  Bose-Einstein condensation (BEC) of Feshbach molecules in a homogeneous Bose
gas is studied at finite temperatures in a single-channel mean-field approach
where the Hartree-Fock energy and pairing gap are determined self-consistently.
In the molecular-BEC state, the atomic excitation is gapped and the molecular
excitation is gapless. The binding energy of Feshbach molecules is shifted from
the vacuum value due to many-body effect. When the scattering length $a_s$ of
atoms is negative, the system is subject to mechanical collapse due to negative
compressibility. The system is stable in most regions with positive scattering
lengths. However at low temperatures near the resonance, the molecular-BEC
state vanishes, and the coherent mixture of atomic and molecular BEC is subject
to mechanical collapse.
