Does the Round Sphere Maximize the Free Energy of (2+1)-Dimensional QFTs?

Abstract

We examine the renormalized free energy of the free Dirac fermion and the free scalar on a (2+1)-dimensional geometry $\mathbb{R}\times \Sigma$, with $\Sigma$ having spherical topology and prescribed area. Using heat kernel methods, we perturbatively compute this energy when $\Sigma$ is a small deformation of the round sphere, finding that at any temperature the round sphere is a local maximum. At low temperature the free energy difference is due to the Casimir effect. We then numerically compute this free energy for a class of large axisymmetric deformations, providing evidence that the round sphere globally maximizes it, and we show that the free energy difference relative to the round sphere is unbounded below as the geometry on $\Sigma$ becomes singular. Both our perturbative and numerical results in fact stem from the stronger finding that the difference between the heat kernels of the round sphere and a deformed sphere always appears to have definite sign. We investigate the relevance of our results to physical systems like monolayer graphene consisting of a membrane supporting relativistic QFT degrees of freedom.

arXiv comment39+17 pages, 16 figures. v2: references added, minor edits

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Cited by in corpus (4)

• Casimir Energy and Modularity in Higher-dimensional Conformal Field Theories
• A Surprising Similarity Between Holographic CFTs and a Free Fermion in $(2+1)$ Dimensions
• Graphene shapes from quantum elasticity
• Disks globally maximize the entanglement entropy in $2+1$ dimensions