## Superconducting Phase in the BCS Model with Imaginary Magnetic Field. II. Multi-Scale Infrared Analysis

J. Math. Sci. Univ. Tokyo
Vol. 28 (2021), No. 2, Page 181-398.

Kashima, Yohei
Superconducting Phase in the BCS Model with Imaginary Magnetic Field. II. Multi-Scale Infrared Analysis
We analyze the reduced BCS model with an imaginary magnetic field in a large domain of the temperature and the imaginary magnetic field. The magnitude of the attractive reduced BCS interaction is fixed to be small but independent of the temperature and the imaginary magnetic field unless the temperature is high. We impose a series of conditions on the free dispersion relation. These conditions are typically satisfied by free electron models with degenerate Fermi surface. For example, our theory applies to the model with nearest-neighbor hopping on 3 or 4-dimensional (hyper-)cubic lattice having degenerate free Fermi surface or the model with nearest-neighbor hopping on the honeycomb lattice with zero chemical potential. We prove that a spontaneous $U(1)$-symmetry breaking (SSB) and an off-diagonal long range order (ODLRO) occur in many areas of the parameter space. The SSB and the ODLRO are proved to occur in low temperatures arbitrarily close to zero in particular. However, it turns out that the SSB and the ODLRO are not present in the zero-temperature limit. The proof is based on Grassmann Gaussian integral formulations and a multi-scale infrared analysis of the formulations. We keep using notations and lemmas of our previous work [Kashima, Y., J. Math. Sci. Univ. Tokyo 28 (2021), 1-179] implementing the double-scale integration scheme. So the multi-scale analysis this paper presents is a continuation of the previous work.