Direct observation of electron standing waves with superconducting Nb tips on Au(111) using low temperature scanning tunnelling microscopy

Using a low temperature scanning tunnelling microscope with normal metal tungsten tips and superconducting niobium tips, we have observed in real space the formation of electron standing waves by the scattering of surface state electrons at surface defects on Au(I 11) surfaces. In the constant-current STM images of Au(I 11) observed at low temperature with both tips, standing waves are clearly seen emanating from step edges and point defects. From the observed structure of the standing waves, the wavenumber of the surface electrons can be obtained as a function of the electron energy. In comparison with the cross-sectional profiles (height versus distance) measured across the standing waves, it can be seen that the amplitude of the standing waves obtained with the niobium tip at 4.2 K is about three times larger than that with the tungsten tip. We have also proposed a simple model to explain these observed effects.