CONICET | Buscador de Institutos y Recursos Humanos

Previous work showed that by means of the Jeener-Broekaert (JB) experiment two quasiequilibriumstatescan be prepared independently in the proton spin system of thermotropic nematicliquid crystals(LC) in a strong magnetic field, and their relaxation rates measured by recording thesignal in phase with the third pulse of the JB sequence. The experimental evidences support theassumption that both quasiinvariants, which we call S and W, stem from a separation in energyscales of the secular dipolar Hamiltonian. In this work we study the nature of these quasiinvariantsin nematic 5CB (4-pentyl-4-biphenyl-carbonitrile) by encoding the quantum states following theJB pulse pair on two orthogonal bases, Z and X. The experiments were also performed in powderadamantane at 301 K which is used as a reference compound having only one dipolar quasiinvariant.We show that the evolution of the quantum states during the build up of the quasi-equilibriumstate is similar in adamantane and in 5CB prepared under the S condition. Also, we find thattheir quasi-equilibrium density operators have the same tensor structure. In contrast, the W state,whose explicit operator form is not known, involves a richer composition of even order coherenceson the X basis, in consistence with the fact that the truncation of the weak contribution to thedipolar energy, inherent in the definition of this dipolar quasiinvariant, modifies the operator formof the W portion of the dipolar energy. We exploited the ocurrence of coherences of high order( ±4, ±6, ±8), which are exclusive of the W order, to measure the spin-lattice relaxation timeTW accurately, so avoiding experimental difficulties that usually impair dipolar order relaxationmeasurement such as Zeeman contamination at high fields, and also superposition of the differentquasiinvariants. In fact, we report the first measurement of TW in a liquid crystal at high magneticfields. The obtained value points out that the relaxation of the W order in liquid crystals isstrongly dependent on the intensity of the external magnetic field, similarly to the case of the Sreservoir. These are very promising results that can bring light on the porperties of the dipolarquasiinvariants and their theoretical expressions as well as a means for accurate measurments oftheir relaxation times.