SURFACE SEGREGATION IN CU3AU(001)

In this work we report X-ray scattering studies of the Cu3Au(001) surf ace for temperatures above the bulk order-disorder transition temperat ure T-0=390 degrees C. The temperature dependence of the asymptotic Br agg tails normal to the surface gives direct evidence for an oscillato ry, exponentially damped Au segregation profile. It turns out that the depth range (lambda) of this surface segregation phenomenon is govern ed 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-s p 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 structur e and the lateral coherence of the surface segregation profiles is fou nd. First experimental results from experiments performed at the ESRF are presented which disclose surface segregation-related diffraction f eatures in higher Brillouin zones normal to the surface. The sensitivi ty of this X-ray probe to details of the surface segregation profile i s discussed and the experimental limits are indicated.