We compute the effects of magnon-phonon coupling on the indirect $K$-edge bimagnon resonant inelastic x-ray scattering (RIXS) intensity spectrum of a square lattice Heisenberg antiferromagnet. We analyze the effects of competing nearest and next-nearest magnetic and magnon-phonon coupling interaction in the RIXS spectrum, for both the antiferromagnetic (AF) and the collinear antiferromagnetic (CAF) phases of the model. Utilizing the Dyson-Maleev representation of spin operators, the Bethe-Salpeter ladder approximation scheme for the bimagnon interacting channel, and considering the lowest-order magnon-phonon-magnon scattering interaction, we highlight distinct features in the x-ray spectrum. Considering damping effects, arising due to the presence of phonons, we find that in the AF phase the RIXS intensity spectrum attains a maximum value primarily localized around the $K\left(\pm \frac{\pi }{2},\pm \frac{\pi }{2}\right)$ point. For the CAF phase, the intensity is broadly distributed with a significant scattering intensity located around the $Y\left(\pm \frac{\pi }{2},0\right)$ point. Furthermore, in the CAF phase for suitable anisotropy, nearest-, and next-nearest-neighbor interaction parameters, the phonon effects can manifest as a distinct peak both below and above the bimagnon peak. Such a feature is in contrast to the antiferromagnetic spectrum where the effect due to the phonon peak was located consistently beyond the bimagnon peak in the high-energy end of the spectrum. Additionally, in the CAF phase we find the RIXS bimagnon-phonon spectrum to be more sensitive to anisotropy compared to its antiferromagnetic counterpart. We conclude that the ultimate effect of magnon-phonon effects in the indirect $K$-edge RIXS spectrum, in both the antiferromagnetic and the collinear antiferromagnetic phases, is an observable effect.

We have studied the effect of cobalt substitution in $L{1}_0-{\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\mathrm{Pt}$ films by means of x-ray magnetic circular dichroism (XMCD) and first-principles calculations. The magnetic moments of Fe ($\sim 2.5{\mu }_{\mathrm{B}}$) and Co ($\sim 1.5{\mu }_{\mathrm{B}}$) deduced using XMCD were almost unchanged upon Co doping, and the net magnetization decreases with increasing Co content. Calculation also showed that the $3d$ electrons that have been added by Co substitution occupy only spin-down bands.

When solid state systems possess active orbital-band structures subject to spin-orbit coupling, their multicomponent electronic structures are often described in terms of effective large-spin fermion models. Their topological structures of superconductivity are beyond the framework of spin singlet and triplet Cooper pairings for spin-$\frac{1}{2}$ systems. Examples include the half-Heusler compound series of RPtBi, where R stands for a rare-earth element. Their spin-orbit coupled electronic structures are described by the Luttinger-Kohn model with effective spin-$\frac{3}{2}$ fermions and are characterized by band inversion. Recent experiments provide evidence to unconventional superconductivity in the YPtBi material with nodal spin-septet pairing. We systematically study topological pairing structures in spin-$\frac{3}{2}$ systems with the cubic group symmetries and calculate the surface Majorana spectra, which exhibit zero energy flat bands, or, cubic dispersion depending on the specific symmetry of the superconducting gap functions. The signatures of these surface states in the quasiparticle interference patterns of tunneling spectroscopy are studied, which can be tested in future experiments.

This work investigates interactions of vortices in superconducting materials between standard types I and II, in the domain of the so-called intertype (IT) superconductivity. Contrary to common expectations, the many-body (many-vortex) contribution is not a correction to the pair-vortex interaction here but plays a crucial role in the formation of the IT vortex matter. In particular, the many-body interactions stabilize vortex clusters that otherwise could not exist. Furthermore, clusters with large numbers of vortices become more stable when approaching the boundary between the intertype domain and type I. This indicates that IT superconductors develop a peculiar unconventional type of the vortex matter governed by the many-body interactions of vortices.

