We investigate the superconducting gap function of the topological superconductor ${\mathrm{PbTaSe}}_2$. The temperature, magnetic field, and three-dimensional (3D) field-angle dependences of the specific heat prove that the superconductivity of ${\mathrm{PbTaSe}}_2$ is fully gapped, with two isotropic $s$-wave gaps. The pair-breaking effect is probed by systematically increasing nonmagnetic disorders through ${\mathrm{H}}^{+}$ irradiation. The superconducting transition temperature $T_{\mathrm{c}}$ is found to be robust against disorders, which suggests that the pairing should be sign preserved rather than sign reversed.

Among topological materials, nodal loop semimetals are arguably the most topologically sophisticated, with their valence and conduction bands intersecting along arbitrarily intertwined nodes. But unlike the well-known topological band insulators with quantized edge conductivities, nodal loop materials possess topologically nontrivial Fermi surfaces, not bands. Hence an important question arises: Are there also directly measurable or even technologically useful physical properties characterizing nontrivial nodal loop topology? In this paper, we provide an affirmative answer by showing that nodal linkages protect the higher harmonic generation (HHG) of electromagnetic signals. Specifically, nodal linkages enforce nonmonotonicity in the intraband semiclassical response of nodal materials, which will be robust against perturbations preserving the nodal topology. These nonlinearities distort incident radiation and produce higher frequency peaks in the teraHertz (THz) regime, as we quantitatively demonstrate for a few known nodal materials. Since THz sources are not yet ubiquitous, our new mechanism for HHG will greatly aid applications like material characterization and nonionizing imaging of object interiors.

Angle-resolved photoemission spectroscopy (ARPES) has revealed peculiar properties of mobile dopants in correlated antiferromagnets (AFMs). But, describing them theoretically, even in simplified toy models, remains a challenge. Here, we study ARPES spectra of a single mobile hole in the $t-J$ model. Recent progress in the microscopic description of mobile dopants allows us to use a geometric decoupling of spin and charge fluctuations at strong couplings, from which we conjecture a one-to-one relation of the one-dopant spectral function and the spectrum of a constituting spinon in the undoped parent AFM. We thoroughly test this hypothesis for a single hole doped into a two-dimensional Heisenberg AFM by comparing our semianalytical predictions to previous quantum Monte Carlo results and our large-scale time-dependent matrix product state calculations of the spectral function. Our conclusion is supported by a microscopic trial wave function describing spinon-chargon bound states, which captures the momentum and $t/J$ dependence of the quasiparticle residue. From our conjecture we speculate that ARPES measurements in the pseudogap phase of cuprates may directly reveal the Dirac-fermion nature of the constituting spinons. Specifically, we demonstrate that our trial wave function provides a microscopic explanation for the sudden drop of spectral weight around the nodal point associated with the formation of Fermi arcs, assuming that additional frustration suppresses long-range AFM ordering. We benchmark our results by studying the crossover from two to one dimension, where spinons and chargons are confined and deconfined, respectively.

We study energy gaps of the $\nu =7/2$ and $\nu =5/2$ fractional quantum Hall states in a series of two-dimensional electron gases containing alloy disorder. We found that gaps at these two filling factors have the same suppression rate with alloy disorder. The dimensionless intrinsic gaps in our alloy samples obtained from the model proposed by Morf and d'Ambrumenil are consistent with numerical results, but are larger than those obtained from experiments on pristine samples published in the literature. The disorder broadening parameter has large uncertainties. However, a modified analysis relying on shared intrinsic gaps yields consistent results for both the $\nu =5/2$ and $7/2$ fractional quantum Hall states and establishes a linear relationship between the disorder broadening parameter and alloy concentration. Furthermore, we find that we can separate contributions to the disorder broadening of the long-range and short-range scattering.

