Transport of strong-coupling polarons in optical lattices

TL;DR

This paper investigates how ultracold impurity atoms in a Bose-Einstein condensate, trapped in an optical lattice, exhibit a transition from coherent to incoherent transport due to phonon interactions, leading to novel current behaviors.

Contribution

The study introduces an extended Hubbard model for polarons in optical lattices and analyzes impurity transport, revealing temperature-dependent crossover and negative differential conductance.

Findings

Crossover from coherent to incoherent impurity transport with increasing temperature

Emergence of a net atomic current across a tilted lattice

Transition from ohmic to negative differential conductance

Abstract

We study the transport of ultracold impurity atoms immersed in a Bose-Einstein condensate (BEC) and trapped in a tight optical lattice. Within the strong-coupling regime, we derive an extended Hubbard model describing the dynamics of the impurities in terms of polarons, i.e. impurities dressed by a coherent state of Bogoliubov phonons. Using a generalized master equation based on this microscopic model we show that inelastic and dissipative phonon scattering results in (i) a crossover from coherent to incoherent transport of impurities with increasing BEC temperature and (ii) the emergence of a net atomic current across a tilted optical lattice. The dependence of the atomic current on the lattice tilt changes from ohmic conductance to negative differential conductance within an experimentally accessible parameter regime. This transition is accurately described by an Esaki-Tsu-type relation with the effective relaxation time of the impurities as a temperature-dependent parameter.