Paper detail

Critical free energy and Casimir forces in rectangular geometries

We study the critical behavior of the free energy and the thermodynamic Casimir force in a $L_\parallel^{d-1} \times L$ block geometry in $2<d<4$ dimensions with aspect ratio $ρ=L/L_\parallel$ above, at, and below $T_c$ on the basis of the O$(n)$ symmetric $ϕ^4$ lattice model with periodic boundary conditions (b.c.). We consider a simple-cubic lattice with isotropic short-range interactions. Exact results are derived in the large - $n$ limit describing the geometric crossover from film ($ρ=0$) over cubic $ρ=1$ to cylindrical ($ρ= \infty$) geometries. For $n=1$, three perturbation approaches are presented that cover both the central finite-size regime near $T_c$ for $1/4 \lesssim ρ\lesssim 3$ and the region outside the central finite-size regime well above and below $T_c$ for arbitrary $ρ$. At bulk $T_c$ of isotropic systems with periodic b.c., we predict the critical Casimir force in the vertical $(L)$ direction to be negative (attractive) for a slab ($ρ< 1$), positive (repulsive) for a rod ($ρ> 1$), and zero for a cube $(ρ=1)$. We also present extrapolations to the cylinder limit ($ρ=\infty$) and to the film limit ($ρ=0$) for $n=1$ and $d=3$. Our analytic results for finite-size scaling functions in the minimal renormalization scheme at fixed dimension $d=3$ agree well with Monte Carlo data for the three-dimensional Ising model by Hasenbusch for $ρ=1$ and by Vasilyev et al. for $ρ=1/6$ above, at, and below $T_c$.