Conformal field theory of critical Casimir forces between surfaces with alternating boundary conditions in two dimensions

TL;DR

This paper uses conformal field theory to analyze critical Casimir forces between surfaces with alternating boundary conditions in two dimensions, deriving exact results and exploring force behaviors and equilibria.

Contribution

It introduces a novel modified Szeg"o formula for exact calculation of Casimir forces with alternating boundary conditions in 2D systems.

Findings

Exact Casimir force expressions at all distances

Existence of a stable equilibrium position for the force

Universal cosine form of lateral Casimir forces at large separations

Abstract

Systems as diverse as binary mixtures and inclusions in biological membranes, and many more, can be described effectively by interacting spins. When the critical fluctuations in these systems are constrained by boundary conditions, critical Casimir forces (CCF) emerge. Here we analyze CCF between boundaries with alternating boundary conditions in two dimensions, employing conformal field theory (CFT). After presenting the concept of boundary changing operators, we specifically consider two different boundary configurations for a strip of critical Ising spins: (I) alternating equi-sized domains of up and down spins on both sides of the strip, with a possible lateral shift, and (II) alternating domains of up and down spins of different size on one side and homogeneously fixed spins on the other side of the strip. Asymptotic results for the CCF at small and large distances are derived. We introduce a novel modified Szeg\"o formula for determinants of real antisymmetric block Toeplitz matrices to obtain the exact CCF and the corresponding scaling functions at all distances. We demonstrate the existence of a surface Renormalization Group flow between universal force amplitudes of different magnitude and sign. The Casimir force can vanish at a stable equilibrium position that can be controlled by parameters of the boundary conditions. Lateral Casimir forces assume a universal simple cosine form at large separations.