Loschmidt echoes in NMR: multiple quantum coherence excitations and decoherence

LISANDRO BULJUBASICH; HORACIO M. PASTAWSKI; CLAUDIA M. SÁNCHEZ; ANA K. CHATTAH

Natal

Workshop; Workshop Quantum Non-Equilibrium Phenomena; 2016

In this work we overview time-reversal NMR experiments in many-spin systems evolving under the dipolar Hamiltonian. The Lochmidt echo (LE) in NMR, is simply the signal obtained when spin excitations are recovered by performing a time-reversed pulse sequence which involves a period of forward evolution with a Hamiltonian H followed by a backward evolution with the Hamiltonian -H. The presence of non-diagonal terms in the non-equilibrium density matrix of the many-body state, was directly monitored by experimentally encoding the multiple quantum coherences. This procedure enabled us to perform spin counting, giving information on the spreading of the excitation through the Hilbert space and the formation of clusters of correlated spins during the evolution. Proton systems with very different spin networks were measured. Adamantane and polycrystalline ferrocene, corresponding to infinite networks of protons. The other system was the liquid crystal MBBA in the nematic mesophase, that can be considered as a finite spin system with a hierarchical set of couplings. A normalization procedure with the LE was used to improve the information obtained for the dynamics of each system. A close connection could be established between the LE decay and the behavior of the spin counting, supporting the hypothesis that the complexity of the system is driven by the coherent dynamics.