Surface critical behavior of bcc binary alloys

TL;DR

This study investigates the surface critical behavior of bcc binary alloys near a continuous order-disorder transition using mean-field theory, revealing how surface symmetry influences the type of critical behavior observed.

Contribution

It provides a theoretical analysis showing that symmetry-breaking surfaces exhibit NORMAL transition behavior, while symmetry-preserving surfaces show ORDINARY behavior, via a Ginzburg-Landau model.

Findings

Symmetry-breaking surfaces display NORMAL critical behavior.

Symmetry-preserving surfaces exhibit ORDINARY critical behavior.

An effective surface field explains the critical behavior differences.

Abstract

The surface critical behavior of bcc binary alloys undergoing a continuous B2-A2 order-disorder transition is investigated in the mean-field (MF) approximation. Our main aim is to provide clear evidence for the fact that surfaces which break the two-sublattice symmetry generically display the critical behavior of the NORMAL transition, whereas symmetry-preserving surfaces exhibit ORDINARY surface critical behavior. To this end we analyze the lattice MF equations for both types of surfaces in terms of nonlinear symplectic maps and derive a Ginzburg-Landau model for the symmetry-breaking (100) surface. The crucial feature of the continuum model is the emergence of an EFFECTIVE ORDERING (``staggered'') SURFACE FIELD, which depends on temperature and the other lattice model parameters, and which explains the appearance of NORMAL critical behavior for symmetry-breaking surfaces.