We report on the phonon spectrum probed at the $\beta$-FeSe(001) surface by means of high-resolution electron energy-loss spectroscopy (HREELS). Single crystals of $\beta$-FeSe are cleaved under ultrahigh-vacuum conditions and are subsequently measured below and above the nematic transition temperature. In total we observe five phonon modes and a phonon cutoff energy of about 40 meV. We identify the origin of each phonon mode based on the selection rules of HREELS and by comparing the experimental results to those of ab initio density functional calculations. The most prominent phonon modes $A_{1g}, B_{1g}$, and $A_{2u}$ appear at energies of about 20.5 and 25.6 and 40 meV, respectively. These phonon modes disperse rather weakly while changing the momentum from zero up to the zone boundary, indicating that they are mainly of optical nature. A comparison between our results and the results of ab initio calculations indicates that there must be a mutual interplay between magnetism and lattice dynamics in this compound, similar to the other Fe-based superconductors. Finally, we comment on the role of temperature on the phonon modes probed at the $\overline{\mathrm{\Gamma }}$ point. It is observed that both the $B_{1g}$ and $A_{2u}$ phonon modes undergo a downward shift while increasing the temperature from 15 to 300 K. In the case of the $A_{2u}$ mode this shift is about 1.5 meV.

We propose a route towards creating a metamaterial that behaves as a photonic Chern insulator, through homogenization of an array of gyromagnetic cylinders. We show that such an array can exhibit nontrivial topological effects, including topologically nontrivial band gaps and one-way edge states, when it can be homogenized to an effective medium model that has the Berry curvature strongly peaked at the wave vector $k=0$. The nontrivial band topology depends only on the parameters of the cylinders and the cylinders’ density, and can be realized in a wide variety of different lattices, including periodic, quasiperiodic, and random lattices. Our system provides a platform to explore the interplay between disorder and topology and also opens a route towards the synthesis of topological metamaterials based on the self-assembly approach.

We theoretically study the dynamics of ferrimagnetic domain walls in the presence of a Dzyaloshinskii-Moriya interaction. We find that an application of a dc magnetic field can induce terahertz spin-wave emission by driving ferrimagnetic domain walls, which is not possible for ferromagnetic or antiferromagnetic domain walls. The Dzyaloshinskii-Moriya interaction is shown to facilitate terahertz spin-wave emission in a wide range of the net angular momentum by increasing the Walker breakdown field. Moreover, we show that spin-orbit torque combined with the Dzyaloshinskii-Moriya interaction also drives fast ferrimagnetic domain wall motion by emitting terahertz spin waves in a wide range of the net angular momentum.

The local atomic geometries in ${\mathrm{Ba}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{TiO}}_3$ thin films grown on MgO(001) substrates have been determined by Ti $1s$ near-edge x-ray-absorption fine-structure measurements and density-functional-theory calculations. The accuracy of the atomic geometries predicted by density-functional theory is demonstrated by simulations of the near-edge x-ray-absorption fine-structure spectra using a Bethe-Salpeter treatment of the Ti $1s$ core-hole interaction in the films. Our results show that films with either $c>a$ or $c<a$ tetragonal lattice distortions have their polarization vectors rotated toward or away from the [001] film-normal or $c$-axis direction, respectively. Both distortions result locally in the monoclinic $r$ phase of the strain-phase diagram of Pertsev et al. [Phys. Rev. Lett. 80, 1988 (1998)], and the polarizations are rotated significantly larger than the geometry would suggest.

We theoretically investigate acoustic metasurfaces consisting of either a single pillar or a line of identical pillars on a thin plate, and we report on the dependence on the geometrical parameters of both the monopolar compressional and dipolar bending modes. We show that for specific dimensions of the resonators, bending and compressional modes may be simultaneously excited. We study their interaction with an antisymmetric Lamb wave, whether or not they occur at the same frequency, with particular consideration for the amplitude and phase of waves emitted by the pillars at resonance. Especially, the analysis of both the amplitude and the phase of the wave at the common resonant frequency downstream from a line of pillars demonstrates that the reemitted waves allow for the transmission with phase shift of π.

The critical exponent of the phase boundary has been examined on the three-dimensional incommensurate ordering phase in the spin-$\frac{1}{2}$ ferromagnetic-leg ladder 3-Br-4-F-V [=3-(3-bromo-4-fluorophenyl)-1,5-diphenylverdazyl]. Using the temperature-window fitting technique, we obtained the critical exponents which agreed with the three-dimensional (3D) Bose-Einstein condensation (BEC) universality class at both sides of the lower critical field and the saturation field. 3-Br-4-F-V thus becomes a new member of the quantum magnets which prove the universality of the 3D BEC exponent.

