A negative temperature state of bosons in an optical kagome lattice*

The kagome lattice exhibits a high degree of geometric frustration giving rise to a flat band. Since kinetic energy plays no role in a flat band, it is interaction and topology that determine the behaviour of the system. The many-body physics of atoms in this system is however not fully understood and the frustration makes simulations with classical computers challenging. We instead use neutral atoms in optical lattices as an analogue quantum simulator. Because the flat band is the highest in energy of the three touching lowest bands it will normally not be populated when ultracold atoms are loaded to the lattice. In our case this problem is solved by preparing atoms in a state with negative absolute temperature, for which the highest energies are preferentially occupied. I will present our work creating, for the first time, a negative temperature state of bosonic ³⁹K atoms in the kagome lattice and report on our observations of the melting of the negative temperature Mott Insulator into the flat band.