffness manifesting from a conformational change to exo-endo and aiding the increasing compressibility. Emergence of new diffraction peaks $\sim 50{}^{\circ }\mathrm{C}$ and even observed at 150 °C, which is well above the elastic anomaly, suggests that the material at high temperature may be isostructural to that at low temperature. In this paper, we establish that the high bulk modulus value of TEX contributes to its lower sensitivity; however, we also clearly demonstrate reduction in elastic modulus accompanied by anomalous behavior of Raman modes. This decrease in elastic modulus leading to increased compressibility may result in increased sensitivity of the explosive material just above room temperature.

The structural and magnetic properties of Zn/Al doped nickel ferrite thin films can be adjusted by changing the Zn and Al content. The films are epitaxially grown by reactive magnetron sputtering using a triple cluster system to sputter simultaneously from three different targets. Upon the variation of the Zn content, the films remain fully strained with similar structural properties, while the magnetic properties are strongly affected. The saturation magnetization and coercivity as well as resonance position and linewidth from ferromagnetic resonance (FMR) measurements are altered depending on the Zn content in the material. The reason for these changes can be elucidated by investigation of the x-ray magnetic circular dichroism spectra to gain site- and valence-specific information with elemental specificity. Additionally, from a detailed investigation by broadband FMR, a minimum in $g$ factor and linewidth could be found as a function of film thickness. Furthermore, the results from a variation of the Al content using the same set of measurement techniques is given. Other than for Zn, the variation of Al affects the strain and even more pronounced changes to the magnetic properties are apparent.

We examine the appearance of superconductivity in the strong-coupling limit of the Hubbard model on the pyrochlore lattice. We focus upon the limit of half-filling, where the normal-state band structure realizes a $j=\frac{3}{2}$ semimetal. Introducing doping, we show that the pairing is favored in a $J=2$ quintet $E_g$ state. The attractive interaction in this channel relies on the fact that $E_g$ pairing on the pyrochlore lattice avoids the detrimental onsite repulsion. Our calculations show that a time-reversal symmetry-breaking superconducting phase is favored, which displays Bogoliubov Fermi surfaces.

The superconductor-insulator transition (SIT) in thin-film disordered superconductors is a hallmark example of a quantum phase transition. Despite being observed more than 30 years ago, its nature is still under a lively debate. One intriguing observation concerns the insulating side of the transition, which exhibits some unusual properties. Among them is its current-voltage relation ($I\text{−}V$ curve), which includes (i) a conductance that changes abruptly by several orders of magnitude with increasing voltage, (ii) hysteretic behavior, and (iii) multiple (sometimes more than 100) smaller current jumps near the transition. Some models have been suggested before, but no model has been successful in accounting for the observed behavior in full. One commonly used approach is to model the disordered sample as a two-dimensional array of conducting islands, where charge carriers tunnel from one island to its neighbors. In those models, fast relaxation is assumed, and the system is treated as always being in electrostatic and thermal equilibrium. Those models are successful in explaining some measurement results, including the phase transition itself, but they fail to reproduce hysteresis in their predicted $I\text{−}V$ curves. Here, we suggest incorporating finite relaxation time into an array model. We show that, in the slow relaxation limit, our model can reproduce hysteresis and multiple jumps in the $I\text{−}V$ curve. Based on our results, we argue that a similar behavior should also be observed in two-dimensional normal (nonsuperconducting) arrays. This claim is supported by past observations. We analyze the role of different parameters in our model, determine the range of relevant timescales in the problem, and compare our results with selected measurements.

We present an experimental study of quantum diffusion of atomic hydrogen in solid ${\mathrm{H}}_2$ films at temperatures below $1\mathrm{K}$. The atoms are generated via electron impact dissociation by running a continuous rf discharge in helium gas above the ${\mathrm{H}}_2$ film for long (up to 30 days) times. We are able to distinguish between the diffusion of the atoms moving towards each other followed by their recombination and the pure spatial diffusion driven by the density gradient. We found that in both cases the flux of phonons generated by the discharge above the surface of molecular film is essential to observe the slow diffusive motion. We obtained rates of pure spatial diffusion of H atoms in normal-${\mathrm{H}}_2$ (75% ortho, 25% para) films which were two orders of magnitude faster than those obtained from the H atom recombination, the quantity used in all previous work to characterize the mobility of H atoms in solid ${\mathrm{H}}_2$. We investigated the influence of the film thickness and its ortho-para composition on recombination and pure spatial diffusion. For thin enough films of 0.16 $\mu \mathrm{m}$ we observed complete diffusion of the H atoms through the entire film thickness. We observed peculiar behavior of the samples with ortho-${\mathrm{H}}_2$ ($o\text{−}{\mathrm{H}}_2$) concentration below $5\%$. The recombination rate in these samples was an order of magnitude faster while the rate of spatial diffusion was somewhat slower than in films containing larger $o\text{−}{\mathrm{H}}_2$ concentrations. The rate of production of H atoms in the low $o\text{−}{\mathrm{H}}_2$ samples turned out to be an order of magnitude larger. We discuss possible explanations of these somewhat contradictory observations.

