Discrete-to-Continuum Limits of Long-Range Electrical Interactions in Nanostructures

Why it matters

Designing a field-responsive material rests on an effective description of how charge at the atomic scale becomes polarization at the continuum scale. Rather than assume that description, this paper derives it for helical nanotubes and curved thin films, and finds the limit is local — but that, unlike continuum dimension reduction, both tangential and normal components of the dipole field survive into it. Knowing which fluctuations reach the continuum is what keeps a later design from being fitted to an artifact of the coarse-graining.

Abstract

We consider electrostatic interactions in two classes of nanostructures embedded in a three dimensional space: (1) helical nanotubes, and (2), thin films with uniform bending (i.e., constant mean curvature). Starting from the atomic scale with a discrete distribution of dipoles, we obtain the continuum limit of the electrostatic energy; the continuum energy depends on the geometric parameters that define the nanostructure, such as the pitch and twist of the helical nanotubes and the curvature of the thin film. We find that the limiting energy is local in nature. This can be rationalized by noticing that the decay of the dipole kernel is sufficiently fast when the lattice sums run over one and two dimensions, and is also consistent with prior work on dimension reduction of continuum micromagnetic bodies to the thin film limit. However, an interesting contrast between the discrete-to-continuum approach and the continuum dimension reduction approaches is that the limit energy in the latter depends only on the normal component of the dipole field, whereas in the discrete-to-continuum approach, both tangential and normal components of the dipole field contribute to the limit energy.