Rotationally Anisotropic Second-Harmonic Generation Studies of the Structure and Thermal Stability of Cu(110)

Abstract

The microscopic structure and thermal stability of interfaces are of fundamental importance in determining a variety of materials properties. Order-disorder transitions on surfaces have recently received considerable attention in the scientific literature since the dynamics of such processes can impose fundamental limits on material performance at elevated temperatures. Many high-index (11n) faces (where n>2) of single-crystal metal surfaces are known to undergo this type of phasetransition. By definition, roughening of an atomically clean surface consists of the proliferation of atomic steps by thermal means at a roughening temperature, tr, where the surface free energy for creation of a step becomes zero. An unresolved question of fundamental importance is whether the roughening temperature of a low-index (110) surface can be lower than the bulk crystal melting temperature. For high index faces, a less stringent definition of roughening involving the proliferation of kinks on the already present step rows that can meander randomly has been suggested. The energy required for creation of a kink atom is lower than that required for generation of a step atom, thus roughening at temperatures lower than the bulk melting temperature can occur for stepped surfaces as opposed to low-index surfaces. Indeed, it has been shown shown both theoretically and experimentally that high-index faces of Cu, Ni, and other metals undergo roughening transitions well below the bulk melting temperatures.1