Evidence for Majorana bound state in an iron-based superconductor

The search for Majorana bound state (MBS) has recently emerged as one of the most active research areas in condensed matter physics, fueled by the prospect of using its non-Abelian statistics for robust quantum computation. A highly sought-after platform for MBS is two-dimensional topological superconductors, where MBS is predicted to exist as a zero-energy mode in the core of a vortex. A clear observation of MBS, however, is often hindered by the presence of additional low-lying bound states inside the vortex core. By using scanning tunneling microscope on the newly discovered superconducting Dirac surface state of iron-based superconductor FeTe1-xSex (x = 0.45, superconducting transition temperature Tc = 14.5 K), we clearly observe a sharp and non-split zero-bias peak inside a vortex core. Systematic studies of its evolution under different magnetic fields, temperatures, and tunneling barriers strongly suggest that this is the case of tunneling to a nearly pure MBS, separated from non-topological bound states which is moved away from the zero energy due to the high ratio between the superconducting gap and the Fermi energy in this material. This observation offers a new, robust platform for realizing and manipulating MBSs at a relatively high temperature.Comment: 27 pages, 11 figures, supplementary information include

Nearly quantized conductance plateau of vortex zero mode in an iron-based superconductor

Majorana zero-modes (MZMs) are spatially-localized zero-energy fractional quasiparticles with non-Abelian braiding statistics that hold a great promise for topological quantum computing. Due to its particle-antiparticle equivalence, an MZM exhibits robust resonant Andreev reflection and 2e2/h quantized conductance at low temperature. By utilizing variable-tunnel-coupled scanning tunneling spectroscopy, we study tunneling conductance of vortex bound states on FeTe0.55Se0.45 superconductors. We report observations of conductance plateaus as a function of tunnel coupling for zero-energy vortex bound states with values close to or even reaching the 2e2/h quantum conductance. In contrast, no such plateau behaviors were observed on either finite energy Caroli-de Genne-Matricon bound states or in the continuum of electronic states outside the superconducting gap. This unique behavior of the zero-mode conductance reaching a plateau strongly supports the existence of MZMs in this iron-based superconductor, which serves as a promising single-material platform for Majorana braiding at a relatively high temperature

Tunable vortex Majorana zero modes in LiFeAs superconductor

The recent realization of pristine Majorana zero modes (MZMs) in vortices of iron-based superconductors (FeSCs) provides a promising platform for long-sought-after fault-tolerant quantum computation. A large topological gap between the MZMs and the lowest excitations enabled detailed characterization of vortex MZMs in those materials. Despite those achievements, a practical implementation of topological quantum computation based on MZM braiding remains elusive in this new Majorana platform. Among the most pressing issues are the lack of controllable tuning methods for vortex MZMs and inhomogeneity of the FeSC Majorana compounds that destroys MZMs during the braiding process. Thus, the realization of tunable vortex MZMs in a truly homogeneous compound of stoichiometric composition and with a charge neutral cleavage surface is highly desirable. Here we demonstrate experimentally that the stoichiometric superconductor LiFeAs is a good candidate to overcome these two obstacles. Using scanning tunneling microscopy, we discover that the MZMs, which are absent on the natural surface, can appear in vortices influenced by native impurities. Our detailed analysis and model calculations clarify the mechanism of emergence of MZMs in this material, paving a way towards MZMs tunable by controllable methods such as electrostatic gating. The tunability of MZMs in this homogeneous material offers an unprecedented platform to manipulate and braid MZMs, the essential ingredients for topological quantum computation.Comment: 21 pages, 10 figures. Suggestions and comments are welcom

Switchable hyperbolic metamaterials based on the graphene-dielectric stacking structure and optical switches design

We numerically demonstrate broadband optical switches which can control the propagation of mid-infrared waves that is incident to the interface between a switchable hyperbolic metamaterials (SHM) and air. The SHM consists of a one-dimensional periodic stacking of graphene layers and dielectric layers. The isofrequency curve of the structure can be switched between hyperbolic shape and elliptical shape by controlling the external gate voltage or the electrostatic field biasing. It is revealed that when the interface between air and the SHM is parallel to the graphene sheets, it can switch between positive refraction and total reflection; when the interface is perpendicular to the graphene sheets, it can switch between positive and negative refraction

A tunable silicon-on-insulator valley Hall photonic crystal at telecommunication wavelengths

Silicon-on-insulator (SOI) combined with topological photonics provides on-chip integrated photonic devices CMOS-compatibility and backscattering-suppression. Here we propose a tunable SOI-based valley Hall topological insulator with two nonequivalent air holes to break inversion symmetry. Due to free-carrier excitation, the real and imaginary parts of the refractive index are modified to cause a blue-shift of bandgap and a reduction of transmittance, respectively. Based on this mechanism, we design a tunable all-optical switch with the merits of high transmittance contrast (about 23) and polarization-resolved characteristics simultaneously. Our proposed approach has potential in broadband tunable integrated photonic devices at telecommunication wavelengths

Tunable directional emission based on graphene hyperbolic metamaterials

We have studied the tunable properties of a hyperbolic metamaterial (HMM) composed of alternative dielectric layers and graphene sheets, and proposed a highly directional emission device working in the terahertz region. By changing the chemical potential ${\mu}_{c}$ , the equifrequency contour (EFC) of the HMM is easily switched between elliptical and hyperbolic types. Especially, when a dipole source is placed inside the structure, highly directional emission is realized provided that the EFC is a flat ellipse. Interestingly, such highly directional emission can be achieved for any frequency within a broad frequency range by tuning the chemical potential ${\mu}_{c}$ . The divergence angle of such a directional beam is lower than 12°. Additionally, the influences of structural parameters (${\varepsilon}_{d}$ and d) on the working bandwidth are also studied. These results have potential in many fields such as light coupling and spontaneous radiation antennas

Revealing photonic Lorentz force as the microscopic origin of topological photonic states

Charged particles like electrons moving in a magnetic field encounter Lorentz force, which governs the formation of electronic topological edge states in quantum Hall effect systems. Here we show that photons transporting in magneto-optical materials and structures also encounter a physical effect called photonic Lorentz force via the indirect interaction with the magneto-optical medium assisted effective magnetic field. This effect can induce half-cycle spiral motion of light at the surface of a homogeneous metallic magneto-optical medium and inhomogeneous magneto-optical photonic crystals, and it governs the intriguing one-way transport properties of robustness and immunity against defects, disorders, and obstacles. Thus, photonic Lorentz force serves as the fundamental microscopic origin of macroscopic photonic topological states, much the same as classical Lorentz force does to electronic topological states