INVESTIGADORES
SEGNORILE Hector Hugo
congresos y reuniones científicas
Título:
Quasi-equilibrium in liquid crystal 1H spins via eigen-selective decoherence
Autor/es:
HÉCTOR H. SEGNORILE; CECILIA E. GONZÁLEZ; CLAUDIO J. BONIN; RICARDO C. ZAMAR
Lugar:
Alta Gracia (Córdoba)
Reunión:
Congreso; Magnetic Resonance in a Cordubensis Perspective VI: New Developments in NMR; 2011
Resumen:
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)