Quantum oscillations of superconductivity in ultra-thin Pb films

1. Quantum oscillations of superconductivity in ultra-thin Pb films In collaboration with Ken Shih’s experimental group at UT Austin, we have studied the superconductivity in thin Pb films [1]. The superconducting energy gap of epitaxially grown Pb films was measured by a low temperature scanning tunneling microscope as a function of the layer thickness in ultra-thin regime (5 -18 ML).The layer-dependent energy gap and transition temperature (Tc) show persistent quantum oscillations down to the lowest thickness without any sign of suppression.By comparison with the quantum well states measured above Tc and the theoretical calculations, we found that the Tc oscillation correlates directly with the DOS oscillation at EF. The oscillation is manifested by the phase matching of the Fermi wavelength and the layer thickness, resulting in a bi-layer periodicity modulated by a longer wavelength quantum beat. [1] "Persistent Superconductivity in Ultrathin Pb Films: A Scanning Tunneling Spectroscopy Study," D. Eom, S. Qin, M. Y. Chou, and C.K. Shih, Phys. Rev. Lett. 96, 027005/1-4 (2006).

2. Fig. 1 (a) A variation of critical temperatures, Tc determined from temperature-dependent gap measurements for 5-15 ML. (b) The calculated density of states near a Fermi level, DOS (EF) as a function of the layer thickness by using a simple infinite well model for free standing Pb film. To correct the phase shift at the surface and interface, a line up of the positions of QWS with those determined experimentallyis made. (c) The positions of QWS determined experimentally. D. Eom, S. Qin, M. Y. Chou, and C.K. Shih, Phys. Rev. Lett. 96, 027005/1-4 (2006).