Recently, thermally created pure spin currents were predicted for Zeeman-split superconductor/normal-metal heterostructures. Here it is shown that this “thermospin” current can lead to an accumulation of a pure spin imbalance in a system. The thermally induced spin imbalance can reach the value of Zeeman splitting of the superconducting density of states and strongly influences superconductivity in the heterostructure. Depending on the temperature difference between the superconductor and the normal reservoir it can enhance the critical temperature of the superconductor or additionally suppress the zero-temperature superconducting state. The last possibility gives rise to an unusual superconducting state, which starts to develop at finite temperature.

We revisit the problem of the surface superconductivity nucleation focusing on the detailed study of the critical field $H_{c3}$ as a function of temperature and disorder. Using the semiclassical Eilenberger formalism, we find that away from the Ginzburg-Landau region the ratio between the nucleation critical field $H_{c3}$ and the upper critical field $H_{c2}$ deviates strongly from the Saint-James–de Gennes limit. In particular, the $H_{c3}/H_{c2}$ is found to be a nonmonotonic function of temperature, which reaches the maximum for a set of parameters corresponding to a crossover region from ballistic to diffusive scattering, when the mean-free path in a bulk of a superconductor is of the same order as zero-temperature superconducting coherence length. We also analyze the robustness of the nucleated phases with respect to diffusive scattering off the sample boundary by solving exactly corresponding eigenvalue problem of an integral equation for the critical field. The implications of these results for the transport in superconductors of various geometries near $H_{c3}$ are briefly discussed. In particular, we present results for the mechanism of magnetoconductivity oscillations due to surface superconductivity effects.

We have studied the evolution of magnetic flux pinning behavior in the Meissner phase and the mixed state for the high-$T_{\mathrm{c}}$ single-crystal ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ ($x=0.09$) superconductor using the polarized neutron imaging method with varying magnetic field and temperature. In the Meissner state expulsion of magnetic field (switched on during the measurements) is visualized, and the signatures of a mixed state with increasing temperature are observed. However, for flux pinning behavior in the range $5\mathrm{K}\le T\le 15\mathrm{K}$ and $H_{\mathrm{ext}}=63.5\mathrm{mT}$ (switched off during the measurements), the evolution of the fringe pattern indicates magnetic flux pinning inside the bulk of the sample. At $25\mathrm{K}\le T\le 32\mathrm{K}$, a continuous decrease in inhomogeneously distributed pinned magnetic flux is observed, with the sample reaching a normal conducting state at $T_{\mathrm{c}}$ ($\approx 32$ K). The flux pinning behavior is also explored as a function of $H_{\mathrm{ext}}$ at $T=5\mathrm{K}$. As expected, with increasing $H_{\mathrm{ext}}$ an increase in fringe density is observed, indicating an increase in magnetic flux pinning in the bulk of the sample. A comparison between calculated and experimentally visualized pinned magnetic fluxes shows good agreement. This implies quantification of pinned magnetic flux inside the sample, which is not possible with any other technique for bulk samples.

We present a tunneling study on single-crystalline parent-cuprate thin films, i.e., a series of ${\mathrm{Pr}}_2{\mathrm{CuO}}_{4\pm \delta }$ with tunable superconducting transition temperature. The zero-bias anomaly in the differential conductance, well-reported in the normal state of ${\mathrm{R}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$ (R = Pr, Nd, La) and named as normal-state gap (NSG), is observed in the Ce-free samples. This NSG behaves quite robustly against magnetic fields up to 16 T, but fades away with increasing temperature. Most importantly, we find that the magnitude of the NSG becomes larger with increasing point-contact junction resistance on the superconducting films, which is further enhanced in the nonsuperconducting samples with more oxygen disorder. The origination of NSG can be understood in the framework of the Altshuler-Aronov-Lee theory, where the disorder-induced electron-electron interactions suppress the density of states and thereby result in a soft Coulomb gap.

We present a method for the exact diagonalization of the $\mathrm{SU}\left(N\right)$ Heisenberg interaction Hamiltonian using Young tableaus to work directly in each irreducible representation of the global $\mathrm{SU}\left(N\right)$ group. This generalized scheme is applicable to chains consisting of several particles per site, with any $\mathrm{SU}\left(N\right)$ symmetry at each site. Extending some of the key results of substitutional analysis, we demonstrate how basis states can be efficiently constructed for the relevant $\mathrm{SU}\left(N\right)$ subsector, which, especially with increasing values of $N$ or numbers of sites, has a much smaller dimension than the full Hilbert space. This allows us to analyze systems of larger sizes than can be handled by existing techniques. We apply this method to investigate the presence of edge states in $\mathrm{SU}\left(N\right)$ Heisenberg and Affleck-Kennedy-Lieb-Tasaki Hamiltonians.

