Persistent Superconductivity in Ultrathin Pb Films: A Scanning Tunneling Spectroscopy Study

Abstract

By using a low temperature scanning tunneling microscope we have probed the superconducting energy gap of epitaxially grown Pb films as a function of the layer thickness in an ultrathin 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. Moreover, by comparison with the quantum-well states measured above Tc and the theoretical calculations, we found that the Tc oscillation correlates directly with the density of states oscillation at E(F) . The oscillation is manifested by the phase matching of the Fermi wavelength and the layer thickness, resulting in a bilayer periodicity modulated by a longer wavelength quantum beat.

Thickness-Dependent Oscillations of the Critical Magnetic Field Induced by the Quantum Size Effect in Superconducting Nanofilms

“…The study of the superconductivity in Pb nanofilms has been extended by Eom et al [8]. By the use of the scanning gate microscopy, T c oscillations in the epitaxially grown crystalline Pb films have been measured in the thickness range 5 − −18 ML [8]. Very recently, it has been observed the superconductivity in a single Pb and In atomic layer [20,23].…”

“…This experiment reopens an extensive research concerning the interplay of the superconductivity and the quantum confinement in nanoscale systems [1,3,11,14,18,19,22]. The study of the superconductivity in Pb nanofilms has been extended by Eom et al [8]. By the use of the scanning gate microscopy, T c oscillations in the epitaxially grown crystalline Pb films have been measured in the thickness range 5 − −18 ML [8].…”

Van Hove singularities as a result of quantum confinement: The origin of intriguing physical properties in Pb thin films

“…More interestingly, the anisotropy of the m-band of a Pb film is smaller than for the band of the bulk, the reason for which needs more detailed studies. We can explain very strong thickness-dependent physical properties observed in Pb thin films [5][6][7][8][9][10][11][12][13][14][15][16][17] in terms of the 1-D-like DOS singularity developing from the 2-D band bending: it is simply because the electronic structure near EF is now dominated by the singularity. Although we have only studied 23 and 24 ML Pb films, we know where the VHS of other Pb films is from previous STS measurements [16].…”

“…These have been explained by an oscillation in electronic density of states (DOS) near the Fermi level (E F ) [5][6][7][8][9][10][11][12][13][14][15][16][17], particularly in the bottom of subbands that pass though the Fermi level [18,19]. More interestingly, the oscillations in the properties of Pb persist for large film thicknesses, and quantized electronic states have even been observed at film thicknesses up to 45 ML [14], a much larger thickness [5][6][7][8][9][10][11][12][13][14][15][16][17] than observed for other films [2][3][4]. These strong oscillations in the properties of thick Pb films are very puzzling since, according to theory, the quantum size effect should quickly become damped with increasing thickness [18].…”

Quantum-well states with image state character for Pb overlayers on Cu(111)

“…The TR-2PPE technique has been used for supported Pb films, [36][37][38][39] and STS experiments have been performed on large well characterized Pb adislands. 14,35,[40][41][42][43][44] Irrespective of the overlayer thickness, a Fermi liquid type parabolic dependence of the decay rate on the energy of the QWSs has been found with absolute values close to those obtained in ab initio and model calculations 35,45,46 for bulk Pb.…”