The physics of the epsilon-near-zero (ENZ) mode, which is supported by a nanolayer at the frequency where the dielectric permittivity vanishes, has recently been a subject of debate. In this Rapid Communication, we thoroughly investigate and clarify the physics of this mode, providing its main characteristics and its domain of existence. This understanding will benefit all the applications that rely on ENZ modes in semiconductor nanolayers, including directional perfect absorption, voltage-tunable devices, and ultrafast thermal emission.

We investigate how the electron-vibron coupling influences electron transport via an anisotropic magnetic molecule, such as a single-molecule magnet (SMM) ${\mathrm{Fe}}_4$, by using a model Hamiltonian with parameter values obtained from density-functional theory (DFT). The magnetic anisotropy parameters, vibrational energies, and electron-vibron coupling strengths of the ${\mathrm{Fe}}_4$ are computed using DFT. A giant spin model is applied to the ${\mathrm{Fe}}_4$ with only two charge states, specifically a neutral state with a total spin $S=5$ and a singly charged state with $S=9/2$, which is consistent with our DFT result and experiments on ${\mathrm{Fe}}_4$ single-molecule transistors. In sequential electron tunneling, we find that the magnetic anisotropy gives rise to new features in the conductance peaks arising from vibrational excitations. In particular, the peak height shows a strong, unusual dependence on the direction as well as magnitude of applied $B$ field. The magnetic anisotropy also introduces vibrational satellite peaks whose position and height are modified with the direction and magnitude of applied $B$ field. Furthermore, when multiple vibrational modes with considerable electron-vibron coupling have energies close to one another, a low-bias current is suppressed, independently of gate voltage and applied $B$ field, although that is not the case for a single mode with a similar electron-vibron coupling. In the former case, the conductance peaks reveal a stronger $B$-field dependence than in the latter case. The new features appear because the magnetic anisotropy barrier is of the same order of magnitude as the energies of vibrational modes with significant electron-vibron coupling. Our findings clearly show the interesting interplay between magnetic anisotropy and electron-vibron coupling in electron transport via the ${\mathrm{Fe}}_4$. Similar behavior can be observed in transport via other anisotropic magnetic molecules.

We investigate the electronic band structure of the ${\mathrm{MoS}}_2/\mathrm{Bi}\left(111\right)$ heterostructure, for which the supercell calculation is performed due to the lattice mismatch between two structures but the effective primitive cell is recovered by using the band unfolding technique. It is found that the strong molybdenum-bismuth band hybridization together with a generation of the interfacial dipole field induces the giant Rashba-type splitting accompanying the proper spin topology in molybdenum-driven bands at the $\Gamma$ point. Similar splittings are also found in the heterostructure with other transition-metal dichalcogenides, i.e., with ${\mathrm{MoSe}}_2,{\mathrm{WS}}_2$, and ${\mathrm{WSe}}_2$.

Great efforts have been devoted to control the polarization state, the transmission direction, and the phase of light within a very confined space in recent decades. Here, we present a two-dimensional metastructure made of an array of unisized split rings with different opening orientations on the surface of a $\mathrm{Si}{\mathrm{O}}_2$-silver bilayer. This structure possesses an unexpectedly high polarization conversion ratio and generates significantly strong anomalous reflection beams (over 70% of incident light intensity) over a broad frequency range (1100–1750 nm). Functionally, it is able to turn either a linearly polarized incident light or natural light into two perfect circularly polarized beams with the same amplitude yet different handedness to different directions. These features demonstrate a clear example of momentum conservation and can be applied to detect/manipulate the propagation of circularly polarized light.

In this contribution we present a theoretical investigation of the energy transfer efficiency between quantum systems placed in proximity of a monolayer of conducting graphene. We calculate the spontaneous emission rate of a quantum system and the energy transfer rate between a donor-acceptor pair, and thus the energy transfer efficiency, using a Green's tensor formalism. The direct interaction between the donor and acceptor dominates when they are close to each other, but is modified from its free-space behavior due to the presence of the graphene monolayer and its interaction with the donor and acceptor. We report on a very large influence of the graphene monolayer on the energy transfer efficiency due to both the Förster mechanism and the propagating graphene plasmon mode. In particular, the Förster radius $R_0$ is modified from its free-space value of 20 nm and can reach values of 120 nm when close to a graphene monolayer. As the donor-acceptor separation is increased, their direct interaction is overshadowed by the interaction via the surface plasmon mode. Due to the large propagation length of the surface plasmon mode on graphene, an energy transfer efficiency as high as 50% can still be achieved for distances as large as 300 nm. The interaction via the surface plasmon mode is tunable via the doping of the graphene monolayer and the surface plasmon channel can also be switched off this way.

