Critical behavior of extremal Kerr-Newman black holes

The previously suggested existence of second-order phase transitions in a series of Kerr-Newman holes is re-examined in the framework of equilibrium black-hole thermodynamics, to distinguish a true transition from another confusing phenomenon. By adopting a physical interpretation unique to the black-hole thermodynamics, various critical exponents are calculated for one side of the transition which is shown actually very likely to occur at the extremal limit.