Thermodynamic phase structure of charged anti-de Sitter scalar-tensor black holes

When electromagnetic field with nonlinear lagrangian acts as a source of gravity the no-scalar-hair theorems can be eluded and black holes with non-trivial scalar field can be found in scalar tensor theories [1]. Black holes with secondary scalar hair exist also when a cosmological constant is added in the theory. The thermodynamics of black holes in anti-de Sitter (Lambda dS) space-time has attracted considerable interest due to the Lambda dS/CFT conjecture. A natural question that arises is whether the non-trivial scalar field would alter the black-hole thermodynamical phase structure. In the current work we present the phase structure of charged hairy black holes coupled to nonlinear Born-Infeld electrodynamics in canonical ensemble which is naturally related to AdS space-time. In certain regions of the parameter space we find the existence of a first-order phase transition between small and very large black holes. An unexpected result is that for a small subinterval of charge values two phase transitions are observed - one of zeroth and one of first order.