We present a methodology to calculate radiative carrier capture coefficients at deep defects in semiconductors and insulators from first principles. Electronic structure and lattice relaxations are accurately described with hybrid density functional theory. Calculations of capture coefficients provide an additional validation of the accuracy of these functionals in dealing with localized defect states. We also discuss the validity of the Condon approximation, showing that even in the event of large lattice relaxations the approximation is accurate. We test the method on GaAs:$V_{\text{Ga}}\text{−}{\text{Te}}_{\text{As}}$ and GaN:${\mathrm{C}}_{\text{N}}$, for which reliable experiments are available, and demonstrate very good agreement with measured capture coefficients.

Using a van der Waals (vdW) vertical heterostructure consisting of monolayer graphene, monolayer hBN and $\mathrm{Nb}{\mathrm{Se}}_2$, we have performed local characterization of induced correlated states in different configurations. At a temperature of 4.6 K, we have shown that both superconductivity and charge density waves can be induced in graphene from $\mathrm{Nb}{\mathrm{Se}}_2$ by proximity effects. By applying a vertical magnetic field, we imaged the Abrikosov vortex lattice and extracted the coherence length for the proximitized superconducting graphene. We further show that the induced correlated states can be completely blocked by adding a monolayer hBN between the graphene and the $\mathrm{Nb}{\mathrm{Se}}_2$, which demonstrates the importance of the tunnel barrier and surface conditions between the normal metal and superconductor for the proximity effect.

Synthetic quantum Hall bilayer (SQHB), realized by optically driven monolayer graphene in the quantum Hall regime, provides a flexible platform for engineering quantum Hall phases as discussed in Ghazaryan et al. [Phys. Rev. Lett. 119, 247403 (2017)]. The coherent driving which couples two Landau levels mimics an effective tunneling between synthetic layers. The tunneling strength, the effective Zeeman coupling, and two-body interaction matrix elements are tunable by varying the driving frequency and the driving strength. Using infinite density matrix renormalization group techniques combined with exact diagonalization, we show that the system exhibits a non-Abelian bilayer Fibonacci phase at filling fraction $\nu =\frac{2}{3}$. Moreover, at integer filling $\nu =1$, the SQHB exhibits quantum Hall ferromagnetism. Using Hartree-Fock theory and exact diagonalization, we show that excitations of the quantum Hall ferromagnet are topological textures known as skyrmions.

Substrates, and layered media in general, are ubiquitous and affect the properties of any object in their vicinity. However, their influence is, in an arbitrary framework, challenging to quantify analytically, especially for large arrays which additionally escape explicit numerical treatment due to the computational burden. In this work, we utilize a versatile $T$-matrix-based framework to generalize the coupled multipole model toward arbitrarily high multipole orders and substrate-supported arrays. We then employ it to study substrate-supported random/amorphous arrays of high index dielectric nanoparticles which are of wide interest due to relatively low losses and a highly tunable optical response, making them promising elements for nanophotonic devices. We discuss how multipole coupling rules evolve in the presence of a substrate in amorphous arrays for three interaction mechanisms: direct coupling between particles, substrate-mediated interparticle coupling, and substrate-mediated self-coupling. We show how the interplay of array density, distance from the substrate, and the latter's refractive index determine the optical response of an array. As an example, we use this framework to analyze refractometric sensing with substrate-supported arrays and demonstrate that the substrate plays a crucial role in determining the array sensitivity.

The composition dependence of the structural transition between the monoclinic $1T^{\prime }$ and orthorhombic $T_d$ phases in the ${\mathrm{Mo}}_{1-x}{\mathrm{W}}_x{\mathrm{Te}}_2$ Weyl semimetal was investigated by elastic neutron scattering on single crystals up to $x\approx 0.54$. First observed in ${\mathrm{MoTe}}_2$, the transition from $T_d$ to $1T^{\prime }$ is accompanied by an intermediate pseudo-orthorhombic phase, $T_d^{*}$. Upon doping with W, the $T_d^{*}$ phase vanishes by $x\approx 0.34$. Above this concentration, a phase coexistence behavior with both $T_d$ and $1T^{\prime }$ is observed instead. The interlayer in-plane positioning parameter $\delta$, which relates to the $1T^{\prime }\beta$ angle, decreases with temperature as well as with W substitution, likely due to strong anharmonicity in the interlayer interactions. The temperature width of the phase coexistence remains almost constant up to $x\approx 0.54$, in contrast to the broadening reported under pressure.

