Surface segregation in Cu3Au(001)

In this work we report X-ray scattering studies of the Cu3Au(001) surface for temperatures above the bulk order-disorder transition temperature T-0=390 degrees C. The temperature dependence of the asymptotic Bragg tails normal to the surface gives direct evidence for an oscillatory, exponentially damped Au segregation profile. It turns out that the depth range (lambda) of this surface segregation phenomenon is governed by the bulk correlation length xi(b) and, thus, depends sensitively upon the temperature, in particular, we find that lambda proportional to(T-T-sp)(-v) upon approaching the ordering spinodal temperature T-sp with v = 0.49. The inplane line shape of the asymptotic Bragg rails exhibits a broadening which changes characteristically with the normal momentum transfer L An intimate coupling between the terrace structure and the lateral coherence of the surface segregation profiles is found. First experimental results from experiments performed at the ESRF are presented which disclose surface segregation-related diffraction features in higher Brillouin zones normal to the surface. The sensitivity of this X-ray probe to details of the surface segregation profile is discussed and the experimental limits are indicated.