The spatiotemporal spin dynamics of two-dimensional electron spins with ballistic motion in a GaAs quantum well is investigated using time- and spatially resolved magneto-optical Kerr rotation measurements. The ballistic movement of photoexcited spins, which maintains the width of the excitation spot size, results in a wave-packet-like motion of the spin distribution. The time evolution of the spin distribution measured at different excitation intensities reveals that the spin diffusion slows down and the wave-packet-like motion disappears at higher photoexcited spin densities, at which the spin distribution converges to the helical spin mode observed in the diffusive regime. The variation in the spatiotemporal spin distribution can be controlled by spin diffusion, while the spin precession length is determined by the strength of the spin-orbit interaction.

The magnetization of the highly air-stable two-dimensional (2D) intrinsic ferromagnet ${\mathrm{FeTa}}_3{\mathrm{S}}_6$ single crystal has been systematically investigated. Magnetization measurement revealed paramagnetic to ferromagnetic (PM-FM) phase transition at around 35 K and a strong magnetic anisotropy along the crystallographic c axis ($H\parallel c$). The critical exponents $\beta$ = 0.189(2), $\gamma$ = 1.423(1), and $\delta =8.531\left(7\right)$ with $T_c$ = 35 K suggest that the spin interaction of a two-dimensional (2D) Ising type in ${\mathrm{FeTa}}_3{\mathrm{S}}_6$ coupled with long-range ($\sigma =1.581$) interaction. A comprehensive magnetic phase diagram based on detailed magnetization measurements and universality scaling of ${\mathrm{FeTa}}_3{\mathrm{S}}_6$ is constructed over three magnetic field regions. The magnetic phase diagram of ${\mathrm{FeTa}}_3{\mathrm{S}}_6$ is analogous to that of ${\mathrm{CrNb}}_3{\mathrm{S}}_6$, which exhibits a chiral magnetic soliton lattice. DFT simulation reveals that the large magnetic anisotropy energy in ${\mathrm{FeTa}}_3{\mathrm{S}}_6$ originates from the combined effects such as considerable orbital magnetic moment, host lattice spin-orbit coupling, and hybridization between host lattice $2\mathrm{H}-{\mathrm{TaS}}_2$ and the intercalated Fe component. Furthermore, the magnetic anisotropy ($\parallel$ c) in ${\mathrm{FeTa}}_3{\mathrm{S}}_6$ is due to the dominant contribution of the spin conserving process $\mathrm{\Delta }{S}_z$ = 0 in magnetic anisotropy energy, which differs from that of ${\mathrm{CrNb}}_3{\mathrm{S}}_6$, or ${\mathrm{MnNb}}_3{\mathrm{S}}_6$, where the spin flipping process $\mathrm{\Delta }{S}_z$ = 1 dominates, causing in-plane ($\perp$ c) magnetic anisotropy.

We theoretically study quantum transport through a quantum dot coupled to Majorana bound states confined at the ends of a topological superconducting nanowire. The topological superconductor forms a loop and is threaded by a tunable magnetic flux, which allows one to control the electron transport in the system. In particular, we investigate phonon-assisted transport properties in the device when the central quantum dot interacts with a single long-wave optical phonon mode. We find that when the two Majorana bound states are unhybridized, the zero-temperature linear conductance has a $2\pi$ periodicity as a function of magnetic flux phase, independent of the electron-phonon interaction, the quantum dot energy, or the finite values of dot-Majorana couplings. For a finite overlap between the Majorana bound states, the linear conductance periodicity generally changes to $4\pi$ either due to a finite electron-phonon coupling strength, or a dot energy level that is tuned away from the Fermi level. Additionally, the differential conductance periodicity changes from $2\pi$ to $4\pi$ when the Majorana bound states hybridize and the electron-phonon coupling is finite. Our results provide insight into transport signatures expected in topological quantum computational platforms that integrate quantum dots as a means for Majorana qubit readout. The energy exchange with an environmental bath, here a single phonon mode, significantly alters the current signatures expected from Majorana modes.

Solving the many-body Schrödinger equation in continuous spaces with the presence of strong correlations is an extremely important and challenging issue in quantum physics. In this work, we propose the functional tensor network (FTN) approach to solve the many-body Schrödinger equation. Provided the orthonormal functional bases, we represent the coefficients of the many-body wave function as a tensor network. The observables, such as energy, can be calculated simply by tensor contractions. Simulating the ground state becomes solving a minimization problem defined by the tensor network. An efficient gradient-decent algorithm based on automatically differentiable tensors is proposed. We here take the matrix product state (MPS), whose complexity scales only linearly with the system size, as an example. We apply our approach to solve the ground state of coupled harmonic oscillators and achieve high accuracy by comparing our results with the exact solutions. Reliable results are also given in the presence of three-body interactions, where the system cannot be decoupled from isolated oscillators. Our approach is simple and, with well-controlled error, essentially different from highly nonlinear neural-network solvers. Our work extends the applications of the tensor network from quantum lattice models to systems in continuous space. The FTN can be used as a general solver of the differential equations with many variables. The MPS exemplified here can be generalized to, e.g., fermionic tensor networks to solve the electronic Schrödinger equation.