Dyonic Black Holes in Lorentz-Violating Gravity with a Background Kalb--Ramond Field

TL;DR

This paper constructs an exact dyonic black hole solution in Lorentz-violating gravity with a Kalb-Ramond field, analyzing its properties, thermodynamics, and phase transitions influenced by Lorentz violation and charges.

Contribution

It introduces a novel exact black hole solution with nonminimal coupling in Lorentz-violating gravity and explores its physical and thermodynamic properties.

Findings

Lorentz-violating parameter and charges significantly affect shadow size.

Black hole exhibits a first-order phase transition between small and large states.

Thermodynamic laws and phase structure are consistent with extended phase space analysis.

Abstract

By introducing a nonminimal coupling between the Kalb--Ramond field and the electromagnetic field, we construct an exact four-dimensional static, spherically symmetric dyonic black hole solution in Lorentz-violating gravity with a background Kalb--Ramond field. The curvature invariants show that the spacetime retains a genuine curvature singularity at $r=0$. We then analyze the geodesic motion of null and timelike particles and obtain the photon-sphere radius, the shadow radius, and the innermost stable circular orbit, demonstrating that both the Lorentz-violating parameter and the dyonic charges can appreciably modify the shadow size and the domain of stable circular motion. In the extended phase space, we derive the thermodynamic quantities and verify the first law of black hole thermodynamics together with the Smarr relation. The system also exhibits a first-order phase transition between small and large black holes, and its phase structure is strongly influenced by the Lorentz-violating parameter and the dyonic charges.