We study effects of pinning on the dynamics of a vortex lattice in a type-II superconductor in the strong-pinning situation and determine the force-velocity (or current-voltage) characteristic combining analytical and numerical methods. Our analysis deals with a small density $n_p$ of defects that act with a large force $f_p$ on the vortices, thereby inducing bistable configurations that are a characteristic feature of strong pinning theory. We determine the velocity-dependent average pinning-force density $\langle F_p\left(v\right)\rangle$ and find that it changes on the velocity scale $v_p\sim f_p/\eta a_0^3$, where $\eta$ is the viscosity of vortex motion and $a_0$ the distance between vortices. In the small pin-density limit, this velocity is much larger than the typical flow velocity $v_c\sim F_c/\eta$ of the free vortex system at drives near the critical force density $F_c=\langle F_p(v=0)\rangle \propto n_p{f}_p$. As a result, we find a generic excess-force characteristic, a nearly linear force-velocity characteristic shifted by the critical force density $F_c$; the linear flux-flow regime is approached only at large drives. Our analysis provides a derivation of Coulomb's law of dry friction for the case of strong vortex pinning.

We present a simple scheme for implementing a one-dimensional (1D) magnetic-flux lattice of ultracold fermionic spin-1/2 atoms. The resulting tight-binding model supports gapped and gapless topological phases and chiral currents for Meissner and vortex phases. Its single-particle spectra exhibit topological flat bands at small flux, and the flatness sensitively depends on hopping strength. An effective $p$-wave interaction arises in a $s$-wave paired superfluid. Treating atomic internal states as forming a synthetic dimension and balancing the interplay of magnetic flux and Zeeman field, our model describes a tunable topological Fermi superfluid, which paves the way towards experimental explorations of non-Abelian topological matter in 1D atomic quantum gases.

Recently, our group proposed a metamaterial laser design based on explicitly coupled dark resonant states in low-loss dielectrics, which conceptually separates the gain-coupled resonant photonic state responsible for macroscopic stimulated emission from the coupling to specific free-space propagating modes, allowing independent adjustment of the lasing state and its coherent radiation output. Due to this functionality, it is now possible to make lasers that can overcome the trade-off between system dimensions and $Q$ factor, especially for surface emitting lasers with deeply subwavelength thickness. Here, we give a detailed discussion of the key functionality and benefits of this design, such as radiation damping tunability, directionality, subwavelength integration, and simple layer-by-layer fabrication. We examine in detail the fundamental design tradeoffs that establish the principle of operation and must be taken into account and give guidance for realistic implementations.

Hole states in electric-field-driven superconductivity at the (110)-surface of diamond are examined by means of first-principles calculations and one-dimensional tight-binding model calculations. It is found that surface-bound hole states confined near the surface by application of an electric field $E$ play a key role in superconductivity. Indeed, there is a critical external electric field $|E_{\mathrm{c}}|$ ($\simeq 0.4$ V/Å) for observing the superconductivity, which can be attributed to the second surface-bound hole state. With McMillan's formula and calculated phonon-electron coupling constants, we demonstrate that, in electric fields $<|E_{\mathrm{c}}|$ which correspond to a surface carrier density of $\sim 2.3\times {10}^{13}{\mathrm{cm}}^{-2}$, superconductivity may not be practically observed while the superconductivity transition temperature suddenly increases at $|E_{\mathrm{c}}|$.

We study finite-size scaling for the magnetic observables of an impurity residing at the end point of an open quantum Ising chain with transverse magnetic field, realized by locally rescaling the field by a factor $\mu \ne 1$. In the homogeneous chain limit at $\mu =1$, we find the expected finite-size scaling for the longitudinal impurity magnetization, with no specific scaling for the transverse magnetization. At variance, in the classical impurity limit $\mu =0$, we recover finite scaling for the longitudinal magnetization, while the transverse one basically does not scale. We provide both analytic approximate expressions for the magnetization and the susceptibility as well as numerical evidences for the scaling behavior. At intermediate values of $\mu$, finite-size scaling is violated, and we provide a possible explanation of this result in terms of the appearance of a second, impurity-related length scale. Finally, by going along the standard quantum-to-classical mapping between statistical models, we derive the classical counterpart of the quantum Ising chain with an end-point impurity as a classical Ising model on a square lattice wrapped on a half-infinite cylinder, with the links along the first circle modified as a function of $\mu$.