The presented study considers the electronic conduction across a $\mathrm{SrTi}{\mathrm{O}}_3\text{/}\mathrm{Pt}$ Schottky electrode in a resistive switching cell. It is generally accepted that the resistive switching effect is based on the migration of oxygen vacancies, which can be understood as mobile donors. In the experimental approach, a $\mathrm{Nb}:\mathrm{SrTi}{\mathrm{O}}_3\text{/}\mathrm{SrTi}{\mathrm{O}}_3\text{/}\mathrm{Pt}$ resistive switching cell is fabricated and tested for its electronic and resistive switching characteristics. Using different voltage stimuli, several analog resistance states are realized. Afterwards, the electrical transport properties under different applied voltages and temperatures are measured for each analog resistive state. To gain physical insight into the analog resistive switching a numerical simulation model is developed. The electronic conduction is calculated based on the single band transport theory and a phonon scattering theory accounting for polar material systems. The simulation model allows testing of the conduction in these resistive switching cells by using different doping (oxygen vacancy) concentrations. Combining the simulation model and the experiment, it delivers a comprehensive physical description for the conduction. By means of simulation, the energy resolved current transport across the Schottky barrier is analyzed. It forms a peaklike distribution, originating from the limited thermal excitation and tunneling probability across the $\mathrm{SrTi}{\mathrm{O}}_3\text{/}\mathrm{Pt}$ Schottky barrier. Thus, the conduction processes in all states are identified as a balance between a thermally assisted tunneling effect and a phonon dominated bulk transport. Applying this understanding, the resistive switching effect is reduced to a modification of the Schottky tunnel barrier, based on the rearrangement of oxygen vacancies. Thus a low vacancy concentration leads to a high and wide tunneling barrier, which is reduced and shortened for higher concentrations. All resistance states in between are understood as an adjustment of intermediate tunneling barriers. The physical insights leading to the realization of analog resistance states is mandatory to realize new types of neuromorphic computing circuits based on resistive switching devices. Furthermore, the obtained results could be easily transferred to other systems where a static doping concentration applies. This makes the results highly interesting to other applications in the field of electronic oxides and Schottky barrier dominated systems.

We study theoretically the band structures and angle-resolved photoemission spectroscopy of both monolayer and bilayer kagome lattices irradiated by variously polarized monochromatic light based on the Floquet-Bloch theory. The polarization of light plays distinctive roles in the different energy regimes of kagome bands. The linearly polarized light predominantly affects the flat bands at the bottom of the kagome band, while the circularly polarized light engenders spin-resolved massive Dirac cones whose spin species can be switched by the polarization direction. The edge states of kagome ribbons exhibit a spatially anisotropic quantum spin Hall effect conforming to the nontrivial topology of kagome bands. In kagome bilayers, we take into account the electric field component of monochromatic light which results in a band gap in flat bands in the case of oblique incidence. This shows that the polarized light provides us a versatile tool to manipulate not only the two-dimensional and quasi-one-dimensional band structures, but also the anomalous Hall conductivity by either the polarization or the incident plane of monochromatic light.

The non-Hermiticity caused breakdown of the bulk-boundary correspondence (BBC) in topological phase transition was cured by the skin effect for the systems with chiral symmetry and translation invariance. However, periodic driving, as an active tool in engineering exotic topological phases, breaks the chiral symmetry, and the inevitable disorder destroys the translation invariance. Here, we propose a scheme to retrieve the BBC and establish a complete description of the topological phases of the periodically driven non-Hermitian system both with and without the translation invariance. The demonstration of our method in the non-Hermitian Su-Schrieffer-Heeger model shows that exotic non-Hermitian topological phases of widely tunable numbers of edge states and Floquet topological Anderson insulator are induced by the periodic driving and the disorder. Our result supplies a useful way to artificially synthesize exotic phases by periodic driving in the non-Hermitian system.

We revisit the problem of bound states in graphene under the influence of point electric monopole and dipole impurity potentials extended to the case in which the membrane of this material is uniformly and uniaxially strained, which leads to a redefinition of the charge and dipole moment, respectively. By considering an anisotropic Fermi velocity, we analytically solve the resulting Dirac equation for each potential. We observe that the effect of the anisotropy is to promote or inhibit the critical behavior known to occur for each kind of impurity, depending on the direction along which strain is applied: both the atomic collapse, for the monopole impurity, and the emergence of cascades of infinitely many bound states with a universal Efimov-like scaling, for the dipole impurity, are phenomena that occur under less or more restrictive conditions due to strain.

We investigate the Andreev reflection in an 8-$Pmmn$ borophene-based normal metal-superconductor junction by solving the Dirac–Bogoliubov–de Gennes equation. There are three unusual features for Andreev reflection of anisotropic massless Dirac fermion: (1) The Andreev retroreflection generates in the interband conversion. (2) The specular Andreev reflection generates in the intraband conversion. (3) The electron-hole conversion with unit efficiency happens at a nonzero incident angle of electron regardless of the magnitude of doping in normal and superconducting regions. These three unique properties are opposite to the ones in a graphene-based normal metal-superconductor junction due to the anisotropic band structure of 8-$Pmmn$ borophene. Interestingly, we can modulate the Andreev reflection by adjusting the direction of junction. Our work gives an insight into the understanding of Andreev reflection of Dirac fermion.

