  New NMR experiments reveal the origin of the "pseudocoherence" that leads to
long tails in some Carr-Purcell-Meiboom-Gill (CPMG) sequences in contrast with
the decay of a standard Hahn echo. We showed in [M.B Franzoni and P.R.
Levstein, Phys. Rev. B 72, 235410 (2005)] that under certain conditions these
CPMG sequences become spoiled by stimulated echoes. The presence of these
echoes evidences that the long tails arise on the storage of magnetization in
the direction of the external magnetic field (z-polarization), involving
spin-lattice relaxation times of several seconds. Hole burning experiments
confirm the presence of a highly inhomogeneous line and show that the flip-flop
processes are able to homogenize the line in times agreeing with those obtained
through a Hahn echo sequence. As expected, the decay of the stimulated echoes
is not sensitive to the quenching of the dipolar interaction through an MREV16
sequence, while the Hahn echo decay increases by a factor of 20. CPMG sequences
with hard pulses and different temporal windows clearly show the relevance of
the dipolar interaction in between the pulses for the generation of the long
tails. In particular, the more time the flip-flop is effective (longer
interpulse separation), the shorter are the characteristic times of the long
tails.
