Gaussian to Exponential Crossover in the Attenuation of Polarization   Echoes in NMR
  An ingenious pulse sequence devised by S. Zhang, B. H. Meier, and R. R. Ernst (Phys. Rev. Lett. {\bf 69}, 2149 (1992)) reverses the time evolution (``spin diffusion'') of the local polarization in a dipolar coupled $^{1}$H spin system. This refocusing originates a Polarization Echo whose amplitude attenuates by increasing the time $t_R$ elapsed until the dynamics is reversed. Different functional attenuations are found for a set of dipolar coupled systems: ferrocene, (C$_5$H$_5$)$_2$Fe, cymantrene, (C$_5$H$_5$)Mn(CO)$_3$, and cobaltocene, (C$_5$H$_5$)$_2$Co. To control a relevant variable involved in this attenuation a pulse sequence has been devised to progressively reduce the dipolar dynamics. Since it reduces the evolution of the polarization echo it is referred as REPE sequence. Two extreme behaviors were found while characterizing the materials: In systems with a strong source of relaxation and slow dynamics, the attenuation follows an exponential law (cymantrene). In systems with a strong dipolar dynamics the attenuation is mainly Gaussian. By the application of the REPE sequence the characteristic time of the Gaussian decay is increased until the presence of an underlying dissipative mechanism is revealed (cobaltocene). For ferrocene, however, the attenuation remains Gaussian within the experimental time scale. These two behaviors suggest that the many body quantum dynamics presents an extreme intrinsic instability which, in the presence of small perturbations, leads to the onset of irreversibility. This experimental conclusion is consistent with the tendencies displayed by the numerical solutions of model systems. 
Gonzalo Usaj (1), Horacio M. Pastawski (1,2), and Patricia R.   Levstein(1)((1)Facultad de Matematica, Astronomia y Fisica, ...