Surface oxide effects on tunneling spectroscopy measured with a cryogenic scanning tunneling microscope

Abstract

We describe a simple, versatile cryogenic (∼4.2 K) scanning tunneling microscope (CSTM) system optimized for spectroscopic studies. We find that spectroscopic data obtained at low temperatures are a function of the local surface characteristics of both the tip probe and sample. The superconducting density of states of lead is masked for tips with a heavy surface oxide (etched tungsten, stainless steel), yet easily observed with nonoxidized tips (gold, platinum). We show direct evidence that small metallic inclusions embedded in some oxides can produce single electron capacitive charging effects that dominate current-voltage characteristics.