We show how a structural modification of graphene, which gives the carbon allotrope graphyne, can induce an energy gap at the $K$ point of the Brillouin zone. Upon adsorption on metallic surfaces, the same mechanism is responsible for a further modification of energy bands which occurs via the charge transfer mechanism. We perform the calculation based on the density functional theory with the novel nonlocal van der Waals–density functional correlation of the adsorption of graphyne on Cu(111), Ni(111), and Co(0001) surfaces and show the dependence of the band change on the charge transfer in the system. The binding of graphyne appears to be stronger than that of graphene on the same surfaces.

The Bloch theorem enables reduction of the eigenvalue problem of the single-particle Hamiltonian that commutes with the translational group. Based on a group theory analysis we present a generalization of the Bloch theorem that incorporates all additional symmetries of a crystal. The generalized Bloch theorem constrains the form of the Hamiltonian which becomes manifestly invariant under additional symmetries. In the case of isotropic interactions the generalized Bloch theorem gives a unique Hamiltonian. This Hamiltonian coincides with the Hamiltonian in the periodic gauge. In the case of anisotropic interactions the generalized Bloch theorem allows a family of Hamiltonians. Due to the continuity argument we expect that even in this case the Hamiltonian in the periodic gauge defines observables, such as Berry curvature, in the inverse space. For both cases we present examples and demonstrate that the average of the Berry curvatures of all possible Hamiltonians in the Bloch gauge is the Berry curvature in the periodic gauge.

Among heavy-fermion metals, ${\mathrm{Ce}}_3{\mathrm{Pd}}_{20}{\mathrm{Si}}_6$ is one of the heaviest-electron systems known to date. Here we used high-resolution neutron spectroscopy to observe low-energy magnetic scattering from a single crystal of this compound in the paramagnetic state. We investigated its temperature dependence and distribution in momentum space, which was not accessible in earlier measurements on polycrystalline samples. At low temperatures, a quasielastic magnetic response with a half-width $\Gamma \approx 0.1\mathrm{meV}$ persists with varying intensity all over the Brillouin zone. It forms a broad hump centered at the (111) scattering vector, surrounded by minima of intensity at (002), (220), and equivalent wave vectors. The momentum-space structure distinguishes this signal from a simple crystal-field excitation at 0.31 meV, suggested previously, and rather lets us ascribe it to short-range dynamical correlations between the neighboring Ce ions, mediated by the itinerant heavy $f$ electrons via the Ruderman-Kittel-Kasuya-Yosida mechanism. With increasing temperature, the energy width of the signal follows the conventional $T^{1/2}$ law, $\Gamma \left(T\right)={\Gamma }_0+A\sqrt{T}$. The momentum-space symmetry of the quasielastic response suggests that it stems from the simple-cubic Ce sublattice occupying the $8c$ Wyckoff site, whereas the crystallographically inequivalent $4a$ site remains magnetically silent in this material.

In this paper, we report the signature of spin gapless semiconductor (SGS) in CoFeMnSi that belongs to the Heusler family. SGS is a new class of magnetic semiconductors which have a band gap for one spin subband and zero band gap for the other, and thus are useful for tunable spin transport based applications. We show various experimental evidences for SGS behavior in CoFeMnSi by carefully carrying out the transport and spin-polarization measurements. SGS behavior is also confirmed by first-principles band-structure calculations. The most stable configuration obtained by the theoretical calculation is verified by experiment. The alloy is found to crystallize in the cubic Heusler structure (LiMgPdSn type) with some amount of disorder and has a saturation magnetization of $3.7{\mu }_B/\mathrm{f}.\mathrm{u}.$ and Curie temperature of ∼620 K. The saturation magnetization is found to follow the Slater-Pauling behavior, one of the prerequisites for SGS. Nearly-temperature-independent carrier concentration and electrical conductivity are observed from 5 to 300 K. An anomalous Hall coefficient of 162 S/cm is obtained at 5 K. Point contact Andreev reflection data have yielded the current spin-polarization value of 0.64, which is found to be robust against the structural disorder. All these properties strongly suggest SGS nature of the alloy, which is quite promising for the spintronic applications such as spin injection as it can bridge the gap between the contrasting behaviors of half-metallic ferromagnets and semiconductors.