We study how the conductance fluctuations change in a disordered ensemble of strongly scattering (non-Born) centers. Diagrammatic calculations of the conductance variance are carried out beyond the standard Born definition for the Hikami vertex. For a system of strong pointlike scatterers, the enhancement of the conductance fluctuations is found in the crossover between ballistic and diffusive regimes. The incoherent contribution arising from random spatial variations in the scatterer concentration is primarily responsible for the enhancement of fluctuations. In the limit of resonant scatterers, the coherent contribution to the conductance variance also peaks in the crossover regime and its maximum exceeds the UCF value.

A concept of quasiplanar one-way transparent terahertz absorbers made of linear isotropic materials is presented. The resulting structure consists of a homogeneous absorbing layer of polar dielectric, GaAs, a dispersion-free substrate, and an ultrathin frequency-selective reflector. It is demonstrated that perfect absorption can be obtained for forward illumination, along with total reflection at backward illumination and transparency windows in the adjacent bands. The design is particularized for the polaritonic gap range where permittivity of GaAs varies in a wide range and includes epsilon-near-zero and transparency regimes. The underlying physics can be explained with the aid of a unified equivalent-circuit (EC) analytical model. Perfect matching of input impedance in forward operation and, simultaneously, strong mismatch in the backward case are the universal criteria of one-way absorption. It is shown that perfect one-way absorption can be achieved at rather arbitrary permittivity values, provided these criteria are fulfilled. The EC results are in good agreement with full-wave simulations in a wide range of material and geometrical parameters. The resulting one-way absorbers are very compact and geometrically simple, and enable transparency in the neighboring frequency ranges and, hence, multifunctionality that utilizes both absorption- and transmission-related regimes.

We present a variational method for approximating the ground state of spin models close to (Richardson-Gaudin) integrability. This is done by variationally optimizing eigenstates of integrable Richardson-Gaudin models, where the toolbox of integrability allows for an efficient evaluation and minimization of the energy functional. The method is shown to return exact results for integrable models and improve substantially on perturbation theory for models close to integrability. For large integrability-breaking interactions, it is shown how (avoided) level crossings necessitate the use of excited states of integrable Hamiltonians in order to accurately describe the ground states of general nonintegrable models.

The peculiar shape of the Fermi surface of topological nodal-line semimetals at low carrier concentrations results in their unusual optical and transport properties. We analytically investigate the linear optical responses of three- and two-dimensional nodal-line semimetals using the Kubo formula. The optical conductivity of a three-dimensional nodal-line semimetal is anisotropic. Along the axial direction (i.e., the direction perpendicular to the nodal-ring plane), the Drude weight has a linear dependence on the chemical potential at both low and high carrier dopings. For the radial direction (i.e., the direction parallel to the nodal-ring plane), this dependence changes from linear into quadratic in the transition from low into high carrier concentration. The interband contribution into optical conductivity is also anisotropic. In particular, at large frequencies, it saturates to a constant value for the axial direction and linearly increases with frequency along the radial direction. In two-dimensional nodal-line semimetals, no interband optical transition could be induced and the only contribution to the optical conductivity arises from the intraband excitations. The corresponding Drude weight is independent of the carrier density at low carrier concentrations and linearly increases with chemical potential at high carrier doping.

The ${\text{N}}_2\mathrm{V}$ defect in diamond is characterized by means of ab initio methods relying on density functional theory calculated parameters of a Hubbard model Hamiltonian. It is shown that this approach appropriately describes the energy levels of correlated excited states induced by this defect. By determining its critical magneto-optical parameters, we propose to realize a long-living quantum memory by ${\text{N}}_2\mathrm{V}$ defect, i.e., $H3$ color center in diamond.

While quantum anomalies are often associated with the breaking of a classical symmetry in the quantum theory, their anomalous contributions to observables remain distinct and well defined even when the symmetry is broken from the outset. This paper explores such anomalous contributions to the current, originating from the axial anomaly in a Weyl semimetal, and in the presence of a generic Weyl node-mixing term. We find that apart from the familiar anomalous divergence of the axial current proportional to a product of electric and magnetic fields, there is another anomalous term proportional to a product of the electric field and the orientation of a spin-dependent node-mixing vector. We obtain this result both by a quantum field-theoretic analysis of an effective Weyl action and solving an explicit lattice model. The extended spin-mixing mass terms, and the enriched axial anomaly they entail, could arise as mean-field or proximity-induced order parameters in spin-density-wave phases in Weyl semimetals or be generated dynamically within a Floquet theory.

