The potential of ${\mathrm{Al}}_x{\mathrm{In}}_{1-x}\mathrm{As}/\mathrm{InP}$ as a flexible candidate for future device applications relies on a thorough understanding of the growth mechanisms of this system during metalorganic vapor-phase epitaxy (MOVPE). We present a systematic study of the morphology of InP films grown on macroscopically lattice-matched ${\mathrm{Al}}_x{\mathrm{In}}_{1-x}\mathrm{As}$ during low-pressure MOVPE. InP and AlInAs alloys are often assumed to be fully compatible, as long as macroscopic lattice matching is preserved. However, thin InP films show a variety of spontaneously formed nanostructures as the growth conditions are varied. We have examined the systematic variation of individual growth conditions (temperature, V/III ratios, growth rate, and misalignment angle and direction) to obtain a comprehensive picture of the morphological evolution of this system. The analysis of these experiments is based on a kinetic model of the deposition and decomposition of polyatomic precursors and the subsequent adatom kinetics, which is used to form characteristic lengths from combinations of kinetic coefficients associated with each process. The orderings of these lengths for particular ranges of growth conditions account for many aspects of the morphological evolution on perfectly oriented and misoriented surfaces. For perfectly oriented surfaces, differences in surface free energies are suggested as the driving force for Volmer-Weber growth and the observed formation of multilayer features. Finally, we examine the transition from quantum rings to quantum dots during postgrowth recovery when the group-V source is switched from phosphine to arsine. A qualitative explanation of similar observations in InAs/InP based on As/P exchange is consistent with our findings.

The exotic topological states have advanced the exploration for topological characteristics, such as invariant topological orders and band inversion. Here, we experimentally construct photonic dimer chains composed of split-ring resonators to exhibit interesting topological properties with different orientation angles. By using the method of near-field detection, we measure the dispersion relation and sublattice pseudospin vectors of different orientated configurations, which immediately determines the associated winding number of bulk bands as the topological invariant. Moreover, the sublattice pseudospin vector inversion observed at band edges indicates the band inversion and the associated topological phase transition, which is reconfirmed by measuring the band-edge mode parity and symmetry of the split-ring resonator cell. Our results not only directly show the topological winding number, band inversion, pseudospin, and the symmetries of band-edge states in experiments, but also reveal the great potential of the split-ring resonator platform to experimentally explore the advanced topological characteristics, such as higher winding numbers or higher Chern numbers in more complex setups.

We study theoretically some unique characteristics of surface electromagnetic waves excited at the interface between two different kinds of general bi-isotropic media, which include Tellegen media and chiral media as special cases. We derive an analytical dispersion relation for those waves, using which we deduce eight different conditions under which they are generated between two Tellegen media and between two chiral media independently of the component of the wave vector along the interface. These make it possible to excite the surface waves for all or a wide range of incident angles in attenuated total reflection experiments on multilayer structures. We generalize the concept of a conjugate matched pair to bi-isotropic media and obtain several conditions under which the omnidirectional total transmission, which we call the super-Klein tunneling, occurs through conjugate matched pairs consisting of Tellegen media and of chiral media. We find that these conditions are closely linked to those for the omnidirectional excitation of surface waves. Using the invariant imbedding method, we perform extensive numerical calculations of the absorptance, the transmittance, and the spatial distribution of the electromagnetic fields for circularly polarized waves incident on bilayer structures and confirm that the results agree perfectly with the analytical predictions.

Quantum point contacts (QPCs) are fundamental building blocks of nanoelectronic circuits. For their emission dynamics as well as for interaction effects such as the 0.7 anomaly the details of the electrostatic potential are important, but the precise potential shapes are usually unknown. Here, we measure the one-dimensional subband spacings of various QPCs as a function of their conductance and compare our findings with models of lateral parabolic versus hard-wall confinement. We find that a gate-defined QPC near pinch-off is compatible with the parabolic saddle-point scenario. However, as the number of populated subbands is increased, Coulomb screening flattens the potential bottom and a description in terms of a finite hard-wall potential becomes more realistic.

