Luca Donini (University of Cambridge)
Frustrated physics in triangular and kagome optical lattices at negative absolute temperature
Ultracold atoms in optical lattices are one of the major platforms for experimental quantum simulations and many-body physics. They simulate solid-state physics as they naturally realise the (fermionic or bosonic) Hubbard model. In the past, most experiments have focused on the simplest lattice geometries, namely the primitive 1D lattice, the square lattice, and the cubic lattice. Going beyond these geometries, the triangular and kagome lattices are of particular interest as they host geometric frustration. This manifests itself in degeneracy at the upper band edge, which in the kagome lattice takes the form of a flat band.
I will present our recent experiments with ultracold bosonic 39K in these two lattices. We access the frustrated physics at the upper band edge by creating states with negative absolute temperature, where the particles preferentially occupy the highest-lying energy states. The paradigmatic feature of the Bose-Hubbard model in conventional lattices is a phase transition between a long-range ordered superfluid (SF) state at weak interactions and a Mott insulator (MI) with short-range correlations at strong interactions. Frustration enriches this picture: in the triangular lattice, the ordered SF state is expected to be chiral, such that an additional Z2 symmetry is broken. We observe how frustration suppresses the critical interaction strength for the χSF-MI transition and changes its dynamics [1]. In the kagome lattice, the situation could potentially be very different, with various exotic phases predicted in different regimes. Here, we prepare for the first time a long-lived state that occupies predominantly the flat band. It presents non-trivial structure in momentum space, so we use a variety of time-of-flight techniques to characterise it.
[1] M. Hasan, L. Donini, S. Shanokprasith, D. Braund, T. Marozsak, M. Epping, D. Reed, M. Melchner, T. Harte, U. Schneider. Superfluid-Mott transition in a frustrated triangular optical lattice. arXiv:2509.20352