Transverse force on a vortex in lattice models of superfluids

TL;DR

This paper develops a continuous approximation theory for lattice superfluids to analyze transverse forces on vortices, revealing a new expression for the Magnus force and conditions for its sign reversal.

Contribution

It introduces a novel general expression for the Magnus force in lattice superfluids and explores its behavior near the superfluid-insulator transition.

Findings

The Lorentz force remains consistent with previous knowledge.

A new expression for the Magnus force was derived.

The Magnus force can reverse sign depending on phase diagram conditions.

Abstract

The paper derives the transverse forces (the Magnus and the Lorentz forces) in the lattice models of superfluids in the continuous approximation. The continuous approximation restores translational invariance absent in the original lattice model, but the theory is not Galilean invariant. As a result, calculation of the two transverse forces on the vortex, Magnus force and Lorentz force, requires the analysis of two balances, for the true momentum of particles in the lattice (Magnus force) and for the quasimomentum (Lorentz force) known from the Bloch theory of particles in the periodic potential. While the developed theory yields the same Lorentz force, which was well known before, a new general expression for the Magnus force was obtained. The theory demonstrates how a small Magnus force emerges in the Josephson-junction array if the particle-hole symmetry is broken. The continuous approximation for the Bose--Hubbard model close to the superfluid-insulator transition was developed, which was used for calculation of the Magnus force. The theory shows that there is an area in the phase diagram for the Bose--Hubbard model, where the Magnus force has an inverse sign with respect to that which is expected from the sign of velocity circulation.