%0 Journal Article %T Phase transition in O_2 %A Klaus Thomsen %J Mathematics %D 2015 %I arXiv %X We exhibit a one-parameter group of automorphism on the Cuntz-algebra O_2 such that the simplex of KMS states changes abruptly at a certain critical temperature from infinitely many to one, and then none. The factor types of the extremal KMS states are also determined. %U http://arxiv.org/abs/1510.07513v2