The $\alpha$ (bcc) phase of single-crystalline iron was compressed along the ${\left[111\right]}_{\alpha }$ direction under quasihydrostatic and nonhydrostatic conditions. The emergence of the $\epsilon$ (hcp) phase variants via the pressure-induced martensitic transformation was investigated using x-ray diffraction and optical observations of the sample surface. In quasihydrostatic compression, the $\alpha \text{−}\epsilon$ transition occurred at approximately 14 GPa accompanied by the sudden appearance of texture structures on the mirror-polished sample surface. The texture structure divided the sample surface into two characteristic regions: distorted regions with undulations and flat regions without undulations. Although martensitic transformation based on the Burgers model allows for the emergence of 12 $\epsilon$ variants from one single-crystalline phase, the transition proceeded according to the Burgers model, but selected variants preferably emerged depending on the regions in the texture structure. The flat region consisted of only three variants whose $c$ axes were parallel to the sample surface. The region near the undulation exhibited a large number of variants whose $c$ axes aligned parallel and nonparallel to the sample surface. The $\alpha \text{−}\epsilon$ transition under nonhydrostatic compression deviated from the Burgers model, and the variant selection was not observed. This study demonstrates that the quasihydrostatic compression along ${\left[111\right]}_{\alpha }$ reduces the number of variants via strong variant selection. The strong preferred orientation is useful for investigating the anisotropy of physical properties in the $\epsilon$ phase.

By band engineering the iron chalcogenide Fe(Se,Te) via ab initio calculations, we search for topological surface states and realizations of Majorana bound states. Proposed topological states are expected to occur for nonstoichiometric compositions on a surface Dirac cone where issues like disorder scattering and charge transfer between relevant electronic states have to be addressed. However, this surface Dirac cone is well above the Fermi level. Our goal is to theoretically design a substituted crystal in which the surface Dirac cone is shifted toward the Fermi level by modifying the bulk material without disturbing the surface. Going beyond conventional density functional theory, we apply the Blackman, Esterling, and Berk coherent potential approximation in a mixed basis pseudopotential framework to scan the substitutional phase space of cosubstitutions on the Se sites. We have identified iodine as a promising candidate for intrinsic doping. Our specific proposal is that ${\mathrm{FeSe}}_{0.325}{\mathrm{I}}_{0.175}{\mathrm{Te}}_{0.5}$ is a very likely candidate to exhibit a Dirac cone right at the Fermi energy without inducing strong disorder scattering.

Effects of bond randomness and site dilution are systematically investigated for the Kitaev model describing a quantum spin liquid with fractional excitations of itinerant Majorana fermions and localized fluxes. We find that, in the high-temperature region where the itinerant Majorana fermions release their entropy, both types of disorders suppress the longitudinal thermal conductivity while keeping the specific heat almost unchanged. This suggests that both disorders reduce the mean-free path of the Majorana fermions. On the other hand, in the low-temperature region, the other specific heat peak associated with the entropy release from the localized fluxes is suppressed for both cases, but it is broadened and shifted to the lower-temperature side by the bond randomness, while the position and the width are almost unchanged against the site dilution. Contrasting behavior is also found in the thermal Hall effect under a magnetic field; the half quantization of the thermal Hall conductivity is fragile against the site dilution, while it remains for the bond randomness despite the reduced onset temperature. We discuss the contrasting behavior from the stability of the topological nature by calculating flux condensation and the Majorana excitation gap.

The pairing temperature of superconducting thin films is expected to display, within the Bardeen–Cooper–Schrieffer theory, oscillations as a function of the film thickness. We show that the pattern of these oscillations switches between two different periodicities at a density-dependent value of the superconducting coupling. The transition is most abrupt in the antiadiabatic regime, where the Fermi energy is less than the Debye energy. To support our numerical data, we provide analytical expressions for the chemical potential and the pairing temperature as a function of thickness, which only differ from the exact solution at weak coupling by exponentially small corrections.

Although there is strong theoretical and experimental evidence for electron-hole superfluidity in separated sheets of electrons and holes at low $T$, extending superfluidity to high $T$ is limited by strong two-dimensional fluctuations and Kosterlitz-Thouless effects. We show this limitation can be overcome using a superlattice of alternating electron- and hole-doped semiconductor monolayers. The superfluid transition in a three-dimensional superlattice is not topological, and for strong electron-hole pair coupling, the transition temperature $T_c$ can be at room temperature. As a quantitative illustration, we show $T_c$ can reach $270\mathrm{K}$ for a superfluid in a realistic superlattice of transition metal dichalcogenide monolayers.

