Entropy of Extremal Dyonic Black Holes

TL;DR

This paper investigates the entropy of extremal dyonic black holes in string theory, revealing that their entropy relates to mass and charges rather than area, and clarifies their classical action properties.

Contribution

It demonstrates that extremal dyonic black holes have entropy proportional to mass or charges and possess zero classical action despite nonzero area.

Findings

Entropy is proportional to mass or charges, not area.

Extremal dyonic black holes have zero classical action.

Entropy can be almost fully determined using thermodynamics and recent literature.

Abstract

For extremal charged black holes, the thermodynamic entropy is proportional not to the area but to the mass or charges. This is demonstrated here for dyonic extremal black hole solutions of string theory. It is pointed out that these solutions have zero classical action although the area is nonzero. By combining the general form of the entropy allowed by thermodynamics with recent observations in the literature it is possible to fix the entropy almost completely.