Slow lattice dynamics in the relaxor ferroelectric 0.7Pb(Mg
1/3Nb
2/3)O
3-0.3PbTiO
3 were investigated by neutron spin echo technique. At much higher temperature than
TC (400 K), we observed oscillating normalized intermediate scattering functions
I(
Q,
t)/
I(
Q,0) for the [1-10] direction, which is ascribed to low frequency vibrational mode with ω∼40 μeV. Such low frequency mode suggests that flat energy surfaces exist in the free energy potential along the [1-10] direction, which could be associated with degeneracy between the rhombohedral and tetragonal ground states at morphotropic phase boundary. On cooling towards
TC, the oscillation in
I(
Q,
t)/
I(
Q,0) decreases and
I(
Q,
t)/
I(
Q,0) becomes time-independent below
TC. This indicates that the low frequency vibrational mode change into the slow relaxational mode showing giant responses.
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