Route toward classical frustration and band flattening via optical lattice distortion

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 that 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 tunneling 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 show a complete loss of coherence. Upon slowly tuning back to the condensate regime, coherence is largely restored. Good agreement with model calculations is obtained.

Automated summary

In this study, we propose and experimentally explore a method for realizing frustrated lattice models using a Bose-Einstein condensate held in an optical square lattice. By introducing a small lattice distortion, an energy gap is opened and the lowest band splits into two. A nearly flat energy-momentum dispersion is achieved along the edge of the first Brillouin zone for both bands. Loading ultracold atoms into the excited band leads to the formation of a classically frustrated XY model. The experimental results show a complete loss of coherence in the momentum spectra when adiabatically tuning from the condensate regime to the regime of frustration, and the coherence is largely restored upon tuning back to the condensate regime. The agreement with model calculations is good.