Critical behavior in (2+1)-dimensional black holes

The critical behavior and phase transition in the (2+1)-dimensional Banados-Teitelboim-Zanelli (BTZ) black holes are discussed. By calculating the equilibrium thermodynamic fluctuations in the microcanonical ensemble, canonical ensemble, and grand canonical ensemble, respectively, we find that the extremal spinning BTZ black hole is a critical point, some critical exponents satisfy the scaling laws of the ''first kind,'' and the scaling laws related to the correlation length suggest that the effective spatial dimension of;extremal black holes is one, which is in agreement with the argument that the extremal black holes are the Bogomol'ni saturated string states. In addition, we find that the massless BTZ black hole is a critical point of spinless BTZ black holes.