Aspect-ratio dependence of thermodynamic Casimir forces

TL;DR

This study investigates how the aspect ratio of a three-dimensional Ising model affects thermodynamic Casimir forces, using Monte Carlo simulations and exact calculations, revealing conditions for force vanishing or repulsion.

Contribution

It provides a comprehensive numerical analysis of aspect-ratio dependence of Casimir forces in 3D Ising models and compares results with 2D exact solutions and field theory.

Findings

Casimir force vanishes at criticality for aspect ratio 1

Force becomes repulsive for aspect ratio greater than 1

Good agreement with field theoretical results above and below T_c

Abstract

We consider the three-dimensional Ising model in a $L_\perp \times L_\parallel \times L_\parallel$ cuboid geometry with finite aspect ratio $\rho = L_\perp/L_\parallel$ and periodic boundary conditions along all directions. For this model the finite-size scaling functions of the excess free energy and thermodynamic Casimir force are evaluated numerically by means of Monte Carlo simulations. The Monte Carlo results compare well with recent field theoretical results for the Ising universality class at temperatures above and slightly below the bulk critical temperature $T_\mathrm{c}$. Furthermore, the excess free energy and Casimir force scaling functions of the two-dimensional Ising model are calculated exactly for arbitrary $\rho$ and compared to the three-dimensional case. We give a general argument that the Casimir force vanishes at the critical point for $\rho = 1$ and becomes repulsive in periodic systems for $\rho > 1$.