Multiple Quantum Coherences in Dipolar Evolution

C. M. SÁNCHEZ; L. BULJUBASICH; H. M. PASTAWSKI; A. K. CHATTAH

Workshop; III Taller de Resonancia Mag- nética: NMR and EPR at the Forefront of Research; 2016

In this work we overview time-reversal Nuclear Magnetic Resonance (NMR) experiments in many-spin systems evolving under the dipolar Hamiltonian. The Loschmidt echo (LE) in NMR is the signal of excitations which, after evolving with a forward Hamiltonian, is recovered by means of a backward evolution Fig. [1]. The presence of non-diagonal terms in the non-equilibrium density matrix is directly monitored by encoding the multiple quantum coherences. This enables a spin counting procedure, giving information on the formation of clusters of correlated spins. Two samples representing different spin systems with coupled networks were used in the experiments. Protons in polycrystalline ferrocene corresponds to an ?infinite ?network. Liquid crystal MBBA in the nematic mesophase represents a finite proton system with a hierarchical set of couplings. A close connection was established between the LE decay and the spin counting measurements, confirming the hypothesis that the complexity of the system is driven by the coherent dynamics. 1. Quantum dynamics of excitations and decoherence in many-spin systems detected with Loschmidt echoes: its relation to their spreading through the Hilbert space, C. M. Sánchez, P. R. Levstein, L.Buljubasich, H. M. Pastawski, A.K.Chattah, Phil. Tran. A, in press (2016)