Density functional theory calculations are carried out to investigate the atomic and electronic structures of the $4H\text{-SiC}\left(0001\right)/{\mathrm{SiO}}_2$ interface. We find two characteristic interface atomic structures in scanning transmission electron microscopy images: One is an interface in which the density of atoms at the first interfacial SiC bilayer is greater than that in the SiC substrate, while the other is an interface where the density of atoms at the first interfacial SiC bilayer is lower. Density functional theory calculations reveal that the difference in the scanning transmission electron microscopy images is a reflection of the atomic structures of these two interfaces. In addition, it has been reported that the floating states, which appear at the conduction band edge of a $4H\text{-SiC}\left(0001\right)/{\mathrm{SiO}}_2$ interface, affect the electronic structure of the interface and cause marked scattering of the electrons flowing along the interface [S. Iwase, C. J. Kirkham, and T. Ono, Phys. Rev. B 95, 041302(R) (2017)]. Interestingly, we find that the floating states do not appear at the conduction band edge of one of the two interfaces. These results provide physical insights into understanding and controlling the electronic structure and carrier mobility of electronic devices using wide-band-gap semiconductors.

Hybrid heterostructures based on bulk GaN and two-dimensional (2D) materials offer novel paths toward nanoelectronic devices with engineered features. Here, we study the electronic properties of a mixed-dimensional heterostructure composed of intrinsic $n$-doped $\mathrm{Mo}{\mathrm{S}}_2$ flakes transferred on $p$-doped GaN(0001) layers. Based on angle-resolved photoemission spectroscopy (ARPES) and high resolution x-ray photoemission spectroscopy (HR-XPS), we investigate the electronic structure modification induced by the interlayer interactions in $\mathrm{Mo}{\mathrm{S}}_2/\mathrm{GaN}$ heterostructure. In particular, a shift of the valence band with respect to the Fermi level for $\mathrm{Mo}{\mathrm{S}}_2/\mathrm{GaN}$ heterostructure is observed, which is the signature of a charge transfer from the 2D monolayer $\mathrm{Mo}{\mathrm{S}}_2$ to GaN. The ARPES and HR-XPS revealed an interface dipole associated with local charge transfer from the GaN layer to the $\mathrm{Mo}{\mathrm{S}}_2$ monolayer. Valence and conduction band offsets between $\mathrm{Mo}{\mathrm{S}}_2$ and GaN are determined to be 0.77 and $-0.51\mathrm{eV}$, respectively. Based on the measured work functions and band bendings, we establish the formation of an interface dipole between GaN and $\mathrm{Mo}{\mathrm{S}}_2$ of 0.2 eV.

The band structure of a general class of buckled square lattice materials is investigated using ab initio calculations along with tight-binding modeling. We show that buckling and spin-orbit interaction give rise to a large Rashba-like splitting in the absence of an external electric field. The generality and the robustness of the effect make this class of materials promising candidates for spintronic applications.

We use time-dependent density functional theory to study laser-pulse induced decomposition of ${\mathrm{H}}_2\mathrm{O}$ molecules above the two-dimensional (2D) materials graphene, hexagonal boron nitride, and graphitic carbon nitride. We examine femtosecond-laser pulses with a full width at half maximum of 10 or 20 fs for laser-field intensity and wavelengths of 800 or 400 nm by varying the intensity of the laser field from 5 to 9 V/Å, with the corresponding range of fluence per pulse up to $10.7\mathrm{J}/{\mathrm{cm}}^2$. For a ${\mathrm{H}}_2\mathrm{O}$ molecule above the graphitic sheets, the threshold for laser-field ${\mathrm{H}}_2\mathrm{O}$ decomposition is reduced by more than 20% compared with that of an isolated ${\mathrm{H}}_2\mathrm{O}$ molecule. We also show that hole doping enhances the water adsorption energy above graphene. The present results indicate that the graphitic materials should support laser-induced chemistry and that other 2D materials that can enhance laser-induced ${\mathrm{H}}_2\mathrm{O}$ decomposition should be investigated.

The present paper theoretically studies the possibility to control the orbital ordering in manganite superlattices. Indeed, favored $d_{z^2}{e}_g$-orbital occupancy is one of the proposed interpretations for the formation of a “dead” layer at the interfaces in manganite thin films and superlattices. We show here that favored $d_{z^2}{e}_g$-orbital occupancy at the interfaces can be prevented by using alkaline-earth simple oxides as alternating layers in very thin superlattices. Such an alternating layer promotes the contraction of the manganite layers at the interfaces and favors a $d_{x^2-y^2}{e}_g$ orbital occupancy. This result holds for different manganites, different alkaline-earth simple oxides, as well as different thicknesses of the two layers. While Boltzmann's transport calculations on different superlattices show unexpectedly only weak dependence of the electrical conductivity on the orbital ordering, the enhanced occupation of the $d_{x^2-y^2}$ orbital should result in an increased Curie temperature.