Dipolar (or spatially indirect) excitons (IXs) in semiconductor double quantum well (DQW) subjected to an electric field are neutral species with a dipole moment oriented perpendicular to the DQW plane. Here, we theoretically study interactions between IXs in stacked DQW bilayers, where the dipolar coupling can be either attractive or repulsive depending on the relative positions of the particles. By using microscopic band structure calculations to determine the electronic states forming the excitons, we show that the attractive dipolar interaction between stacked IXs deforms their electronic wave function, thereby increasing the inter-DQW interaction energy and making the IX even more electrically polarizable. Many-particle interaction effects are addressed by considering the coupling between a single IX in one of the DQWs to a cloud of IXs in the other DQW, which is modeled either as a closed-packed lattice or as a continuum IX fluid. We find that the lattice model yields IX interlayer binding energies decreasing with increasing lattice density. This behavior is due to the dominating role of the intra-DQW dipolar repulsion, which prevents more than one exciton from entering the attractive region of the inter-DQW coupling. Finally, both models shows that the single IX distorts the distribution of IXs in the adjacent DQW, thus inducing the formation of an IX dipolar polaron (dipolaron). While the interlayer binding energy reduces with IX density for lattice dipolarons, the continuous polaron model predicts a nonmonotonous dependence on density in semiquantitative agreement with a recent experimental study [cf. Hubert et al., Phys. Rev. X 9, 021026 (2019)].

We consider plasmonic metasurfaces constituted by an arbitrary periodic arrangement of spherical metallic nanoparticles. Each nanoparticle supports three degenerate dipolar localized surface plasmon (LSP) resonances. In the regime where the interparticle distance is much smaller than the optical or near-infrared wavelength associated with the LSPs, the latter couple through the dipole-dipole interaction and form collective plasmonic modes which extend over the whole metasurface. Within a Hamiltonian model which we solve exactly, we derive general expressions which enable us to extract analytically the quasistatic plasmonic dispersion for collective modes polarized within the plane and perpendicular to the plane of the metasurface. Importantly, our approach allows us not only to consider arbitrary Bravais lattices, but also non-Bravais two-dimensional metacrystals featuring nontrivial topological properties, such as, e.g., the honeycomb or Lieb lattices. Additionally, using an open quantum system approach, we consider perturbatively the coupling of the collective plasmons to both photonic and particle-hole environments, which lead, respectively, to radiative and nonradiative frequency shifts and damping rates, for which we provide closed-form expressions. The radiative frequency shift, when added to the quasistatic dispersion relation, provides an approximate analytical description of the fully retarded band structure of the collective plasmons. While it is tempting to make a direct analogy between the various systems which we consider and their electronic tight-binding equivalents, we critically examine how the long-range retarded and anisotropic nature of the dipole-dipole interaction may quantitatively and qualitatively modify the underlying band structures and discuss their experimental observability.

Surface nanopatterns formed by ion beam sputtering (IBS) frequently include a high density of structural defects, which seriously limit their practical applications. Recent theoretical work [M. P. Harrison and R. M. Bradley, Phys. Rev. E 93, 040802(R) (2016)], based on the anisotropic Kuramoto-Sivashinsky (aKS) equation, proposes that rocking a substrate during IBS can produce defect-free patterns under certain requirements. We find experimentally, via low-energy ${\mathrm{Kr}}^{+}$ irradiation of Si and amorphous carbon targets, that rocking the substrates generally improves the order of the ripple patterns. Indeed, order is improved even when conditions required in the aKS model may not be satisfied. Through numerical simulations, we show that a suitable generalization of this equation, in which conserved nonlinear effects are incorporated, reproduces satisfactorily the pattern order in our experimental conditions.

