Van der Waals torque and force between anisotropic topological insulator slabs

TL;DR

This paper studies the van der Waals torque and force between anisotropic topological insulator slabs, revealing how anisotropy and orientation influence the force's sign and magnitude in the non-retardation regime.

Contribution

It is the first to analyze the vdW interaction involving vector potential effects in the non-retarded limit for topological insulators with anisotropy.

Findings

Anisotropy can change the vdW force from repulsive to attractive.

The vdW force oscillates with the angular difference of optic axes.

vdW torque in TIs is weaker than in ordinary dielectrics.

Abstract

We investigate the character of the van der Waals (vdW) torque and force between two coplanar and dielectrically anisotropic topological insulator (TI) slabs separated by a vacuum gap in the non-retardation regime, where the optic axes of the slabs are each perpendicular to the normal direction to the slab-gap interface and also generally differently oriented from each other. We find that in addition to the magnetoelectric coupling strength, the anisotropy can also influence the sign of the vdW force, viz., a repulsive vdW force can become attractive if the anistropy is increased sufficiently. In addition, the vdW force oscillates as a function of the angular difference between the optic axes of the TI slabs, being most repulsive/least attractive (least repulsive/most attractive) for angular differences that are integer (half-integer) multiples of $\pi$. Our third finding is that the vdW torque for TI slabs is generally weaker than that for ordinary dielectric slabs. Our work provides the first instance in which the vector potential appears in a calculation of the vdW interaction for which the limit is non-retarded or static.