Spectroscopic study of the tunnel-current reduction due to adsorbed helium

We present measurements of the tunnel current and dynamic conductance of mechanically controllable break junctions of Pt, Au, and Al at low temperatures in He-4 and He-3 gas environments. The study reveals that the previously observed bias dependence of a reduction of the tunnel current due to the presence of adsorbed He has a typical width of 15-20 meV at 4.2 K, and is centered around zero bias. The tunnel resistance value at which the relative reduction is maximal varies between 100 M Ohm and 1 G Ohm, and the maximum value seems to be both material and sample dependent. The latter may be connected to differences in the exact electrode-surface geometries.