Low-temperature breakdown of many-body perturbation theory for thermodynamics

It is shown analytically and numerically that the finite-temperature many-body perturbation theory in the grand canonical ensemble has zero radius of convergence at zero temperature when the energy ordering or degree of degeneracy for the ground state changes with the perturbation strength. When the degeneracy of the reference state is partially or fully lifted at the first-order Hirschfelder-Certain degenerate perturbation theory, the second-order grand potential and internal energy diverge as T→0. Contrary to earlier suggestions of renormalizability by the chemical potential μ, this nonconvergence, first suspected by Kohn and Luttinger, is caused by the nonanalytic nature of the Boltzmann factor e-E/kBT at T=0, also plaguing the canonical ensemble, which does not involve μ. The finding reveals a fundamental flaw in perturbation theory, which is deeply rooted in the mathematical limitation of power-series expansions and is unlikely to be removed within its framework.