Scale dependence at the level of the effective action is a generic result of quantum field theory. Allowing for scale dependence of the gravitational couplings leads to a generalization of the corresponding field equations. In this work, those equations are solved by imposing the "null energy condition" in three-dimensional space time with stationary spherical symmetry. The constants of integration are given in terms of the classical BTZ parameters plus one additional constant, that parametrizes the strength of the scale dependence. The properties such as asymptotics, horizon structure, and thermodynamics are discussed. It is found that the black hole entropy shows a remarkable transition from the usual "area law" to an "area × radius" law.PACS numbers: 04.60., 04.70.
I. INTRODUCTIONGravity in (2 + 1) dimensions is a vibrant field of research. This is in part due to the fact that the absence of propagating degrees of freedom makes things simpler than in (3 + 1) dimensions, in particular when dealing with the challenge of formulating a quantization of this theory. Another important feature of gravity in (2 + 1) dimensions is the deep connection to Chern-Simons theory [1][2][3]. This by itself makes the black hole solution [4,5] found by Bañados, Teitelboim, and Zanelli (BTZ) an extremely interesting research object, which has been generalised in many directions. An additional component that motivates the research on black holes in three dimensions is their prominent role in the context of the AdS/CFT correspondence [6][7][8][9].Despite of some progress, the consistent formulation of quantum gravity remains an open task which is attacked in many different ways [10-26] (for a review see [27]). Even though many approaches to quantum gravity are very different, most of them have the common feature that the resulting effective action of gravity acquires a scale dependence. This means that the couplings appearing in the quantum-effective action (such as Newtons coupling G 0 , or the cosmological term Λ 0 ) become scale dependent quantities (G 0 → G k , Λ 0 → Λ k ). There is quite some evidence that this scaling behavior is in agreement with Weinberg's Asymptotic Safety program [28][29][30][31][32][33][34][35]. In particular, the effective action and running couplings in three dimensions have been studied in [36,37]. In any case, scale dependent couplings can be expected to produce differences to classical general relativity, such as modifications of classical black hole backgrounds .In this paper the possible effects of scale dependence on the black hole in three dimensional gravity will be investigated in the light of the effective action approach. We will use the scale-field method applied to the Einstein-Hilbert truncation, which allows to derive generalized Einstein equations for the case of scale dependent couplings [59][60][61][62]. The theoretical uncertainty concerning the functional form of the scale dependence of G k and Λ k will be avoided. Instead, the most general stationary spherically symmetric solution w...