# Charged black holes with axionic-type couplings: classes of solutions and dynamical scalarisation

## TL;DR

This paper explores charged black holes with axionic couplings, revealing the existence of scalarised solutions, their stability, and the conditions under which they coexist or dominate over scalar-free black holes.

## Contribution

It introduces new classes of dyonic black hole solutions with axionic couplings and analyzes their stability, coexistence, and dynamical scalarisation phenomena.

## Key findings

- All charged black holes with canonical axionic coupling have axion hair.
- Scalar-free and scalarised black holes can coexist, with scalarised solutions being entropically favored.
- Scalarised black holes can emerge dynamically from instabilities of scalar-free solutions.

## Abstract

We consider an augmented Einstein-Maxwell-scalar model including an axionic-type coupling between the scalar and electromagnetic field. We study dyonic black hole solutions in this model. For the canonical axionic coupling emerging from high energy physics, all charged black holes have axion hair. We present their domain of existence and investigate some physical properties. For other axionic-type couplings, two classes of black hole solutions may co-exist in the model: scalar-free Reissner-Nordstr\"om black holes and scalarised black holes. We show that in some region of the parameter space, the scalar-free solutions are unstable. Then, there is non-uniqueness since new scalarised black hole solutions with the same global charges also exist, which are entropically preferred over the scalar-free solutions and, moreover, emerge dynamically from the instability of the former.