Thermodynamics and phase structure of rotating Hayward-AdS black holes

Abstract

We use the modified Newman--Janis algorithm to obtain the rotating ``mother'' black hole solution of the rotating Hayward-AdS black hole. We further derive the fundamental thermodynamic quantities of the ``mother'' black hole in the full unconstrained phase space $(S,J,q,α,P)$, and impose the regularity condition $M_1=q^3/α$ to obtain the thermodynamic quantities of the rotating Hayward-AdS regular black hole. This order of operations gives a Hawking temperature identical to the surface gravity temperature and produces the correct response functions of the regular black hole. For $P<P_c$, the Hawking temperature $T_2$ possesses a local maximum and a local minimum, which divides the black hole into small, intermediate, and large black hole branches, and the $P$--$V$ isotherms display oscillatory behavior characteristic of a liquid/gas phase transition. The small and large black hole branches are locally stable, whereas the intermediate black hole branch has negative heat capacity and is locally unstable. Increasing the rotation parameter $a$, magnetic charge $q$, or pressure $P$ reduces the region of the intermediate branch. The $G_2$--$T_2$ diagram shows that the phase structure, in contrast to the standard swallowtail of the Reissner--Nordström-AdS and Kerr--Newman-AdS black holes, is 8-shaped or C-shaped. These two shapes are divided by the characteristic pressure $P_z$. For $P<P_z$, the curve is 8-shaped, corresponding to a first-order small/large black hole transition. For $P_z<P<P_c$, the curve is C-shaped, corresponding to a zeroth-order transition. These results show that the 8-shaped/C-shaped phase structure characteristic of regular AdS black holes persists under rotation, indicating that it stems from the restructuring of the thermodynamic phase space by the regularity condition rather than from spherical symmetry.