Quantum phases in frustrated triangular and kagome optical lattices at negative absolute temperatures*

We report on recent experiments investigating the phase diagram of bosons in the triangular and kagome optical lattices. We explore the frustrated physics of these lattices by creating negative absolute temperature states where the sign of the tunnelling is effectively inverted. Notably, in this scheme the Hamiltonian remains static and there is no Floquet heating, leading to long lifetimes. In the triangular lattice, we observe that in the frustrated case the critical interaction strength for the bosonic superfluid to Mott insulator transition is strongly reduced. We also investigate signatures of the chiral nature of the superfluid and the dynamics of the transition. In the kagome lattice, we have prepared a long-lived state of ultracold bosons in the flat band. We study the melting of the Mott insulator into the flat band and report on our latest measurements characterising the flat-band state with a variety of time-of-flight techniques.