Superconducting density of states at the border of an amorphous thin film grown by focused-ion- beam

We present very low temperature Scanning Tunneling Microscopy and Spectroscopy (STM/S) measurements of a W based amorphous thin film grown with focused-ionbeam. In particular, we address the superconducting properties close to the border, where the thickness of the superconducting film decreases, and the Au substrate emerges. When approaching the Au substrate, the superconducting tunneling conductance strongly increases around the Fermi level, and the quasiparticle peaks do not significantly change its position. Under magnetic fields, the vortex lattice is observed, with vortices positioned very close to the Au substrate. In a recent experiment we have shown that W based thin films grown using a precusor gas and a focused ion beam on top of a conducting Au substrate are excellent superconducting systems for making STM/S down to atomic level and at very low temperatures[1]. The superconducting properties are very homogeneous, and the vortex lattice can be observed with beautiful detail. It is of interest to understand the behavior of the superconductor (S) close to its border, where the height of the thin film continuously decreases, and the normal substrate (N) appears. This gives a curious N-S hybrid system, where the superconducting properties must gradually disappear due to the proximity of the N metal, and where it is possible to follow the positions of the vortices close to the border of the thin film, when the magnetic field is applied perpendicular to the thin film. Note that the situation is exactly the inverse as in Refs.[2, 3, 4], where an Au layer was deposited on top of a superconductor, and the local density of states (LDOS) on the N layer measured as a function of its thickness. In those cases, the vortex lattice was not studied. The thin films are grown as described elsewhere, have a concentration of W=40±7%, C=43±4%, Ga=10±3% and O=7±2%, a thickness of 200 nm, and Tc = 4.15 K [1, 5, 6]. Measurements are made in a STM/S system in a dilution refrigerator, which cools sample and tip down to about 100 mK[1, 7, 8]. We position the tip at the border of the W deposit using an in-situ displacement mechanism. In Fig.1 we schematically describe our experiment. We focus on the border of the thin film, where topography is smooth, as well within the film [1], and there is a continuous height decrease towards Au that disappears when reaching the Au surface, 25th International Conference on Low Temperature Physics (LT25) IOP Publishing Journal of Physics: Conference Series 150 (2009) 052064 doi:10.1088/1742-6596/150/5/052064 c © 2009 IOP Publishing Ltd 1