(2+1)-dimensional black holes in f (R,φ) gravity

We consider a f(R) gravity theory in (2 + 1)-dimensions with a self-interacting scalar field non-minimally coupled to gravity. Without specifying the form of the f (R) function, solving the field equations we find that the Ricci scalar receives a nonlinear correction term which breaks the conformal invariance and leads to a massless black hole solution. When the nonlinear term decouples, we get a well-known hairy black hole solution with the scalar field conformally coupled to gravity. We also fmd that the entropy of our black hole may be higher than the corresponding conformal black hole which indicates that our solution may be thermodynamically preferred.