CPM Seminar

Ohm's law on the nanoscale

R. Möller¹, Ch. Bobisch², B. Weyers¹, A. Bannani¹, E. Zubkov¹, A. Bernhart¹, M. Kaspers¹,J. Homoth³, M. Wenderoth³, T. Druga³, L. Winking³, R.G. Ulbrich³

¹Department of Physics, University of Duisburg-Essen
²Department of Physics & Astronomy, University of California, Irvine
³Physikalisches Institut, University of Göttingen

By restraining an electric current to a surface state Ohm's law may be studied by means of scanning tunneling potentiometry in great detail. Here we show for the Si(111) sqrt(3) x sqrt(3) Ag surface how the electrochemical potential varies on the nanometer scale. At a step edge the local electrochemical potential exhibits a rather localized monotonous transition. The major variation occurs in a very short range of about 1.2 nm corresponding roughly to 1/4 Fermi wavelength. Our measurements cover the transition from the microscopic transport processes, i.e. quantum mechanical tunnelling at step edges and diffusive scattering in the two dimensional electron gas, to the macroscopic resistance. In spite of the underlying quantum mechanical processes the experimental data can be very well described by a network of Ohmic resistances. For perfectly flat surfaces with small islands on top the analysis provides specific conductivities for the plane surfaces and monoatomic steps. However, the comparison of the electronic transport parallel and perpendicular to the steps on vicinal surfaces reveals size effects as well as the angular dependence of the transmission at step edges. Our data explain how the results of previous experiments in which the electric transport was analyzed on a mesoscopic scale may be understood as the average over many elementary contributions.