Steven W. Morgan, Vadim Oganesyan, Gregory S. Boutis
Coherent time evolution of the many-body density matrix is characterized by onset of multispin correlations absent in any thermal distribution. As this process is usually accompanied by readily observed (free induction) decay (FID) of transverse magnetization, the concept of spin temperature has been invoked historically to describe the asymptotic state of the system on time scales, t, intermediate between T2 << T1. Using transverse phase encoding techniques we quantify non-thermal correlations in the density matrix during the conventional FID and also following the "magic echo," a time suspension technique used to partially reverse time evolution induced by dipolar interactions. In samples of calcium fluoride and adamantane, we observe that the generation and subsequent decay of spin correlations following a magic echo are very similar to those of a free induction decay even though the overall signal amplitude has decreased by about ~50% for the longest magic echoes measured. In addition, the long-time portion of the single-spin, single-quantum coherence detected is observed to decay at the same rate for both the FID and magic echoes. Furthermore, in the long-time regime the multiple spin correlations are also observed to decay at the same rates for each respective order. We also report an extension of the theoretical analysis of the magic echo to second order in average Hamiltonian treatment. The degradation of time-reversal with increasing evolution time is predicted theoretically, and verified via simulation and experiment, to be strongly influenced by finite pulse width effects.
View original:
http://arxiv.org/abs/1205.7039
No comments:
Post a Comment