QUANTUM DECOHERENCE AND QUASI-EQUILIBRIUM IN 1H-NMR OF LIQUID CRYSTALS

H. H. SEGNORILE; C. E. GONZÁLEZ; C.J. BONIN; R.C. ZAMAR

Alta Gracia

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

ISMAR - FaMAF

Due to the typical anisotropic molecular orientation  together with the rapid liquid-like individual motions,  the proton spin system of  LCs can be considered as dipole-interacting spin clusters,  magnetically isolated in the average between them. Despite of the low dimension of the  Hilbert space of the units, previous work1 showed that quasi-invariant states (QI) associated with the residual dipolar Hamiltonian  can be prepared through the Jeener-Broekaert (JB) experiment2, which have a similar phenomenology than in solids.  It is well known that in closed systems of finite dimension, the evolution under the system Hamiltonian does not produce quasi-equilibrium (QE) states3; approaches based on the spin-diffusion, usually introduced in solids to justify the occurrence of a spin-temperature, are not justified in small systems, either.  Selective refocusing experiments of multiple quantum coherences conducted in LC's, showed that irreversible decoherence occurs over an intermediate timescale between those of  Liouvillian interference (corresponding to the 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 the ?pure dephasing? mechanisms4. Hence, the quantum openness of the spin system becomes an essential ingredient for explaining the experimental results. Theoretically, the noticeable separation between decoherence and relaxation  timescales and the highly correlated nature of the molecular environment, suggests that in LC decoherence 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 which are of a very different nature and give rise to distinct timescales of the spin dynamics:  reversible or adiabatic quantum decoherence and irreversible or essentially-adiabatic quantum decoherence, which is in accordance with the recent experimental results.  A fingerprint of quantum decoherence is the  process we call ?eigen-selectivity?, by which  the diagonal-in-block structure of the density matrix is preserved over decoherence, allowing the occurrence of a ?free-decoherence? or ?free-noise? subspace, which defines defining the quasi-invariant structure of the QE  states. Therefore, this approach to decoherence 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. 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 conclusion. The effect of eigen-selectivity is  experimentally shown through the evolution of the NMR spectrum under refocusing.