FFLO- and Néel States in finite-size Systems

The general structure of the $s$-wave fermionic superfluid pairing order parameter is discussed for systems in thermal equilibrium. We demonstrate that for finite-size systems with fixed boundary conditions the pairing-amplitude may always be chosen as a {\it real} function in space, in contrast to systems underlying periodic boundary conditions, with drastical consequences for several postulated Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states. Using a simple mapping, we also investigate the consequences of our results for antiferromagnetic equilibrium states in a repulsive Hubbard model.