Spin-orbit coupled ultracold gases in optical lattices: High-band physics and insufficiency of tight-binding models

TL;DR

This paper investigates the effects of spin-orbit coupling in ultracold Fermi gases within optical lattices, emphasizing the importance of high-band physics and demonstrating the limitations of tight-binding models in predicting superfluid-insulator transitions.

Contribution

The study provides a full-spectrum analysis of spin-orbit coupled ultracold gases, revealing the insufficiency of tight-binding models and uncovering phenomena like band-gap closing and superfluidity reentrance.

Findings

High-band contributions are crucial for accurate single-particle physics.

Raman-induced SOC can close band gaps in 2D optical lattices.

Superfluidity reentrance occurs at integer filling due to band-gap closing.

Abstract

We study the interplay effect of spin-orbit coupling(SOC) and optical lattice to the single-particle physics and superfluid-insulator transition in ultracold Fermi gases. We consider the type of SOC that has been realized in cold atoms experiments via two-photon Raman processes. Our analyses are based on the knowledge of full single-particle spectrum in lattices, without relying on any tightbinding approximation.We evaluate existing tight-binding models and point out their limitations in predicting the correct single-particle physics due to the missed high-band contributions. Moreover, we show that the Raman field (creating SOC) can induce band-gap closing in a two-dimensional optical lattice, leading to the intriguing phenomenon of superfluidity-reentrance for interacting fermions at integer filling. We present the superfluid-insulator phase diagram in a wide parameter regime of chemical potentials and Raman fields. All these results are far beyond any tight-binding model can predict, and can be directly probed in current cold atoms experiments.