We study hybridization of collective plasmon and Carlson-Goldman-Artemenko-Volkov modes in a hybrid system, consisting of two-dimensional layers of electron gas in the normal state and superconductor, coupled by long-range Coulomb forces. The interaction between these collective modes is not possible in a regular single-layer two-dimensional system since they exist in nonoverlapping domains of dimensionless parameter $\omega \tau$, where $\omega$ is the external electromagnetic field frequency and $\tau$ is electron-scattering time. Thus, in a single-layer structure, these modes are mutually exclusive. However, the coupling may become possible in a hybrid system consisting of two separated in space materials with different properties, in particular, the electron-scattering time. We investigate the electromagnetic power absorption by the hybrid system and reveal the conditions necessary for the hybridization of collective modes.

We explore in detail the electronic phases of a system consisting of three noncolinear arrays of coupled quantum wires, each rotated ${120}^{\circ }$ with respect to the next. A perturbative renormalization-group analysis reveals that multiple correlated states can be stabilized: a $s$-wave or $d\pm id$ superconductor, a charge density wave insulator, a two-dimensional Fermi liquid, and a 2D Luttinger liquid (also known as smectic metal or sliding Luttinger liquid). The model provides an effective description of electronic interactions in small-angle twisted bilayer graphene and we discuss its implications in relation to the recent observation of correlated and superconducting ground states near commensurate densities in magic-angle twisted samples, as well as the “strange metal” behavior at finite temperatures as a natural outcome of the 2D Luttinger liquid phase.

In coexistence with bulk water, wetting in the hydrophobic grain boundary is unavailable at ambient conditions, because of less energy gain when water is on top of a hydrophobic surface. However, this study discloses a high-pressure driven wetting between hydrophobic surfaces. The dewetting-wetting transition pressure is higher when temperature increases, due to a competition between enthalpy gain and entropy loss. The stably wetting water monolayer exhibits a continuous phase transition from a liquidlike to a square-ice-like form, through interconversions between liquid and square-ice components. The diagram of dewetting-wetting and liquidlike-icelike transitions is delivered here.

Quantum turbulence in superfluid ${}^4\mathrm{He}$ in narrow channels often takes the form of moving localized vortex tangles. Such tangles, called turbulent plugs, also serve as building blocks of quantum turbulence in wider channels. We report on a numerical study of various aspects of the dynamics and structure of turbulent plugs in a wide range of governing parameters. The unrestricted growth of the tangle in a long channel provides a unique view on a natural tangle structure including superfluid motion at many scales. We argue that the edges of the plugs propagate as turbulent fronts, following the advection-diffusion-reaction dynamics. This analysis shows that the dynamics of the two edges of the tangle have distinctly different nature. While bearing similarity to the dynamics of the patches of turbulent fluctuations during the transition to turbulence in the classical pipe flows, the superfluid turbulent fronts exhibit richer behavior. We provide an analytic solution of the equation of motion for the fronts that define their shape, velocities, and effective diffusivity, and analyze these parameters for various flow conditions.

In this paper, we report an experimental observation of the large anomalous Hall effect (AHE) in a hexagonal ferromagnetic $\mathrm{F}{\mathrm{e}}_5\mathrm{S}{\mathrm{n}}_3$ single crystal with current along the $b$ axis and a magnetic field normal to the $bc$ plane. The intrinsic contribution of the anomalous Hall conductance ${\sigma }_{AH}^{\mathrm{int}}$ was approximately $613{\mathrm{\Omega }}^{-1}\mathrm{c}{\mathrm{m}}^{-1}$, which was more than 3 times the maximum value in the frustrated kagome magnet $\mathrm{F}{\mathrm{e}}_3\mathrm{S}{\mathrm{n}}_2$ and nearly independent of the temperature over a wide range between 5 and 350 K. The analysis results revealed that the large AHE was dominated by a common, intrinsic term, while the extrinsic contribution, i.e., the skew scattering and side jump, turned out to be small. In addition to the large AHE, it was found that the types of majority carriers changed at approximately 275 and 30 K, consistent with the critical temperatures of the spin reorientation. These findings suggest that the hexagonal ferromagnetic $\mathrm{F}{\mathrm{e}}_5\mathrm{S}{\mathrm{n}}_3$ single crystal is an excellent candidate to use for the study of the topological features in ferromagnets.

