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We report transport measurements on Josephson junctions consisting of Bi2Te3 topological insulator (TI) thin films contacted by superconducting Nb electrodes. For a device with junction length L = 134 nm, the critical supercurrent Ic can be modulated by an electrical gate which tunes the carrier type and density of the TI film. Ic can reach a minimum when the TI is near the charge neutrality regime with the Fermi energy lying close to the Dirac point of the surface state. In the p-type regime the Josephson current can be well described by a short ballistic junction model. In the n-type regime the junction is ballistic at 0.7 K < T < 3.8 K while for T < 0.7 K the diffusive bulk modes emerge and contribute a larger Ic than the ballistic model. We attribute the lack of diffusive bulk modes in the p-type regime to the formation of p–n junctions. Our work provides new clues for search of Majorana zero mode in TI-based superconducting devices. [ABSTRACT FROM AUTHOR]

We report high transition temperature superconductivity in one unit-cell (UC) thick FeSe films grown on a Seetched SrTiO3(001) substrate by molecular beam epitaxy (MBE). A superconducting gap as large as 20 meV and the magnetic field induced vortex state revealed by in situ scanning tunneling microscopy (STM) suggest that the superconductivity of the 1 UC FeSe films could occur around 77 K. The control transport measurement shows that the onset superconductivity temperature is well above 50 K. Our work not only demonstrates a powerful way for finding new superconductors and for raising TC, but also provides a well-defined platform for systematic studies of the mechanism of unconventional superconductivity by using different superconducting materials and substrates. [ABSTRACT FROM AUTHOR]

Atomically flat thin films of topological semimetal Na3 Bi are grown on double-layer graphene formed on 6H–SiC(0001) substrates by molecular beam epitaxy. By combined techniques of molecular beam epitaxy, scanning tunneling microscopy and angle resolved photoelectron spectroscopy, the growth conditions for Na3 Bi thin films on double-layer graphene are successfully established. The band structure of Na3 Bi grown on graphene is mapped along Γ̄–M̄ and Γ̄–K̄ directions. Furthermore, the energy band of Na3 Bi at higher energy is uncovered by doping Cs atoms on the surface. [ABSTRACT FROM AUTHOR]