This paper predicts that individual Bloch points, topological point defects with vanishing local magnetization, can be created and stabilized by magnetostatic and chiral interactions in nanocuboids, confined in between two chiral bobbers of opposing polarity. The Bloch point can be moved by an external magnetic field of moderate strength but only if the field strength is enough to overcome a pinning potential that results from intrinsic exchange forces and extrinsic surface effects. The Bloch point can be driven by the external field reversibly, in a direction opposing the field, and it remains stable up to moderate field strengths. At a critical field strength the Bloch point escapes through one of the surfaces, leaving behind a collinear magnetization configuration, and upon removing the field a new Bloch point is formed. These findings highlight the topological diversity in nanostructures and show that a Bloch point, despite its zero dimensionality, couples to external fields via a substantial magnetic volume around it. The control of topological point defects has technological implications with regards to reversibly movable nanomagnetic textures and their associated emergent electrodynamics.

Filled skutterudite compound ${\mathrm{EuFe}}_4{\mathrm{As}}_{12}$ shows the highest magnetic ordering temperature of $T_{\mathrm{C}}=154\mathrm{K}$ among Eu-based skutterudite compounds, but its magnetic ground state has not been determined yet. Here, we performed ${}^{153}\mathrm{Eu}$ nuclear magnetic resonance (NMR) and ${}^{75}\mathrm{As}$ nuclear quadrupole resonance (NQR) measurements on ${\mathrm{EuFe}}_4{\mathrm{As}}_{12}$ to reveal its magnetic ground state as well as the physical properties from a microscopic point of view. From the temperature and magnetic field dependence of ${}^{153}\mathrm{Eu}$ NMR spectrum in the magnetically ordered state, we found that the Eu ions are in ${\mathrm{Eu}}^{2+}$ state with a nearly 7 ${\mu }_{\mathrm{B}}$ corresponding to $S=7/2$ spins. Combined with the magnetization measurements, which show the reduced saturation moments of 4.5 ${\mu }_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$, we determined the ground magnetic structure in ${\mathrm{EuFe}}_4{\mathrm{As}}_{12}$ to be ferrimagnetic where the ${\mathrm{Eu}}^{2+} 4f$ and the $\mathrm{Fe}3d$ ordered moments are ferromagnetically aligned in each sublattice but the moments between the sublattices are antiferromagnetically aligned. We also found the local distortion at the Eu site from the cubic symmetry in the magnetically ordered state. The relationship between the rattling motion of Eu atoms and the local symmetry of the Eu ions is discussed. From the ${}^{75}\mathrm{As}$ NQR nuclear spin-lattice relaxation time measurements as well as ${}^{153}\mathrm{Eu}$ NMR measurements, we found that the $4f$ electrons of the Eu ions are well described by the local moment picture in both the magnetic and paramagnetic metallic states.

Spectrum of spin eigenmodes localized on a ferromagnetic skyrmion pinned by a geometrical defect (bump) of magnetic films is studied theoretically. By means of direct numerical solution of the corresponding eigenvalue problem and finite element micromagnetic simulations we demonstrate that the curvature can induce localized modes with higher azimuthal and radial quantum numbers, which are absent for planar skyrmions (for the same parameters). The eigenfrequencies of all modes, except the breathing and gyromodes decreases with increasing curvature. Due to the translational symmetry break, the zero translational mode of the skyrmion gains a finite frequency and forms the gyromode, which describes the uniform rotation of skyrmions around the equilibrium position. To treat the gyromotion analytically we developed a Thiele-like collective variable approach. We show that Néel skyrmions in curvilinear films experience a driving force originating from the gradient of the mean curvature. The gyrofrequency of the pinned skyrmion is proportional to the second derivative of the mean curvature at the point of equilibrium.

${}^{133}\mathrm{Cs}$ and ${}^{77}\mathrm{Se}$ NMR measurements were conducted on a single crystal to study the local magnetic and electronic properties of the Fe-based compound ${\mathrm{CsFe}}_2{\mathrm{Se}}_3$ with a two-leg ladder structure. From the experimental results of the ${}^{133}\mathrm{Cs}$ and ${}^{77}\mathrm{Se}$ NMR spectra and the ${}^{133}\mathrm{Cs}$ nuclear spin-lattice relaxation rate, we found successive magnetic transitions from a paramagnetic phase to an incommensuratelike antiferromagnetic (AFM1) phase with a distributed internal field at $T_{\mathrm{N}1}=176.5\mathrm{K}$ and then from the AFM1 phase to another commensurate antiferromagnetic (AFM2) phase at $T_{\mathrm{N}2}=148.5-152.0\mathrm{K}$. Internal fields of the Cs and Se sites obtained from the angular dependence of the NMR spectra are discussed in terms of the transferred hyperfine tensors governed by site symmetry. For the AFM2 phase, we show that the stripe-type spin structure occurs in two domains, which have magnetic wave vectors of $\mathbf{Q}=(1/2,1/2,0)$ and $(1/2,-1/2,0)$. On the contrary, we observed a characteristic modulation of the internal field with magnetic fluctuation between the ladders in the AFM1 phase. To clarify why ${\mathrm{CsFe}}_2{\mathrm{Se}}_3$ has a large charge gap although it is a ${\mathrm{Fe}}^{2.5+}$ mixed valent compound, we investigated the hyperfine coupling constants and found that a hole is doped into the Fe site instead of the Se site. Based on this fact, molecular orbital and double exchange models are proposed for the electronic state.

