Quantum Decoherence and Quasi-equilibrium in 1H-NMR of Liquid Crystals

H. H. SEGNORILE, C. E. GONZÁLEZ ,C. J. BONIN AND R.C. ZAMAR

Alta Gracia, Cordoba

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

Fa.M.A.F. - U.N.C.

The typical anisotropic molecular orientation together with the rapid liquid-like individual motions, allow considering the proton spins of each liquid crystal (LC) molecule as a dipole-interacting spin cluster, magnetically isolated in the average from other molecules. Previous work [1] showed that despite the low dimension of the Hilbert space of the molecular units, quasi-equilibrium states (QE) associated with the residual dipolar Hamiltonian can be prepared through the Jeener-Broekaert (JB) experiment, which have a similar phenomenology than in solids. It is well known that in closed systems of finite dimension, the evolution under the self Hamiltonian does not produce QE states [2]; approaches based on the spin-diffusion, usually introduced in solids to justify the occurrence of a spin-temperature, are not justified in small systems. Selective refocusing experiments of multiple quantum coherences conducted in LC's, showed that irreversible decoherence occurs over an intermediate timescale between Liouvillian interference regime (closed system) and thermalization with the lattice. The dissimilar behavior of the different coherence orders observed in the experiment indicates that decoherence is controlled by a full quantum spin-environment interaction that is essentially different from usual coupling models based on thermal fluctuations of the local field, as “pure dephasing” mechanisms [3]. Hence, the quantum openness of the spin system becomes an essential ingredient of any comprehensive theoretical deescription. The noticeable separation of decoherence and relaxation timescales and the highly correlated nature of the molecular environment, suggest that decoherence in LC is ruled by an energy conserving spin-environment interaction, where the molecular orientational order plays a main role. By analyzing a hypothetical time reversal experiment within a scheme where the angular variables are quantum operators, we identify two sources of coherence loss, of a very different nature, which give rise to distinct timescales of the spin dynamics: reversble or adiabatic quantum decoherence and irreversible or essentially-adiabatic quantum decoherence, in accordance with the recent experimental results. A fingerprint of quantum decoherence is the process we call “eigen-selectivity” [4], by which the diagonal-in-block structure of the density matrix is preserved over decoherence, allowing the occurrence of a “decoherence-free” subspace, and defines the quasi-invariant structure of the QE states. Therefore, this approach, relying on plausible hypotheses of general character valid for a large class of LC's, also explains the build-up of the QE states in these mesophases. The occurrence of an irreversible trend towards QE is experimentally demonstrated through spin dynamics refocusing 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 conclusion. The effect of eigen-selectivity is experimentally shown through the evolution of the NMR spectrum under refocusing. REFERENCES: 1. O. Mensio, C. E. González, and R. C. Zamar, Phys. Rev. E 71, 011704 (2005). 2. R. Bruchsweiler and R. Ernst, Chem. Phys. Lett. 264, 393 (1997). 3. C.E.González, H.H.Segnorile and R.C.Zamar, Phys.Rev.E, 83, 011705 (2011). 4. H.H.Segnorile and R.C. Zamar, submitted. H.H. Segnorile, PhD Thesis, Universidad Nacional de Córdoba, Argentina (2009)