The formation of a novel surface reconstruction upon Co deposition on freshly cleaved Ge(111)2x1 surfaces is studied by means of scanning tunneling microscopy (STM) at low temperature. The deposited Co atoms are immobile at substrate temperatures of
4.5K, while they can diffuse along the upper pi-bonded chains at a temperature of 80K and higher. This mobility results in accumulation of Co atoms at atomic steps, at domain boundaries as well as on atomically flat Ge terraces at, e.g., vacancies or adatoms, where reconstructed Co/Ge intermixing layers are formed. Voltage dependent STM images reveal that the newly reconstructed surface locally exhibits a highly ordered atomic structure, having the same periodicity as that of the initial 2x1 reconstruction. In addition, it shows a double periodicity along the [2-1-1] direction, which can be related to the modified electronic properties of the pi-bonded chains.
We report on the experimental observation by scanning tunneling microscopy at low temperature of ring-like features that appear around Co metal clusters deposited on a clean (110) oriented surface of cleaved p-type InAs crystals. These features are v
isible in spectroscopic images within a certain range of negative tunneling bias voltages due to the presence of a negative differential conductance in the current-voltage dependence. A theoretical model is introduced, which takes into account non-equilibrium effects in the small tunneling junction area. In the framework of this model the appearance of the ring-like features is explained in terms of interference effects between electrons tunneling directly and indirectly (via a Co island) between the tip and the InAs surface.
