Strong coupling s-wave superconductors in the extreme Pauli limit: I.The breached pair and metastable FFLO phases

Abstract

We study s-wave superconductivity in the attractive Hubbard model in an applied magnetic field and assume the extreme Pauli limit where the orbital critical field is much greater than the Zeeman critical field. We work at a coupling corresponding roughly to the peak $T_c$ in the BCS to BEC crossover window and retain the crucial amplitude and phase fluctuations. At low field, as expected, the superconductor undergoes a second order thermal transition to the normal state, and is only weakly magnetized near $T_c$. At intermediate fields the thermal transition is still second order, but the magnetization is significantly larger, characteristic of a `breached pair' state. At strong field, the thermal transition is first order and our Monte Carlo reveals the presence of a metastable Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. At even higher fields we observe the true FFLO ground state. We present the full `field-temperature' phase diagram of this strong coupling problem, revealing $T_c$ scales an order of magnitude below the mean field estimate, compute the superconducting and magnetic order parameters, and map out the directly measurable magnetic structure factor. We compare these trends to results on the Pauli limited heavy fermion CeCoIn$_5$, and the cold atomic Fermi gas at unitarity. This paper focuses primarily on the homogeneous superconducting state, another deals with thermal effects in the FFLO regime.