We use neutron scattering and bulk property measurements to determine the single-ion crystal-field Hamiltonians of delafossites ${\mathrm{KErSe}}_2$ and ${\mathrm{CsErSe}}_2$. These two systems contain planar equilateral triangular Er lattices arranged in two stacking variants: rhombohedral (for K) or hexagonal (Cs). Our analysis shows that regardless of the stacking order both compounds exhibit an easy-plane ground-state doublet with large $J_z=1/2$ terms and the potential for significant quantum effects, making them candidates for quantum-spin-liquid or other exotic ground states.

Many magnetic point-group symmetries induce a topological classification on crystalline insulators, dividing them into those that have a nonzero quantized Chern-Simons magnetoelectric coupling (“axion-odd” or “topological”) and those that do not (“axion-even” or “trivial”). For time-reversal or inversion symmetries, the resulting topological state is usually denoted as a “strong topological insulator” or an “axion insulator,” respectively, but many other symmetries can also protect this “axion $Z_2$” index. Topological states are often insightfully characterized by choosing one crystallographic direction of interest and inspecting the hybrid Wannier (or equivalently, the non-Abelian Wilson-loop) band structure, considered as a function of the two-dimensional Brillouin zone in the orthogonal directions. Here, we systematically classify the axion-quantizing symmetries and explore the implications of such symmetries on the Wannier band structure. Conversely, we clarify the conditions under which the axion $Z_2$ index can be deduced from the Wannier band structure. In particular, we identify cases in which a counting of Dirac touchings between Wannier bands, or a calculation of the Chern number of certain Wannier bands, or both, allows for a direct determination of the axion $Z_2$ index. We also discuss when such symmetries impose a “flow” on the Wannier bands, such that they are always glued to higher and lower bands by degeneracies somewhere in the projected Brillouin zone, and the related question of when the corresponding surfaces can remain gapped, thus exhibiting a half-quantized surface anomalous Hall conductivity. Our formal arguments are confirmed and illustrated in the context of tight-binding models for several paradigmatic axion-odd symmetries, including time reversal, inversion, simple mirror, and glide mirror symmetries.

Hopf insulators are exotic topological states of matter outside the standard tenfold-way classification based on discrete symmetries. Its topology is captured by an integer invariant that describes the linking structures of the Hamiltonian in the three-dimensional momentum space. In this paper, we investigate the quantum dynamics of Hopf insulators across a sudden quench and show that the quench dynamics is characterized by a ${\mathbb{Z}}_2$ invariant $\nu$ which reveals a rich interplay between quantum quench and static band topology. We construct the ${\mathbb{Z}}_2$ topological invariant using the loop unitary operator and prove that $\nu$ relates the pre- and postquench Hopf invariants through $\nu =(\mathcal{L}-{\mathcal{L}}_0)mod2$. The ${\mathbb{Z}}_2$ nature of the dynamical invariant is in sharp contrast to the $\mathbb{Z}$ invariant for the quench dynamics of Chern insulators in two dimensions. The nontrivial dynamical topology is further attributed to the emergence of $\pi$ defects in the phase band of the loop unitary. These $\pi$ defects are generally closed curves in the momentum-time space, for example, as nodal rings carrying Hopf charge.

Using density functional theory based first-principles calculations, we predict the dynamically stable 1H phase of a Janus single layer composed of S-Mo-O atomic layers. It is an indirect band gap semiconductor exhibiting strong polarization arising from the charge difference on the two surfaces. In contrast to 1H phases of ${\mathrm{MoS}}_2$ and ${\mathrm{MoO}}_2$, Janus MoSO is found to possess four Raman active phonon modes and a large out-of-plane piezoelectric coefficient which is absent in fully symmetric single layers of ${\mathrm{MoS}}_2$ and ${\mathrm{MoO}}_2$. We investigated the electronic and phononic properties under applied biaxial strain and found an electronic phase transition with tensile strain while the conduction band edge displays a shift when under compressive strain. Furthermore, single-layer MoSO exhibits phononic stability up to 5% of compressive and 11% of tensile strain with significant phonon shifts. The phonon instability is shown to arise from the soft in-plane and out-of-plane acoustic modes at finite wave vector. The large strain tolerance of Janus MoSO is important for nanoelastic applications. In view of the dynamical stability even under moderate strain, we expect that Janus MoSO can be fabricated in the common 1H phase with a strong out-of-plane piezoelectric coefficient.

