ON THE TRANSITION FROM TUNNELING REGIME TO POINT-CONTACT - GRAPHITE

In this paper we present a model of the transition from tunneling regime to point-contact that shows how density-of-states features may appear in current-voltage characteristics between these two limits. This transition is gradual and there is no clear-cut point separating both situations. We also give a semiquantitative account of the variation of conductance with tip and sample distance in STS (scanning tunneling spectroscopy performed with a STM) and apply this model to the interpretation of spectroscopic results obtained in graphite, We also show that it is possible to have atomic resolution with very low tunneling resistances (approximately 1 k-OMEGA) and high currents (approximately 10-mu-A). Under these conditions the forces between tip and sample appear clearly in the topographic images as distortions. These forces vary qualitatively with temperature in the range 4-300 K.