Critical behavior at the interface between two systems belonging to different universality classes

TL;DR

This paper analyzes the critical behavior at interfaces between two different universality classes using mean-field theory, phenomenological scaling, and numerical simulations, revealing various interface critical phenomena including special transitions.

Contribution

It provides a comprehensive analytical and numerical study of interface critical behavior between different universality classes, introducing the concept of special interface transitions with new critical exponents.

Findings

Weak interface couplings do not alter critical behavior.

Strong interface couplings keep the interface ordered at the transition.

Special interface transitions involve new critical exponents expressed in terms of bulk and surface exponents.

Abstract

We consider the critical behavior at an interface which separates two semi-infinite subsystems belonging to different universality classes, thus having different set of critical exponents, but having a common transition temperature. We solve this problem analytically in the frame of mean-field theory, which is then generalized using phenomenological scaling considerations. A large variety of interface critical behavior is obtained which is checked numerically on the example of two-dimensional q-state Potts models with q=2 to 4. Weak interface couplings are generally irrelevant, resulting in the same critical behavior at the interface as for a free surface. With strong interface couplings, the interface remains ordered at the bulk transition temperature. More interesting is the intermediate situation, the special interface transition, when the critical behavior at the interface involves new critical exponents, which however can be expressed in terms of the bulk and surface exponents of the two subsystems. We discuss also the smooth or discontinuous nature of the order parameter profile.