Quasi-equilibrium in liquid crystal 1H spins via eigen-selective decoherence

HÉCTOR H. SEGNORILE; CECILIA E. GONZÁLEZ; CLAUDIO J. BONIN; RICARDO C. ZAMAR

Alta Gracia (Córdoba)

Congreso; Magnetic Resonance in a Cordubensis Perspective VI: New Developments in NMR; 2011

A quasi-equilibrium (QE) state is a stage of the spin dynamics that can be characterized by a spin-temperature, that is, by a diagonal density operator in a timescale much shorter than relaxation towards thermal equilibrium with the lattice. This state can be observed in solids  and nematic liquid crystals (LC)  by  NMR Jeener-Broekaert1 (JB) experiment, for example. The existence of QE is controversial: it is claimed that in solids, dephasing of a  huge amount of spin states over a short time enables the use of a diagonal density matrix when calculating the observables2. However, in a LC the number of effectively interacting spins (8 spins in PAAd6) seems too small in this view. A full-quantum(FQ) theoretical approach justifying the QE in LC is presented3. In this work spins are treated as an open quantum system, where mechanical molecular operators are included in the dipolar Hamiltonian, together with the spin operators. FQ description, allows to disentangle different timescales in the dynamics:  Liouvillian evolution of a closed spin system, reversible adiabatic quantum decoherence, irreversible quantum decoherence and relaxation.  Our theoretical approach predicts the occurrence of a decay process we called eigen-selectivity. We present an experiment which clearly shows this effect on the multiple quantum coherences spectra. Experiments showing the occurrence of an irreversible trend towards QE are presented: time reversal of the spin dynamics with MREV8 and magic echo pulse sequences starting from the JB initial condition.  Numerical calculation of the dipolar signal on a LC molecule supports this conclusion3,4. In summary, we have obtained: •         Identification of the timescales associated with characteristic processes in the dynamics of an observed system coupled to an unobserved system or environment. •         The quantum adiabatic decoherence is reversible and consistent with a description of the molecular dynamics based in average field theories. •         Understanding of the irreversible character of the quantum decoherence processes. •         Explanation of the quasi-equilibrium states in terms of a combination of eigen-selectivity effects and the irreversible nature of the quantum decoherence. •         Corroboration of the quasi-equilibrium as an intrinsec state of the molecular interactions, which is attained simultaneously by all molecules of the sample, being possible a description in terms of quasi-invariants obtained from the diagonal (in blocks) part of the density matrix. In this poster, we show the evolution of the spin dynamics in a LC from a perturbation of the thermal equilibrium by a radio frequency excitation in NMR experiments. We describe the different processes of the dynamics and the successive schematic representations of the system through the evolution toward the thermal-equilibrium. 1. J. Jeener, P. Broekaert, Phys. Rev., 157,  232 - 240 (1967). 2. W.-K. Rhim, A. Pines and J. S. Waugh, Phys. Rev. B, 3, 684 - 696 (1971). 3. H. H. Segnorile, PhD Thesis, Universidad Nacional de Córdoba (2009) http://www.famaf.unc.edu.ar/publicaciones/documents/serie_d/DFis138.pdf. 4. H. H. Segnorile, C. J. Bonin, C. E. González, R. H. Acosta and R. C. Zamar, Solid State Nuclear Magnetic Resonance, 36, 77 - 85 (2009)