Negative-mass effects in spin-orbit coupled Bose-Einstein condensates

TL;DR

This paper investigates negative effective masses in spin-orbit coupled Bose-Einstein condensates, revealing how negative masses influence wave packet dynamics and lead to novel self-interference phenomena.

Contribution

It clarifies the role of individual effective mass parameters in observed fringes and predicts new counter-propagating self-interfering states when both masses are negative.

Findings

Fringes are due to negativity of one effective mass parameter.

Negative mass can halt condensate expansion and produce density fringes.

Both mass parameters being negative lead to counter-propagating self-interfering packets.

Abstract

Negative effective masses can be realised by engineering the dispersion relation in a variety of quantum systems. A recent experiment with spin-orbit coupled Bose-Einstein condensates has shown that a negative effective mass can halt the free expansion of the condensate and produce fringes in the density [M. Khamehchi et al., Phys. Rev. Lett. 118, 155301 (2017)]. Here, we show that the observed fringes are due to the negativity of only one of the two effective mass parameters that characterise the dispersion, which leads to previously predicted self-interference of the wave packet. We show how confgurations are nevertheless accessible to spin-orbit coupled Bose-Einstein condensates where both mass parameters controlling the propagation and diffusion of the condensate are negative, resulting in the novel phenomenon of counter-propagating self-interfering packets.