We present a stability analysis on a driven-dissipative electron-hole condensate in the BCS (Bardeen-Cooper-Schrieffer)–BEC (Bose-Einstein condensation) crossover region. Extending the combined BCS-Leggett theory with the generalized random phase approximation to the nonequilibrium case by employing the Keldysh formalism, we show that the pumping and decay of carriers causes a depairing effect on excitons. This phenomenon gives rise to an attractive interaction between excitons in the BEC regime, as well as a supercurrent that anomalously flows antiparallel to $\mathbf{\nabla }\theta \left(\mathit{r}\right)$ [where $\theta \left(\mathit{r}\right)$ is the phase of the condensate] in the BCS regime, both leading to dynamical instabilities of an exciton BEC. Our results suggest that a substantial region of the exciton-BEC phase in the phase diagram (in terms of the interaction strength and the decay rate) is unstable.

We develop a theory of plasmon-assisted tunneling in graphene-insulator-graphene heterostructures and reveal the manifestations of such process in current-voltage curves, plasmon emission spectra, and junction electroluminescence. We present a unified framework for evaluation of tunneling due to carrier-carrier Coulomb scattering and due to emission of plasmons; the latter mechanism generally dominates the full inelastic current. Moreover, the plasmon-assisted current and plasmon emission rates possess resonant peaks at voltages providing equal energies, momenta, and group velocities of collective and single-particle interlayer excitations. This resonance is unique to the tunnel-coupled 2d systems of massless electrons and is deeply related to strong interactions between collinear carriers in graphene. The predicted effect can be used for design of efficient nanoscale voltage-tunable sources of photons and surface plasmons.

We investigate theoretical aspects of the detection of Majorana bound states in Josephson junctions using the semiclassical resistively capacitively shunted junction (RCSJ) model of junction dynamics. The influence of a $4\pi$-periodic supercurrent contribution can be detected through its effect on the width of the Shapiro steps and the Fourier spectrum of the voltage signal. We explain how the inclusion of a capacitance term results in a strong quenching of the first step when the junction is underdamped, while the higher odd steps are less affected. Remarkably, this feature has been observed experimentally. We examine the emission spectrum of phase-locked solutions, showing that the presence of period doubling may make the measurement of the $4\pi$-periodic contribution from the Fourier spectrum difficult. Finally, we study the voltage response in the quasiperiodic regime and indicate how the Fourier spectra and the first-return maps in this regime reflect the change of periodicity in the supercurrent in the presence of Majorana bound states.

The magnetic properties of the compound ${\mathrm{Fe}}_5{\mathrm{Si}}_{1-x}{\mathrm{P}}_x{\mathrm{B}}_2$ have been studied, with a focus on the Curie temperature $T_{\mathrm{C}}$, saturation magnetization $M_{\mathrm{S}}$, and magnetocrystalline anisotropy. Field and temperature dependent magnetization measurements were used to determine $T_{\mathrm{C}}\left(x\right)$ and $M_{\mathrm{S}}\left(x\right)$. The saturation magnetization at 10 K (300 K) is found to monotonically decrease from $1.11\mathrm{MA}/\mathrm{m}$ $\left(1.03\mathrm{MA}/\mathrm{m}\right)$ to $0.97\mathrm{MA}/\mathrm{m}$ $\left(0.87\mathrm{MA}/\mathrm{m}\right)$, as $x$ increases from 0 to 1. The Curie temperature is determined to be 810 and 615 K in ${\mathrm{Fe}}_5{\mathrm{SiB}}_2$ and ${\mathrm{Fe}}_5{\mathrm{PB}}_2$, respectively. The highest $T_{\mathrm{C}}$ is observed for $x=0.1$, while it decreases monotonically for larger $x$. The Curie temperatures have also been theoretically determined to be 700 and 660 K for ${\mathrm{Fe}}_5{\mathrm{SiB}}_2$ and ${\mathrm{Fe}}_5{\mathrm{PB}}_2$, respectively, using a combination of density functional theory and Monte Carlo simulations. The magnitude of the effective magnetocrystalline anisotropy was extracted using the law of approach to saturation, revealing an increase with increasing phosphorus concentration. Low-field magnetization vs temperature results for $x=0,0.1,0.2$ indicate that there is a transition from easy-axis to easy-plane anisotropy with decreasing temperature.