Spin models in three dimensions: Adaptive lattice spacing

TL;DR

This paper explores implementing adaptive lattice spacing in three-dimensional spin models, specifically the Ising model, to study boundary effects and critical phenomena, addressing the challenges of parameter transformations and boundary coupling.

Contribution

It introduces a method for coupling sectors with different lattice spacings in 3D Ising models and demonstrates how to adjust boundary couplings to reduce corrections.

Findings

Boundary coupling tuning reduces corrections

Magnetization profiles match homogeneous results

Thermodynamic Casimir force is accurately determined

Abstract

Aiming at the study of critical phenomena in the presence of boundaries with a non-trivial shape we discuss how lattices with an adaptive lattice spacing can be implemented. Since the parameters of the Hamiltonian transform non-trivially under changes of the length-scale, adapting the lattice spacing is much more difficult than in the case of the numerical solution of partial differential equations, where this method is common practice. Here we shall focus on the universality class of the three-dimensional Ising model. Our starting point is the improved Blume-Capel model on the simple cubic lattice. In our approach, the system is composed of sectors with lattice spacing a, 2 a, 4 a, ... . We work out how parts of the lattice with lattice spacing a and 2 a, respectively, can be coupled in a consistent way. Here, we restrict ourself to the case, where the boundary between the sectors is perpendicular to one of the lattice-axis. Based on the theory of defect planes one expects that it is sufficient to tune the coupling between these two regions. To this end we perform a finite size scaling study. However first numerical results show that slowly decaying corrections remain. It turns out that these corrections can be removed by adjusting the strength of the couplings within the boundary layers. As benchmark, we simulate films with strongly symmetry breaking boundary conditions. We determine the magnetization profile and the thermodynamic Casimir force. For our largest thickness L_0=64.5, we find that results obtained for the homogeneous system are nicely reproduced.