Observation of a first-order structural phase transition in a monolayer of Pd on Nb(110) and its relationship to electronic structure

It is shown that a commensurate 1 x 1 monolayer of Pd on a Nb(110) surface undergoes a first-order structural phase transition to an incommensurate monolayer Pd(111) structure when roughly-equal18% more Pd atoms are added to the surface. We find, however, that this phase transition has no substantial effect on the surface electronic structure. A steady shift toward the Fermi level of the local density of states at the surface is found as the Pd coverage is increased beyond about one monolayer. This shift correlates quite well with a previously observed increase in the hydrogen uptake rate