Low-temperature thermal conductivity measurements have been conducted on an oxygen-annealed single crystal of ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ from 60 mK to 50 K and in magnetic fields up to 8 T applied in the [111] crystallographic direction. The temperature dependence of the conductivity in zero field shows a significant peak in thermal conductivity at $T\sim 13$ K and a sharp anomaly at $T_c\sim 0.2$ K suggesting that the sample's behavior is representative of the high-purity limit, with low levels of disorder. The magnetic field dependence of the thermal conductivity close to $T_c$ reveals a reentrant magnetic phase for a field in the [111] direction. With this information, analysis of the very low magnetic field behavior of the thermal conductivity suggests the presence of significant fluctuations close to the phase line.

In this study, molecular beam epitaxy is utilized to stabilize a nanostructured thin-film magnet consisting of a soft magnetic ${\mathrm{Y}}_2{\mathrm{Co}}_{17}$ exchange coupled to hard magnetic ${\mathrm{YCo}}_5$. While, typically, a phase decomposition can be obtained in rare-earth cobalt systems only by the addition of further elements like Cu, Fe, and Zr and complex heat treatments, here we directly induce phase separation by growth kinetics. The resulting nanoscale architecture, as revealed by cross-sectional transmission electron microscopy, is composed of a network of coherently interlinked and aligned ${\mathrm{Y}}_2{\mathrm{Co}}_{17}$ and ${\mathrm{YCo}}_5$ building blocks. The formation of coherent precipitations is facilitated by the perfectly matching lattice constants, atomic species, and crystal symmetry of the two phases with vastly different magnetocrystalline anisotropies. The hard magnetic phase induces an aligned uniaxial anisotropy in ${\mathrm{Y}}_2{\mathrm{Co}}_{17}$, resulting in substantial coercivity associated with enhanced energy products. This work highlights the importance of thin-film epitaxy in understanding magnetic hardening mechanisms and suggests strategies for a rational design of future sustainable magnetic systems.

Variations of critical temperature $T_c$ and in-plane critical field $H_{c2}$ of ${\text{Sr}}_2\text{Ru}{\text{O}}_4$ under uniaxial stress were recently reported. We compare the strain dependence of $T_c$ and $H_{c2}$ in various pairing channels ($d$ wave, extended $s$ wave, and $p$-wave) with the experimental observations by studying a three-band tight-binding model that includes effects of spin-orbit and Zeeman couplings and a separable pairing interaction. Our study helps narrow down the possibility of pairing channels. The importance of the multiband nature of ${\text{Sr}}_2\text{Ru}{\text{O}}_4$ is also highlighted.

Large-scale nonequilibrium molecular dynamics (MD) simulations of shock waves in nanocrystalline iron show evidence of plasticity before the polymorphic transformation takes place. The atomistic structure in the shock direction shows an elastic precursor, plastic deformation, and shock-induced phase transformation from bcc to hcp iron. In this Rapid Communication, large-scale MD models show that the shock response of iron is highly related to the ramp time of the applied shocks. For long ramp times we observe significant plastic relaxation and formation of microstructure defects. Pressure-induced phase transformations in iron are accompanied by stress relaxation achieving almost fully relaxed three-dimensional hydrostatic final states. The evolution of the stress relaxation is in agreement with theory and experiments. Analysis of the x-ray diffraction patterns calculated from the atomistic structure using the Debye equation revealed pronounced anisotropy of the line broadening that is caused by stacking faults in hcp Fe and by dislocations in bcc Fe.