We experimentally compare two types of interface structures with magnetic and nonmagnetic Weyl semimetals. They are the junctions between a gold normal layer and magnetic Weyl semimetal ${\mathrm{Ti}}_2\mathrm{MnAl}$, and a ferromagnetic nickel layer and nonmagnetic Weyl semimetal ${\mathrm{WTe}}_2$, respectively. Due to the ferromagnetic side of the junction, we investigate spin-polarized transport through the Weyl semimetal surface. For both structures, we demonstrate similar current-voltage characteristics, with hysteresis at low currents and sharp peaks in differential resistance at high ones. Despite this behavior resembling the known current-induced magnetization dynamics in ferromagnetic structures, evolution of the resistance peaks with magnetic field is unusual. We connect the observed effects with current-induced spin dynamics in Weyl topological surface states.

We study the rectification of near-field radiative heat transfer between two InSb nanoparticles due to the presence of nonreciprocal surface modes in a nearby InSb sample when an external magnetic field is applied and its dependence on the magnetic field strength. We reveal the spin-spin coupling mechanism of the localized particle resonances and the surface mode resonances which is substantiated by the directional heat flux in the given setup. We discuss further the interplay of the frequency shift, the propagation length, and local density of states on the strength and directionality of the rectification as well as the nonreciprocal heating effect of the nanoparticles.

A large anomalous Hall effect (AHE) has been observed in ferromagnetic ${\mathrm{Fe}}_3{\mathrm{Sn}}_2$ with breathing kagome bilayers. To understand the underlying mechanism for this, we investigate the electronic structure of ${\mathrm{Fe}}_3{\mathrm{Sn}}_2$ by angle-resolved photoemission spectroscopy (ARPES). In particular, we use both vacuum ultraviolet light (VUV) and soft x ray (SX), which allow surface-sensitive and relatively bulk-sensitive measurements, respectively, and distinguish bulk states from surface states, which should be unlikely related to the AHE. While VUV-ARPES observes two-dimensional bands mostly due to surface states, SX-ARPES reveals three-dimensional band dispersions with a periodicity of the rhombohedral unit cell in the bulk. Our data show a good consistency with a theoretical calculation based on density functional theory, suggesting a possibility that ${\mathrm{Fe}}_3{\mathrm{Sn}}_2$ is a magnetic Weyl semimetal.

Two-dimensional layered semiconductor black phosphorus (BP), a promising pressure-induced Dirac system as predicted by band structure calculations, has been studied by ${}^{31}\mathrm{P}$ nuclear magnetic resonance. Band calculations have been also carried out to estimate the density of states $D\left(E\right)$. The temperature and pressure dependences of the nuclear spin lattice relaxation rate $1/T_1$ in the semiconducting phase are well reproduced using the derived $D\left(E\right)$, and the resultant pressure dependence of the semiconducting gap is in good accordance with previous reports, giving a good confirmation that the band calculation on BP is fairly reliable. The present analysis of $1/T_1$ data with complemental theoretical calculations allows us to extract essential information, such as the pressure dependences of $D\left(E\right)$ and the chemical potential, as well as to decompose the observed $1/T_1$ into intrinsic and extrinsic contributions. An abrupt increase in $1/T_1$ at $P=1.63\mathrm{GPa}$ indicates that the semiconducting gap closes, resulting in the enhancement of conductivity.

Ultrafast transient reflectivity across the unusual three-dimensional Peierls-like insulator-metal (IM) transition in ${\mathrm{CuIr}}_2{\mathrm{S}}_4$ was measured as a function of temperature. The low-temperature insulating-phase transient response is dominated by broken-symmetry-induced coherent lattice oscillations that abruptly vanish at the IM transition. The coherent mode spectra are consistent with Raman spectra reported in literature. The origin of the broken-symmetry-induced modes and the photoexcited carrier recombination pathway through in-gap defect states are also briefly discussed.