SCANNING TUNNELING SPECTROSCOPY OF RARE-EARTH-METALS - GADOLINIUM ON YTTRIUM

Scanning tunneling microscopy images of ultrathin Gd films on Y(0001) show a strong chemical contrast depending on the tunneling voltage. Fo r small voltages the tunneling spectra for Gd reveal pronounced featur es which are attributed to occupied and unoccupied electronic surface states as found previously in photoemission experiments. These Gd surf ace states give rise to a large tunneling current and the image contra st between Gd and Y is enhanced by a factor of two because low-energy surface states are absent in Y. On the other hand, for a high positive bias of +2 V the contrast is reversed due to tunneling into unoccupie d Y electronic states presumably originating from the Y 5d band. The c orresponding current increase with respect to Gd exceeds the current i ncrease induced by a monolayer step.