R. C. Zamar

Relaxation properties of proton magnetic spin quasi-invariants in liquid crystals

Abstract We present an experimental study of the thermodynamic properties of proton pairs in two thermotropic nematic liquid crystals: chain deuterated 4-n-pentyl-4′-cyanobyphenyl, 5CBd11, and normal 5CB. In the first sample, we find the existence of pure intra-pair and inter-pair magnetic quasi-invariants. In the second, the dipolar signal is more complex, due to the nonequivalence of protons pairs in the molecule, but it is still possible to prepare states for which only one kind of order strongly dominates. In both compounds, the dipolar quasi-invariants relax independently to thermal equilibrium with the lattice. Finally, we discuss the temperature dependence of the characteristic relaxation times in terms of local and long-range cooperative molecular dynamics.

Intramolecular character of the intrapair dipolar order relaxation in the methyl deuterated nematic para-azoxyanisole

Abstract The proton intra-pair dipolar order relaxation time ( T 1D ) was measured for nematic methyl-deuterated para -azoxyanisole (PAA d6 ) as a function of both the temperature and concentration in perdeuterated PAA (PAA d14 ), at 27 MHz. Since the results coincide for all measured concentrations in all the nematic temperature range, we conclude that the intermolecular contribution to the relaxation of the dipolar energy in this compound is negligible, at the studied frequency. The observed temperature dependence of T 1D is typical of the order fluctuations of the director (ODF), which clearly indicates that the ODF is the relevant mechanism producing dipolar order relaxation, in accordance with previous field cycling experiments.

Molecular motions in thermotropic liquid crystals studied by NMR spin-lattice relaxation

Nuclear magnetic resonance relaxation experiments with field cycling techniques proved to be a valuable tool for studying molecular motions in liquid crystals, allowing a very broad Larmor frequency variation, sufficient to separate the cooperative motions from the liquidlike molecular diffusion. In new experiments combining NMR field cycling with the Jeener Broekaert order-transfer pulse sequence, it is possible to measure the dipolar order relaxation time (T1D), in addition to the conventional Zeeman relaxation time (T1Z) in a frequency range of several decades. When applying this technique to nematic thermotropic liquid crystals, T1D showed to depend almost exclusively on the order fluctuation of the director mechanism in the whole frequency range. This unique characteristic of T1D makes dipolar order relaxation experiments specially useful for studying the frequency and temperature dependence of the spectral properties of the collective motions.

Mechanisms of irreversible decoherence in solids

Refocalization sequences in Nuclear Magnetic Resonance (NMR) can in principle reverse the coherent evolution under the secular dipolar Hamiltonian of a closed system. We use this experimental strategy to study the effect of irreversible decoherence on the signal amplitude attenuation in a single crystal hydrated salt where the nuclear spin system consists in the set of hydration water proton spins having a strong coupling within each pair and a much weaker coupling with other pairs. We study the experimental response of attenuation times with temperature, crystal orientation with respect to the external magnetic field and rf pulse amplitudes. We find that the observed attenuation of the refocalized signals can be explained by two independent mechanisms: (a) evolution under the non-secular terms of the reversion Hamiltonian, and (b) an intrinsic mechanism having the attributes of irreversible decoherence induced by the coupling with a quantum environment. To characterize (a) we compare the experimental data with the numerical calculation of the refocalized NMR signal of an artificial, closed spin system. To describe (b) we use a model for the irreversible adiabatic decoherence of spin-pairs coupled with a phonon bath which allows evaluating an upper bound for the decoherence times. This model accounts for both the observed dependence of the decoherence times on the eigenvalues of the spin-environment Hamiltonian, and the independence on the sample temperature. This result, then, supports the adiabatic decoherence induced by the dipole-phonon coupling as the explanation for the observed irreversible decay of reverted NMR signals in solids.