Thermodynamically Stable Phases of Asymptotically Flat Lovelock Black Holes

We present the first examples of phase transitions in asymptotically flat black hole solutions. We analyze the thermodynamic properties of black holes in order $N\ge 3$ Lovelock gravity, with zero cosmological constant. We find a new type of “inverted” swallowtail indicative of stable temperature regions for an otherwise unstable neutral black hole, and demonstrate multiple such stable phases can exist and coexist at multi-critical points. We also find that for charged black holes, ordinary swallowtails can exist on the stable Gibbs free energy branch, allowing for multiple first order phase transitions as seen for AdS black holes. A triple point for $N=5$ and a quadruple point for $N=7$ are presented explicitly. We investigate changes in the Gibbs free energy as the lowest order Lovelock constant is varied, and draw comparisons to pressure changes for AdS black hole systems.