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Route toward classical frustration and band flattening via optical lattice distortion

Pil Saugmann1, José Vargas2,3, Yann Kiefer2,3, Max Hachman2,3, Raphael Eichberger2,3, Andreas Hemmerich2,3,4, and Jonas Larson1

  • 1Department of Physics, Stockholm University, SE-106 91 Stockholm, Sweden
  • 2Institut für Laserphysik, Universität Hamburg, D-22761 Hamburg, Germany
  • 3Zentrum für Optische Quantentechnologien and Institut für Laser-Physik, Universität Hamburg, D-22761 Hamburg, Germany
  • 4The Hamburg Center for Ultrafast Imaging, Luruper Chaussee 149, D-22761 Hamburg, Germany

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Phys. Rev. A 106, L041302 – Published 27 October, 2022

DOI: https://doi.org/10.1103/PhysRevA.106.L041302

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 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.

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Phys. Rev. A 106, L041302– Published 27 October, 2022
  • Received 17 March 2022
  • Accepted 7 October 2022
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