  We observe evidence of first-order superfluid to Mott-insulator quantum phase
transitions in a lattice-confined antiferromagnetic spinor Bose-Einstein
condensate. The observed signatures include hysteresis effect and significant
heatings across the phase transitions. The nature of the phase transitions is
found to strongly depend on the ratio of the quadratic Zeeman energy to the
spin-dependent interaction. Our observations are qualitatively understood by
the mean field theory, and in addition suggest tuning the quadratic Zeeman
energy is a new approach to realize superfluid to Mott-insulator phase
transitions.
