  Mixing transformations in quantum field theory are non-trivial, since they
are intimately related to the unitary inequivalence between Fock spaces for
fields with definite mass and fields with definite flavor. Considering the
superposition of two neutral scalar (spin-0) bosonic fields, we investigate
some features of the emerging condensate structure of the flavor vacuum. In
particular, we quantify the flavor vacuum entanglement in terms of the von
Neumann entanglement entropy of the reduced state. Furthermore, in a suitable
limit, we show that the flavor vacuum has a structure akin to the thermal
vacuum of Thermo Field Dynamics, with a temperature dependent on both the
mixing angle and the particle mass difference.
