Rumpled relaxation of TiC(001) and TaC(001) determined by high-resolution medium-energy ion scattering spectroscopy

The rumpled relaxations of TiC(001) and TaC(001) with strong covalent bonds were determined precisely by medium energy ion scattering using a toroidal electrostatic analyzer with an excellent energy resolution of Delta E/E= 9 X 10(-4). It allows a monolayer depth resolution and thus a simple triangu lation method using the ion shadowing effect achieved the accuracy of 0.01 Angstrom in depth for the displacement of atoms near a surface region. The surface relaxation (+: outwards) and rumpling [+: displacement of C toward vacuum side larger than that of Ti(Ta)] were estimated to be +0.1+/-0.3% an d +3.5+/-0.5% for TiC(001) and +1.4+/-0.3% and +4.9+/-0.5% for TaC(001). Th e present result is consistent with the recently reported calculation using the full-potential linear muffin-tin orbital method but inconsistent with the predictions using the tight-binding model and the full-potential linear ized augmented plane-wave method.