  We observe an experimental signature of the role of the phonons in spin
relaxation between triplet and singlet states in a two-electron quantum dot.
Using both the external magnetic field and the electrostatic confinement
potential, we change the singlet-triplet energy splitting from 1.3 meV to zero
and observe that the spin relaxation time depends non-monotonously on the
energy splitting. A simple theoretical model is derived to capture the
underlying physical mechanism. The present experiment confirms that spin-flip
energy is dissipated in the phonon bath.
