ELECTRONIC-STRUCTURE OF TRANSITION-METAL TIPS AND TIP SURFACE INTERACTIONS

We studied the electronic structure of pyramidal supported tips and the interaction of such tips with perfect surfaces. We are interested in general trends, so that we use the simplicity of the tight-binding scheme and of the real-space recursion method. We consider perfect and truncated pyramidal tips of height h = nd0 (n = 1, 2.... ; d0 is the interplanar distance) deposited on a surface S of the same metal (bcc W, S = (100) and (111)). The apex "d" densities of states (DOS) are quite different from the corresponding DOS on finite clusters or on S; they are nearly independent of h for n greater-than-or-equal-to 4 and do not present strong peaks associated with surface states. We examine the interaction between such tips and perfect surfaces ot the same metal in the contact regime (distance d less-than-or-equal-to 3d0). The strong interaction between the sample surface states and the tip states leads to bonding and antibonding states. The corresponding decrease of the DOS in the middle of the "d" bands induces an increase of the interaction energies and forces. The energy distance curve is calculated when the tip is located on a tip or hollow site of the sample surface. Finally, we discuss in this regime the interaction energies and their variations with the distance and with the sample material.