Observing the Onset of Effective Mass

TL;DR

This paper experimentally demonstrates that particles initially respond with their bare mass in a periodic potential, with effective mass behavior emerging only after rapid oscillations over longer timescales.

Contribution

The study provides direct experimental evidence showing the transition from bare mass response to effective mass behavior in a Bose-Einstein condensate within an optical lattice.

Findings

Initial response characterized by bare mass

Observation of rapid oscillations in particle response

Effective mass description valid over longer timescales

Abstract

The response of a particle in a periodic potential to an applied force is commonly described by an effective mass which accounts for the detailed interaction between the particle and the surrounding potential. Using a Bose-Einstein condensate of 87-Rb atoms initially in the ground band of an optical lattice, we experimentally show that the initial response of a particle to an applied force is in fact characterized by the bare mass. Subsequently, the particle response undergoes rapid oscillations and only over timescales long compared to that of the interband dynamics is the effective mass observed to be an appropriate description.