Yao-Yi Li

Majorana Zero Mode Detected with Spin Selective Andreev Reflection in the Vortex of a Topological Superconductor.

Recently, theory has predicted a Majorana zero mode (MZM) to induce spin selective Andreev reflection (SSAR), a novel magnetic property which can be used to detect the MZM. Here, spin-polarized scanning tunneling microscopy or spectroscopy has been applied to probe SSAR of MZMs in a topological superconductor of the Bi_{2}Te_{3}/NbSe_{2} heterostructure. The zero-bias peak of the tunneling differential conductance at the vortex center is observed substantially higher when the tip polarization and the external magnetic field are parallel rather than antiparallel to each other. This spin dependent tunneling effect provides direct evidence of MZM and reveals its magnetic property in addition to the zero energy modes. Our work will stimulate MZM research on these novel physical properties and, hence, is a step towards experimental study of their statistics and application in quantum computing.

Coexistence of Topological Edge State and Superconductivity in Bismuth Ultrathin Film

Ultrathin freestanding bismuth film is theoretically predicted to be one kind of two-dimensional topological insulators. Experimentally, the topological nature of bismuth strongly depends on the situations of the Bi films. Film thickness and interaction with the substrate often change the topological properties of Bi films. Using angle-resolved photoemission spectroscopy, scanning tunneling microscopy or spectroscopy and first-principle calculation, the properties of Bi(111) ultrathin film grown on the NbSe2 superconducting substrate have been studied. We find the band structures of the ultrathin film is quasi-freestanding, and one-dimensional edge state exists on Bi(111) film as thin as three bilayers. Superconductivity is also detected on different layers of the film and the pairing potential exhibits an exponential decay with the layer thicknesses. Thus, the topological edge state can coexist with superconductivity, which makes the system a promising platform for exploring Majorana Fermions.

Development of in situ two-coil mutual inductance technique in a multifunctional scanning tunneling microscope

Superconducting thin films have been a focal point for intensive research efforts since their reduced dimension allows for a wide variety of quantum phenomena. Many of these films, fabricated in UHV chambers, are highly vulnerable to air exposure, making it difficult to measure intrinsic superconducting properties such as zero resistance and perfect diamagnetism with ex situ experimental techniques. Previously, we developed a multifunctional scanning tunneling microscope (MSTM) containing in situ four-point probe (4PP) electrical transport measurement capability in addition to the usual STM capabilities [Ge et al., Rev. Sci. Instrum. 86, 053903 (2015)]. Here we improve this MSTM via development of both transmission and reflection two-coil mutual inductance techniques for in situ measurement of the diamagnetic response of a superconductor. This addition does not alter the original STM and 4PP functions of the MSTM. We demonstrate the performance of the two-coil mutual inductance setup on a 10-nm-thick NbN thin film grown on a Nb-doped SrTiO3(111) substrate.

Metastable Face-Centered Cubic Structure and Structural Transition of Sn on 2H-NbSe

Surface structures and properties of Sn islands grown on superconducting substrate 2H-NbSe2(0001) are studied using low temperature scanning tunneling microscopy or spectroscopy. The pure face-centered cubic (fcc) structure of Sn surface is obtained. Superconductivity is also detected on the fcc-Sn(111) surface, and the size of superconducting gap on the Sn surface is nearly the same as that on the superconducting substrate. Furthermore, phase transition occurs from fcc-Sn(111) to β-Sn(001) by keeping the sample at room temperature for a certain time. Due to the strain relaxation on the β-Sn islands, both the in-plane unit cell and out-of-plane structures distort, and the height of surface atoms varies periodically to form a universal ripple structure.

_{2}

HgTe (111) surface is comprehensively studied by scanning tunneling microscopy/spectroscopy (STS). In addition to the primitive (1 × 1) hexagonal lattice, six reconstructed surface structures are observed: (2 × 2), 2 × 1, 4 × 1, , and . The (2 × 2) reconstructed lattice maintains the primitive hexagonal symmetry, while the lattices of the other five reconstructions are rectangular. Moreover, the topographic features of the reconstruction are bias dependent, indicating that they have both topographic and electronic origins. The STSs obtained at different reconstructed surfaces show a universal dip feature with size ~100 mV, which may be attributed to the surface distortion. Our results reveal the atomic structure and complex reconstructions of the cleaved HgTe (111) surfaces, which paves the way to understand the rich properties of HgTe crystal.

(0001)

2H- and -phase monolayer MoTe2 films on highly oriented pyrolytic graphite are studied using scanning tunneling microscopy and spectroscopy (STM/STS). The phase transition of MoTe2 can be controlled by a post-growth annealing process, and the intermediate state during the phase transition is directly observed by STM. For 2H-MoTe2, inversion domain boundaries are presented as bright lines at high sample bias, but as dark lines at lower sample bias. The dI/dV mappings reveal the distinct distributions of electronic states between domain boundaries and interiors of domains. It should be noted that a 2 × 2 periodic structure is clearly discernable inside the domains, where the STS measurement shows a small dip of size ~150 meV at the vicinity of the Fermi level, indicating that the 2 × 2 periodic structure may be an incommensurate charge density wave. Moreover, a 4 × 4 periodic structure appears in 2H-MoTe2 grown at a higher substrate temperature.

Surface Structure and Reconstructions of HgTe (111) Surfaces

The electronic structure of single-crystalline Ba(Zn0.875Mn0.125)2As2, the parent compound of the recently found high-temperature ferromagnetic semiconductor–(Ba,K)(Zn,Mn)2As2, was studied by high-resolution angle-resolved photoemission spectroscopy. A flat band related to the 3d states of the doped Mn atoms was observed at the binding energy of ∼ −1.6 eV besides the previously reported feature at about −3.3 eV. The spectral weight of this feature is much stronger than that obtained from density functional theory calculations. We proposed that the large spectral weight could originate from the hybridization between Mn and As orbitals. The overall band dispersions agree well with the first-principles calculations of undoped BaZn2As2 except that the total bandwidth of the valence band is larger than that calculated. The increased bandwidth after Mn doping is compatible with the proposed theoretical model.

Charge Density Wave States in 2H-MoTe2 Revealed by Scanning Tunneling Microscopy

Atomically flat superconducting NbN thin films have been grown on the SrTiO3 (111) substrate by plasma-assisted molecular beam epitaxy for the first time. Using scanning tunneling microscopy and in situ reflection high energy electron diffraction, we investigate the surface structure of epitaxial NbN thin films on the SrTiO3 (111) substrate. The orientation [101¯]NbN//[112¯]STO is dominating at substrate temperature above 700 °C. The quality of the as-grown films can be further improved by annealing at elevated temperatures. The homogeneous spatial distribution of superconducting gaps and magnetic vortices further demonstrates the high quality of the NbN films.