IFEG   20353
INSTITUTO DE FISICA ENRIQUE GAVIOLA
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
QUANTUM DECOHERENCE AND QUASI-EQUILIBRIUM IN 1H-NMR OF LIQUID CRYSTALS
Autor/es:
H. H. SEGNORILE; C. E. GONZÁLEZ; C.J. BONIN; R.C. ZAMAR
Lugar:
Alta Gracia
Reunión:
Workshop; Magnetic Resonance in a Cordubensis Perspective: New Developments in NMR; 2011
Institución organizadora:
ISMAR - FaMAF
Resumen:
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.