On coherences and pseudocoherences observed through the Carr-Purcell-Meiboom-Gill sequences.

FRANZONI M. B.; LEVSTEIN P. R.

Gotemburgo, Suecia

Congreso; EUROMAR09; 2009

Lately, quantum information has become a very interesting and growing field. Various physical systems and different techniques were proposed as potential hardware for quantum computing, and it is crucial the knowledge and correct understanding of the system decoherence processes. Nuclear Magnetic Resonance studies of coherence in C60, 29Si and Y2O3, as good candidates for quantum information, showed very unusual behaviors. Commonly used pulse sequences for a spin-spin decay time (T2) measurement are the Hahn Echo, and multipulse sequences like CPMG (with different pulse phase alternations). From analytic calculations it is well known that these sequences, when applied to solids, should give the same result. The Hahn echo experiment performed in C60 yielded a T2HE = 15 ms. However unexpected long tails (T2 approx 2 s) appeared when the CPMG1 ({Y,Y}) or the CP2 ({X,-X}) sequences were applied. Other unusual observations were oscillations in the magnetization decay for CPMG2 ({Y,-Y}) and a magnetization decay faster than 15 ms for CP1 ({X,X}). After a careful experimental and analytical study we were able to understand these phenomena. The unexpected behaviors arise on highly inhomogeneous lines compared with the dipolar couplings leading to slow spin diffusion. The inhomogeneity produces different tilting angles in different sites of the sample that the flip-flop interaction is unable to average out. As a consequence, there is an unwanted magnetization stored in the direction of the external magnetic field (z polarization). This polarization is brought back to the plane and observed either as a long tail or as oscillations. So, the long tails should not be interpreted as long coherence times. In this work, we present a complete experimental and numerical study of the magnetization behavior as a function of the interpulse separation for the different sequences. We observed that for longer interpulses the unusual behaviors tend to disappear leading to T2HE . Our results are important because the interpulse time can be directly associated to the effective dipolar interaction capable to average out the inhomogeneities