The extended phase space thermodynamics and Ehrenfest scheme for the Kerr-Sen AdS black holes

TL;DR

This paper analyzes the thermodynamics and phase transitions of Kerr-Sen-AdS black holes, confirming second-order phase transitions through Ehrenfest equations and exploring related physical properties and dual CFT quantities.

Contribution

It provides a systematic numerical and analytical study of phase transitions in Kerr-Sen-AdS black holes, including critical point analysis and verification of second-order transition nature.

Findings

Critical points match Kerr-AdS black hole thermodynamics in certain limits.

All three thermodynamic quantities diverge at critical points.

Prigogine-Defay ratio equals unity, indicating second-order phase transitions.

Abstract

In the present work, we numerically investigate the horizon structure of the Kerr-Sen black holes in anti-de Sitter (AdS) spacetime. Further, we investigate the phase transitions and critical phenomena in Kerr-Sen-AdS black holes at the critical points. Such black holes are characterized by its mass ($M$), the dilaton charge ($Q$), and the negative cosmological constant, $\Lambda(<0)$. We define a dimensionless parameter $\epsilon=\bar{J}/{\bar{Q}^2}$ and express the mass, temperature, volume, and Gibbs free energy in terms of $\epsilon$ and its polynomials. Moreover, we numerically fit the data for the critical points and find that in the appropriate limit, the expressions for critical points would correspond to the respective critical points of the Kerr-AdS black hole thermodynamics. Such a study involves a systematic analysis of temperature, Gibbs free energy, and volume in the extended phase space. We provide an analytical verification of the nature of the phase transitions at the critical points by introducing the Ehrenfest equations. We also check that all three quantities, e.g., the specific heat at constant pressure, $C_P$, the volume expansion coefficient, $\alpha$, and the isothermal compressibility, $\kappa_T$, diverge at the critical points. We find the $Prigogine$-$Defay$ ratio using the expressions of $C_P$, $\alpha$, and $\kappa_T$, and find that it identically equals unity. Hence, the phase transition behavior of the Kerr-Sen-AdS black holes at their critical points is of second order. In addition, we propose investigating the energy extraction process via the Penrose process. Later, we calculate the speed of sound and adiabatic compressibility for the rotating Kerr-Sen-AdS black holes. Finally, on a specific note, we calculate the thermodynamic quantities of the boundary conformal field theory (CFT) dual to the extended phase space.