Atomic collapse can be observed in graphene because of its large “effective” fine structure constant, which enables this phenomenon to occur for an impurity charge as low as $Z_c\sim 1–2$. Here we investigate the effect of the sublattice symmetry on molecular collapse in two spatially separated charge tunable vacancies, which are located on the same (A-A type) or different (A-B type) sublattices. We find that the broken sublattice symmetry: (1) does not affect the location of the main bonding and antibonding molecular collapse peaks, (2) but shifts the position of the satellite peaks, because they are a consequence of the breaking of the local sublattice symmetry, and (3) there are vacancy characteristic collapse peaks that only occur for A-B type vacancies, which can be employed to distinguish them experimentally from the A-A type. As the charge, energy, and separation distance increase, the additional collapse features merge with the main molecular collapse peaks. We show that the spatial distribution around the vacancy site of the collapse states allows us to differentiate the molecular from the frustrated collapse.

We study the spectral rigidity of the non-Hermitian analog of the Anderson model suggested by Tzortzakakis, Makris, and Economou (TME). This is a $L\times L\times L$ tightly bound cubic lattice, where both real and imaginary parts of onsite energies are independent random variables uniformly distributed between $-W/2$ and $W/2$. The TME model may be used to describe a random laser. In a recent paper we proved that this model has the Anderson transition at $W=W_c\simeq 6$ in three dimension. Here we numerically diagonalize TME $L\times L\times L$ cubic lattice matrices and calculate the number variance of eigenvalues in a disk of their complex plane. We show that on the metallic side $W<6$ of the Anderson transition, complex eigenvalues repel each other as strongly as in the complex Ginibre ensemble only in a disk containing $N_c(L,W)$ eigenvalues. We find that $N_c(L,W)$ is proportional to $L$ and grows with decreasing $W$ similarly to the number of energy levels $N_c$ in the Thouless energy band of the Anderson model.

Ultrasound focusing manifests itself as the accumulation of ultrasound energy on the target. It attracts extensive interest owing to its substantial advantage in engineering detection and biomedical science. However, the conventional focusing technique leads to the undesirable off-target influence in the nonfocusing region, hindering its application such as ultrasound surgery. Here we propose an ultrasound beam with idiosyncratic sharp autofocusing properties and implement it with the binary metasurface. The beam shows a sharp accumulation at the focus with an abrupt increase of intensity by three orders of magnitude while keeping the unintended region intact. The sharp autofocusing is realized by the controlled transverse self-acceleration and collapse of caustic at the focus in a nonlinear fashion. The intensity contrast, width, and position of the beam are highly tunable to realize the freewheeling focusing with high efficiency. Remarkably, the possibility to synthesize the abrupt autofocusing by the phase-only modulation is revealed. As an implementation, we demonstrate that the precise control of the sharp converging beam can be realized by an extremely simple metasurface with binary elements. The sharp autofocusing beams would open an avenue to realize the diverse ultrasound focusing with promising applications in medical ultrasound imaging, therapy, and nondestructive evaluation.

Iridium is a very promising material for spintronic applications due to its interesting magnetic properties such as large Ruderman-Kittel-Kasuya-Yosida exchange coupling as well as its large spin-orbit coupling value. Ir is for instance used as a spacer layer for perpendicular synthetic antiferromagnetic or ferrimagnet systems. However, only a few studies of the spintronic parameters of this material have been reported. In this paper, we present inverse spin Hall effect–spin pumping ferromagnetic resonance measurements on CoFeB/Ir based bilayers to estimate the values of the effective spin Hall angle, the spin diffusion length within iridium, and the spin mixing conductance in the CoFeB/Ir bilayer. In order to have reliable results, we performed the same experiments on CoFeB/Pt bilayers, whose behavior is well known due to numerous reported studies. Our experimental results show that the spin diffusion length within iridium is 1.3 nm for resistivity of 250 nΩ m, the spin mixing conductance $g_{\mathrm{eff}}^{\uparrow \downarrow }$ of the CoFeB/Ir interface is $30{\mathrm{nm}}^{-2}$, and the spin Hall angle of iridium has the same sign as the one of platinum and is evaluated at 26% of the one of platinum. The value of the spin Hall angle found is 7.7% for Pt and 2% for Ir. These relevant parameters shall be useful to consider Ir in new concepts and devices combining spin-orbit torque and spin-transfer torque.

We propose a quantum state readout mechanism of a weakly coupled qubit in a dispersive regime. The hybrid system consists of a ferromagnetic insulator and a weakly coupled quantum system, e.g., single electron spin, in a microwave cavity. The enhancement of the measurement sensitivity is achieved by exerting torque on the ferromagnetic insulator magnetization, which compensates the damping of the system leading to an exceptional point. The proposed mechanism allows us to measure the qubit state either via the transmission of the cavity or the FMR signal of the magnetic material.

