Hole burning in polycrystallineC60: An answer to the long pseudocoherent tails

Abstract

NMR experiments in ${\text{C}}_{60}$ 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 a previous work [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 amplitudes of the long tails decrease and their characteristic times become shorter.