Route towards classical frustration and band flattening via optical lattice distortion

TL;DR

This paper demonstrates how optical lattice distortion can create frustrated lattice models with flat bands and degeneracies, enabling the study of classical frustration and band flattening using ultracold atoms in a Bose-Einstein condensate.

Contribution

It introduces a method to realize frustrated lattice models via optical lattice distortion and experimentally explores the transition between coherent condensate and frustrated regimes.

Findings

Nearly flat energy-momentum dispersion achieved at Brillouin zone edges

Observation of coherence loss during frustration regime

Restoration of coherence upon returning to condensate regime

Abstract

We propose and experimentally explore a method for realizing frustrated lattice models using a Bose-Einstein condensate held in an optical square lattice. A small lattice distortion opens up an energy gap such the lowest band splits into two. Along the edge of the first Brillouin zone for both bands a nearly flat energy-momentum dispersion is realized. For the excited band a highly degenerate energy minimum arises. By loading ultracold atoms into the excited band, a classically frustrated $XY$ model is formed, describing rotors on a square lattice with competing nearest and next nearest tunnelling couplings. Our experimental optical lattice provides a regime, where a fully coherent Bose-Einstein condensate is observed, and a regime where frustration is expected. If we adiabatically tune from the condensate regime to the regime of frustration, the momentum spectra shows a complete loss of coherence. Upon slowly tuning back to the condensate regime, coherence is largely restored. Good agreement with model calculations is obtained.