If a current of electrons flows through a normal conductor (in contrast to a superconductor), it is impeded by local scattering at defects as well as phonon scattering. Both effects contribute to the voltage drop observed for a macroscopic complex system as described by Ohm's law. Although this concept is well established, it has not yet been measured around individual defects on the atomic scale. We have measured the voltage drop at a monatomic step in real space by restricting the current to a surface layer. For the Si(111)-( [see text]3 x [see text]3)-Ag surface a monotonous transition with a width below 1 nm was found. A numerical analysis of the data maps the current flow through the complex network and the interplay between defect-free terraces and monatomic steps.

We have developed a new scanning tunneling potentiometry technique which can-with only minor changes of the electronic setup-be easily added to any standard scanning tunneling microscope (STM). This extension can be combined with common STM techniques such as constant current imaging or scanning tunneling spectroscopy. It is capable of performing measurements of the electrochemical potential with microvolt resolution. Two examples demonstrate the versatile application. First of all, we have determined local variations of the electrochemical potential due to charge transport of biased samples down to angstrom length scales. Second, with tip and sample at different temperatures we investigated the locally varying thermovoltage occurring at the tunneling junction. Aside from its use in determining the chemical identity of substances at the sample surface our method provides a controlled way to eliminate the influence of laterally varying thermovoltages on low-bias constant current topographies.

We present scanning tunneling microscopy data on clean Ag͑111͒ surfaces in UHV. Standing-wave patterns of the surface state are investigated to study the influence of a thermal step across the tunnel junction on constant current topographies. When constant current topographies are measured at bias voltages below 10 mV with temperature differences of more than a few kelvins between tip and sample, the apparent corrugation is drastically increased. Constant current topographies at low bias voltages and finite temperature differences do not reveal the true local density of states of the surface. The apparent corrugation is caused by a spatial varying thermovoltage superimposed to the bias voltage while scanning the sample's local density of states. With the help of scanning tunneling potentiometry, the correct local density of states of the sample can be reconstructed.

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