Cooling and Near-equilibrium Dynamics of Atomic Gases Across the Superfluid-Mott Insulator Transition

TL;DR

This paper investigates the near-equilibrium thermodynamics of bosonic atoms in a 2D optical lattice during the superfluid to Mott insulator transition, revealing slow thermalization and novel cooling mechanisms.

Contribution

It provides detailed experimental insights into local and global thermalization timescales and demonstrates potential cooling effects during the transition, with new measurements supporting the findings.

Findings

Global thermalization exceeds one second.

Localized cooling can occur during non-adiabatic ramps.

Final global temperatures as low as 9 nK were achieved.

Abstract

We study near-equilibrium thermodynamics of bosonic atoms in a two-dimensional optical lattice by ramping up the lattice depth to convert a superfluid into an inhomogeneous mixture of superfluid and Mott insulator. Detailed study of in situ density profiles shows that, first, locally adiabatic ramps do not guarantee global thermal equilibrium. Indeed, full thermalization for typical parameters only occurs for experiment times which exceed one second. Secondly, ramping non-adiabatically to the Mott insulator regime can result in strong localized cooling at short times and global cooling once equilibrated. For an initial temperature estimated as 20 nK, we observe local temperatures as low as 1.5 nK, and a final global temperature of 9 nK. Possible cooling mechanisms include adiabatic decompression, modification of the density of states near the quantum critical regime, and the Joule-Thomson effect. **NOTE: Following submission of arXiv:0910.1382v1, a systematic correction was discovered in the density measurement, stemming from three-body losses during the imaging process. New measurements were performed, and the result is in support of the claim on the slow global dynamics. Due to the substantially altered methods and analysis, a new text has been posted as arXiv:1003.0855.