Observation of Two-Dimensional Mott Insulator and π-Superfluid Quantum Phase Transition in Shaking Optical Lattice

The Mott insulator and superfluid phase transition is one of the most prominent phenomena in ultracold atoms. We report the observation of a novel 2D quantum phase transition between the Mott insulator and π superfluid in a shaking optical lattice. In the deep optical lattice regime, the lowest S band can be tuned to Mott phase, while the higher p_x,y bands are itinerant for having larger bandwidth. Through a shaking technique coupling the s-orbital to p_x,y-orbital states, we experimentally observe the transition between the states of the S and p_x,y bands, leading to a quantum phase transition from two-dimensional s-orbital Mott phase to the p_x,y-orbital superfluid which condensed at (π, π) momentum. Using the band-mapping method, we also observe the changes of atomic population in different energy bands during the transition, and the experimental results are well consistent with theoretical expectations.