We model the injection of elastic waves into a ferromagnetic film (F) by a nonmagnetic transducer (N). We compare the configurations in which the magnetization is normal and parallel to the wave propagation. The lack of axial symmetry in the former results in the emergence of evanescent interface states. We compute the energy-flux transmission across the $\mathrm{N}|\mathrm{F}$ interface and sound-induced magnetization dynamics in the ferromagnet. We predict efficient acoustically induced pumping of spin current into a metal contact attached to F.

Replacing semiconductors with topological insulators, we propose the problem of dilute magnetic topological semiconductors. Performing the renormalization group analysis for an effective field theory, where doped magnetic impurities give rise to a spatially modulated random axion term, we find a novel insulator-metal transition from either a topological or band insulating phase to an inhomogeneously distributed Weyl metallic state with such insulating islands, where extremely broad distributions of ferromagnetic clusters combined with strong spin-orbit interactions are responsible for the emergence of randomly distributed Weyl metallic islands. Since electromagnetic properties in a Weyl metal are described by axion electrodynamics, the role of random axion electrodynamics in transport phenomena casts an interesting problem beyond the physics of percolation in conventional disorder-driven metal-insulator transitions.

We investigate theoretically the interface between a ferroelectric $\mathrm{BaTi}{\mathrm{O}}_3$ film and a nonpolar insulating $\mathrm{SrTi}{\mathrm{O}}_3$ substrate. We find that thin $\mathrm{BaTi}{\mathrm{O}}_3$ (under 5 nm) can stabilize a nonpolarized state, and an additional metastable polarized state. While the nonpolarized state is insulating, for the polarized heterostructure, we discover the existence of two-dimensional charge carrier gases. In this case, the heterostructure undergoes an electronic reconstruction in order to prevent the polar catastrophe. The two-dimensional gases, formed as a result, screen the polarization, leading to a substantially reduced potential drop across the ferroelectric film. We emphasize that the two-dimensional electron and hole gases are created by the polarization of the sample, and are not due to the polar nature of the material or to doping.

Despite continuous efforts, a conceptual perception on the origin of quasi-two-dimensional electron gas (q-2DEG) in oxide heterostructures remains far from any generalization. Along this perspective, a systematic study identifying systems that exhibit heterointerface conductivity (${\mathrm{LaAlO}}_3{/\mathrm{SrTiO}}_3$) and that do not (${\mathrm{LaCrO}}_3{/\mathrm{SrTiO}}_3$) is undertaken. High quality ${\mathrm{LaAl}}_{1-x}{\mathrm{Cr}}_x{\mathrm{O}}_3$ ($0\le x\le 1$) films were fabricated on ${\mathrm{TiO}}_2$ terminated ${\mathrm{SrTiO}}_3$ (001) substrates using a pulsed laser deposition technique and its growth was monitored by reflection high energy electron diffraction (RHEED). The transport and photoemission spectroscopy experiments reveal that the the origin of heterointerface q-2DEG is associated with ${\mathrm{Ti}}^{3+}$ states and that it ceases to exist with increasing Cr ion concentration in the overlayer films. Following experimental evidences of the Cr concentration dependent metal-insulator electronic phase transition at the heterointerface, explanation is sought within the realms of a polar catastrophe model. Based on our transport and spectroscopy measurements, we propose that an electronic reconstruction drives the formation of q-2DEG for pristine LAO/STO and Al rich samples due to the formation of ${\mathrm{Ti}}^{3+}{/\mathrm{Ti}}^{4+}$ mixed valent state, while for heavily doped Cr samples, the required electron count necessary to solve the polar catastrophe instability are trapped in the ${\mathrm{LaAl}}_{1-x}{\mathrm{Cr}}_x{\mathrm{O}}_3$ overlayers in the Cr $3d$ states. These trapped electrons in the overlayers are manifested in the form of ${\mathrm{Cr}}^{2+}$ ions.