Direct experimental investigations of the low-energy electronic structure of the ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ iridate insulator are sparse and draw two conflicting pictures. One relies on flat bands and a clear gap, the other involves dispersive states approaching the Fermi level, pointing to surface metallicity. Here, by a combination of angle-resolved photoemission, photoemission electron microscopy, and x-ray absorption, we show that the correct picture is more complex and involves an anomalous band, arising from charge transfer from Na atoms to Ir-derived states. Bulk quasiparticles do exist, but in one of the two possible surface terminations the charge transfer is smaller and they remain elusive.

Magnetoconductance (MC) at low temperature was measured to investigate spin-related transport affected by spin-orbit interaction (SOI) in III-VI compound $n$-type GaSe thin films. Results reveal that MC shows weak antilocalization (WAL). Its temperature and gate voltage dependences reveal that the dominant spin relaxation is governed by the D’yakonov-Perel’ mechanism associated with the Rashba SOI. The estimated Rashba SOI strength in GaSe is much stronger than that of III-V compound GaAs quantum wells, although the energy gap and spin split-off band in GaSe closely resemble those in GaAs. The angle dependence of WAL amplitude in the in-plane magnetic field direction is almost isotropic. This isotropy indicates that the strength of the Dresselhaus SOI is negligible compared with the Rashba SOI strength. The SOI effect in $n$-GaSe thin films differs greatly from those of III-V compound semiconductors and transition-metal dichalcogenides.

We use time-dependent nonequilibrium dynamical mean-field theory with weak-coupling auxiliary-field continuous time quantum Monte Carlo as an impurity solver to study the thermalization behavior of the mass-imbalanced single-band Hubbard model after a quench of the Coulomb interaction from the noninteracting limit to a finite positive value. When the Coulomb interaction in our model is increased under equilibrium conditions, the quasiparticle weights for spin-up and spin-down (the mass imbalance) electrons approach zero simultaneously, indicating the absence of a spin-selective Mott transition. By contrast, our out-of-equilibrium study of the mass-imbalanced Hubbard model suggests that there exists the spin-selective dynamical phase transition (one spin orientation undergoes a fast thermalization at its critical Coulomb interaction strength while the other spin orientation shows prethermalization behavior). The spin-selective dynamical phase transition is characterized by the relaxation behavior of the spin-resolved kinetic energy and the spin-resolved momentum-dependent occupation. To connect with possible experiments, we calculate the spin-resolved two-time optical conductivity, which confirms the spin-selective thermalization plateau. We find that the critical Coulomb interaction of each spin orientation for the spin-selective thermalization grows as the mass imbalance decreases.

Spin polarized density functional theory calculations on a predicted ${\mathrm{BiVO}}_3$ crystal structure is presented. An orthorhombic phase with space group $Pnma$ is observed to be highly stable compared to the aristotype cubic structure. An optical band gap of 1.92 eV and a strong optical absorption at 2.25 eV—which lie in the visible region of the solar spectra—are estimated at the coupled-perturbed hybrid density-functional theory level. In addition, the band-structure analysis somewhat shows dispersion at the valence and conduction bands, and the binding energy of the excitons is calculated to be quite low, which can be possibly dissociated at room temperature. ${\mathrm{BiVO}}_3$ is, therefore, expected to be a promising candidate worth being investigated for visible light driven photocatalytic applications. Simulated infrared and Raman spectra are reported, which could represent a guideline for future experiments, along with a full characterization of structural, electronic, and dynamic properties.

Contact resistance is a severe performance bottleneck for electronic devices based on two-dimensional (2D) layered semiconductors, whose contacts are Schottky rather than Ohmic. Although there is a general consensus that the injection mechanism changes from thermionic to tunneling with gate biasing, existing models tend to oversimplify the transport problem, by neglecting the 2D transport nature and the modulation of the Schottky barrier height, the latter being of particular importance in back-gated devices. In this paper, we develop a semianalytical model based on Bardeen's transfer Hamiltonian approach to describe both effects. Remarkably, our model is able to reproduce several experimental observations of a metallic behavior in the contact resistance, i.e., a decreasing resistance with decreasing temperature, occurring at high gate voltages.