Understanding the temperature dependence of thermal boundary resistance, or Kapitza resistance, between liquid helium and sintered metal has posed a problem in low-temperature physics for decades. In the ballistic regime of superfluid ${}^3\mathrm{He}\text{−}B$, we find the Kapitza resistance can be described via scattering of thermal excitations (quasiparticles) with a macroscopic geometric area, rather than the sintered metal's microscopic area. We estimate that a quasiparticle needs on the order of 1000 collisions to successfully thermalize with the sinter. Finally, we find that the Kapitza resistance is approximately doubled with the addition of two monolayers of solid ${}^4\mathrm{He}$ on the sinter surface, which we attribute to an extra magnetic channel of heat transfer being closed as the nonmagnetic solid ${}^4\mathrm{He}$ replaces the magnetic solid ${}^3\mathrm{He}$.

We discuss a way to construct a commuting projector Hamiltonian model for a ($3+1$)-dimensional topological superconductor in class DIII. The wave function is given by a sort of string net of the Kitaev wire, decorated on the time-reversal ($T$) domain wall. Our Hamiltonian is provided on a generic three-dimensional (3D) manifold equipped with a discrete form of the spin structure. We will see how the 3D spin structure induces a 2D spin structure (called a “Kasteleyn” direction on a 2D lattice) on $T$ domain walls, which makes it possible to define fluctuating Kitaev wires on them. Upon breaking the $T$ symmetry in our model, we find the unbroken remnant of the symmetry, which is defined on the time-reversal domain wall. The domain wall supports the 2D nontrivial symmetry-protected topological (SPT) phase protected by the unbroken symmetry, which allows us to determine the SPT classification of our model, based on the recent quantum field theory argument by Hason, Komargodski, and Thorngren.

Higgs-like modes in condensed-matter physics have drawn attention because of analogies to the Higgs bosons of particle physics. Here we use a microscopic time-dependent mean-field theory to study the collective mode spectra of two-dimensional spatially indirect exciton (electron-hole pair) condensates, focusing on the Higgs-like modes, i.e., those that have a large weight in electron-hole pair amplitude response functions. We find that in the low exciton density (Bose-Einstein condensate) limit, the dominant Higgs-like modes of spatially indirect exciton condensates correspond to adding electron-hole pairs that are orthogonal to the condensed pair state. We comment on the previously studied Higgs-like collective excitations of superconductors in light of this finding.

We observe third-harmonic generation (THG) in boron-doped silicon (Si:B) upon pumping with picosecond 1.56-THz pulses from a free-electron laser with a peak electric field strength of up to 12 kV/cm. The measurements are performed at cryogenic temperatures where the majority of holes are bound to the acceptor dopants. The dependence of the THG on the pump intensity exhibits a threshold-free power-law behavior with an exponent close to 4. The observations can be explained by THz emission by free holes accelerated in the nonparabolic valence band, under the assumption that the density of free holes increases with the pump intensity. A quantitative treatment supports that these carriers are generated by impact ionization, initiated by the population of thermally ionized carriers, as opposed to direct tunneling ionization. In addition, we also observe intracavity THG by embedding the Si:B in a one-dimensional photonic crystal cavity. The THG efficiency is increased by a factor of eight due to the field enhancement in the cavity, with the potential to reach a factor of more than 100 for pump pulses with a spectrum narrower than the linewidth of the cavity resonance.

The adhesion strength of lateral heterostructures composed of transition metal dichalcogenides ($\mathrm{Mo}{\mathrm{S}}_2, \mathrm{Mo}{\mathrm{Se}}_2, {\mathrm{WS}}_2$, and $\mathrm{W}{\mathrm{Se}}_2$) monolayers is investigated with first-principles electronic structure calculations. Our density functional theory calculations demonstrate that the adhesion strength, which is gauged by the ideal work of separation ($W_{\mathrm{sep}}$), strongly depends on the local atomic configuration, and that $W_{\mathrm{sep}}$ becomes enhanced (diminished) at the interface where a chalcogen atom forms bonds with one (two) W and two (one) Mo atoms compared to those of the homogeneous cases. It is further shown that the increase (decrease) in $W_{\mathrm{sep}}$ from the homogeneous value is caused by the charge redistribution among the interfacial bonds, which is determined by the differences in the electronegativity of the transition metal species at the interface. Such geometrically controlled interfacial strength presents a route to control the materials' mechanical characteristics through structural engineering.