We investigate the interplay between graphene and different, weakly interacting metal substrates by measuring the local density of states of the surface with scanning tunneling spectroscopy. Energy-resolved Friedel oscillations, confined states, and a prominent signal in point spectra are found after intercalating several monolayers of silver between graphene and Ir(111) and correspond to the shifted surface state of silver. These features outweigh spectroscopic signatures of graphene, which are retrieved when the amount of silver is reduced to one monolayer. Hence, suppressing the surface states of the metal substrate enhances the sensitivity to the Dirac states of quasi-free-standing graphene.

Dynamical localization is one of the most startling manifestations of quantum interference, where the evolution of a simple system is frozen out under a suitably tuned coherent periodic drive. Here we show that, although any randomness in the interactions of a many-body system kills dynamical localization eventually, spectacular remnants survive even when the disorder is strong. We consider a disordered quantum Ising chain where the transverse magnetization relaxes exponentially with time with a decay time-scale $\tau$ due to random longitudinal interactions between the spins. We show that, under external periodic drive, this relaxation slows down ($\tau$ shoots up) by orders of magnitude as the ratio of the drive frequency $\omega$ and amplitude $h_0$ tends to certain specific values (the freezing condition). If $\omega$ is increased while maintaining the ratio $h_0/\omega$ at a fixed freezing value, then $\tau$ diverges exponentially with $\omega .$ The results can be easily extended for a larger family of disordered fermionic and bosonic systems.

We introduce the concept of boundary degeneracy, as the ground state degeneracy of topologically ordered states on a compact orientable spatial manifold with gapped boundaries. We emphasize that the boundary degeneracy provides richer information than the bulk degeneracy. Beyond the bulk-edge correspondence, we find the ground state degeneracy of the fully gapped edge modes depends on boundary gapping conditions. By associating different types of boundary gapping conditions as different ways of particle or quasiparticle condensations on the boundary, we develop an analytic theory of gapped boundaries. By Chern-Simons theory, this allows us to derive the ground state degeneracy formula in terms of boundary gapping conditions, which encodes more than the fusion algebra of fractionalized quasiparticles. We apply our theory to Kitaev's toric code and Levin-Wen string-net models. We predict that the $Z_2$ toric code and $Z_2$ double-semion model [more generally, the $Z_k$ gauge theory and the $U{\left(1\right)}_k\times U{\left(1\right)}_{-k}$ nonchiral fractional quantum Hall state at even integer $k$] can be numerically and experimentally distinguished, by measuring their boundary degeneracy on an annulus or a cylinder.

We study the conductance fluctuations in the crossover between ballistic and diffusive regimes of phase coherent transport. Within a diagrammatic approach, the conductance variance for a quasi-one-dimensional disordered system is calculated beyond the diffusion approximation. The result obtained establishes the interrelation between conductance fluctuations in the crossover regime and the inhomogeneity characteristics of the disordered system. In the case of highly forward scattering by large inhomogeneities, we find the enhancement of conductance fluctuations at subdiffusion length scales.

We have performed ab initio band-structure calculations on more than 2000 half-Heusler compounds in order to search for new candidates for topological insulators. Herein, LiAuS and NaAuS are found to be the strongest topological insulators with the bulk band gaps of 0.20 and 0.19 eV, respectively, different from the zero band-gap feature reported in other Heusler topological insulators. Due to the inversion asymmetry of the Heusler structure, their topological surface states on the top and bottom surfaces exhibit $p$-type and $n$-type carriers, respectively. Thus, these materials may serve as an ideal platform for the realization of topological magnetoelectric effects as polar topological insulators. Moreover, these topological surface states exhibit the right-hand spin texture in the upper Dirac cone, which distinguishes them from currently known topological insulator materials. Their topological nontrivial character remains robust against in-plane strains, which makes them suitable for epitaxial growth of films.

The site-specific magnetic properties of thin films of the Heusler compound ${\text{Mn}}_2{\text{Ru}}_x\text{Ga}$, $0.6<x<1.0$, are studied using x-ray absorption and dichroism spectroscopy. There is evidence of half metallicity up to $x=0.7$, and compensation of the Mn $4a$ and $4c$ moments in this range, leading to a zero-moment half metal. We also also discuss the effect of substrate-induced strain on the magnetic properties. By tuning the biaxial strain, simultaneous perfect magnetic compensation and half-metallic character is achievable at, below, or above room temperature.