Anisotropic diffusion in square lattice potentials: Giant enhancement and control

¹ Universität Bielefeld, Fakultät für Physik - 33615 Bielefeld, Germany, EU
² NORDITA - Roslagstullsbacken 23, 10691 Stockholm, Sweden, EU

Received: 8 December 2011
Accepted: 22 February 2012

Abstract

The unbiased thermal diffusion of an overdamped Brownian particle in a square lattice potential is considered in the presence of an externally applied ac driving. The resulting diffusion matrix exhibits two orthogonal eigenvectors with eigenvalues D₁> D₂>0, indicating anisotropic diffusion along a “fast” and a “slow principal axis”. For sufficiently small temperatures, D₁ may become arbitrarily large and at the same time D₂ arbitrarily small. The principal diffusion axis can be made to point into (almost) any direction by varying either the driving amplitude or the coupling of the particle to the potential, without changing any other property of the system or the driving.