Depth-dependent ordering, two-length-scale phenomena and crossover behavior in a crystal featuring a skin-layer with defects

TL;DR

This paper investigates how surface defects influence the ordering and critical behavior in crystals, revealing depth-dependent crossover phenomena and proposing a dislocation-based theoretical explanation.

Contribution

It introduces a theory linking edge dislocations to two-length-scale phenomena and crossover behavior in defected crystal surfaces.

Findings

Depth-dependent transition and crossover temperatures decay with depth.

Crossover from non-mean-field to mean-field critical behavior occurs near the surface.

Near-surface regions exhibit cylindrical growth and weak first-order transitions.

Abstract

Structural defects in a crystal are responsible for the "two length-scale" behavior, in which a sharp central peak is superimposed over a broad peak in critical diffuse X-ray scattering. We have previously measured the scaling behavior of the central peak by scattering from a near-surface region of a V2H crystal, which has a first-order transition in the bulk. As the temperature is lowered toward the critical temperature, a crossover in critical behavior is seen, with the temperature range nearest to the critical point being characterized by mean field exponents. Near the transition, a small two-phase coexistence region is observed. The values of transition and crossover temperatures decay with depth. An explanation of these experimental results is here proposed by means of a theory in which edge dislocations in the near-surface region occur in walls oriented in the two directions normal to the surface. The strain caused by the dislocation lines causes the ordering in the crystal to occur as growth of roughly cylindrically shaped regions. After the regions have reached a certain size, the crossover in the critical behavior occurs, and mean field behavior prevails. At a still lower temperature, the rest of the material between the cylindrical regions orders via a weak first-order transition.