Proton NMR signals of intra- and interpair dipolar order in liquid crystals

An experimental study of NMR spin decoherence in nematic liquid crystals is presented. Decoherence dynamics can be put in evidence by means of refocusing experiments of the dipolar interactions. The experimental technique used in this work is based on the MREV8 pulse sequence. The aim of the work is to detect the main features of the irreversible quantum decoherence in liquid crystals, on the basis of the theory presented by the authors recently. The focus is laid on experimentally probing the eigen-selection process in the intermediate time scale, between quantum interference of a closed system and thermalization, as a signature of the quantum spin decoherence of the open quantum system, as well as on quantifying the effects of non-idealities as possible sources of signal decays which could mask the intrinsic decoherence. In order to contrast experiment and theory, the theory was adapted to obtain the decoherence function corresponding to the MREV8 reversion experiments. Non-idealities of the experimental setting, like external field inhomogeneity, pulse misadjustments and the presence of non-reverted spin interaction terms are analysed in detail within this framework, and their effects on the observed signal decay are numerically estimated. It is found that, though all these non-idealities could in principle affect the evolution of the spin dynamics, their influence can be mitigated and they do not present the characteristic behaviour of the irreversible spin decoherence. As unique characteristic of decoherence, the experimental results clearly show the occurrence of eigen-selectivity in the intermediate timescale, in complete agreement with the theoretical predictions. We conclude that the eigen-selection effect is the fingerprint of decoherence associated with a quantum open spin system in liquid crystals. Besides, these features of the results account for the quasi-equilibrium states of the spin system, which were observed previously in these mesophases, and lead to conclude that the quasi-equilibrium is a definite stage of the spin dynamics during its evolution towards equilibrium.

Section: Non-ideal Behaviors and Approximations In The Experimentsmentioning

confidence: 99%

Quantum irreversible decoherence behaviour in open quantum systems with few degrees of freedom: Application to 1H NMR reversion experiments in nematic liquid crystals

Segnorile

Zamar²

2013

“…Two different quasi-equilibrium states can be selectively prepared in LC in the nematic phase by suitably selecting the preparation time t 12 of the JB sequence [7,8]. It was found that the attributes of the dipolar echoes in nematic 5CB (4'-pentyl-4-biphenyl-carbonitrile), in the alkyl deuterated 5BC d11 and in PAA d6 at 16 and 27 MHz, as well as 300 MHz [8][9][10], are similar to those of the 'intrapair' and 'interpair' quasiinvariants measured in crystal hydrates [11][12][13].…”

Section: Introductionmentioning

confidence: 99%

Quasiequilibrium states in thermotropic liquid crystals studied by multiple-quantum NMR

Buljubasich

Monti

Acosta

et al. 2009

Previous work showed that by means of the Jeener-Broekaert (JB) experiment, two quasiequilibrium states can be selectively prepared in the proton spin system of thermotropic nematic liquid crystals (LC) in a strong magnetic field. The similarity of the experimental results obtained in a variety of LC in a broad Larmor frequency range, with crystal hydrates, supports the assumption that also in LC the two spin reservoirs into which the Zeeman order is transferred, originate in the dipolar energy and that they are associated with a separation in energy scales: a constant of motion related to the stronger dipolar interactions (S), and a second one (W) corresponding to the secular part of the weaker dipolar interactions with regard to the Zeeman and the strong dipolar part. We study the nature of these quasiinvariants in nematic 5CB (4'-pentyl-4-biphenylcarbonitrile) and measure their relaxation times by encoding the multiple quantum coherences of the states following the JB pulse pair on two orthogonal bases, Z and X. The experiments were also performed in powder adamantane 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-equilibrium state in 5CB prepared under the S condition is similar to the case of powder adamantane and that their quasi-equilibrium density operators have the same tensor structure.In contrast, the second constant of motion, whose explicit operator form is not known, involves a richer composition of multiple quantum coherences on the X basis of even order, in consistency with the truncation inherent in its definition. We exploited the exclusive presence coherences ±4, ±6, ±8, besides 0 and ±2 under the W condition to measure the spin-lattice relaxation time T W accurately, so avoiding experimental difficulties that usually impair dipolar order relaxation measurement such as Zeeman contamination at high fields, and also superposition of the different quasiinvariants. This procedure opens the possibility of measuring the spin-lattice relaxation of a quasiinvariant independent of the Zeeman and S reservoirs, so incorporating a new relaxation parameter useful for studying the complex molecular dynamics in mesophases. In fact, we report the first measurement of T W in a liquid crystal at high magnetic fields. The comparison of the obtained value with the one corresponding to a lower field (16 MHz) points out that the relaxation of the W-order strongly depends on the intensity of the external magnetic field, similarly to the case of the S reservoir, indicating that the relaxation of the W-quasiinvariant is also governed by the cooperative molecular motions.

“…On the contrary, only two constants of motion were observed in gypsum when the magnetic field B [010] and only one at an orientation that makes the protons at the water molecules equivalent. The fact that compounds having very different numbers of interacting spins (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25) in LCs and infinite in gypsum) admit the preparation of more than one dipolar QI, indicates that their occurrence is not exclusively determined by the size of the dipolar network. Instead, both the occurrence of multiple QIs and the many-spin character they show, do depend strongly on the topology of the spin network, since it determines the quantum dynamics of the spin system.…”

Section: Discussionmentioning

confidence: 99%

“…Two dipolar QE states, were instead observed both on hydrated salts, 5,12 and on liquid crystals (LCs). 13,14 The occurrence of "intra-pair" and "inter-pair" QE states in hydrated salts was associated with the distribution of the proton pairs of the water molecules in the lattice, conforming a system of weakly interacting spin pairs. The spin system of LCs instead, comprises the few proton spins within each molecule, which display a hierarchy of dipolar couplings.…”

Section: Introductionmentioning

confidence: 99%

Evidence for several dipolar quasi-invariants in liquid crystals

Bonin¹,

González²,

Segnorile³

et al. 2013

The quasi-equilibrium states of an observed quantum system involve as many constants of motion as the dimension of the operator basis which spans the blocks of all the degenerate eigenvalues of the Hamiltonian that drives the system dynamics, however, the possibility of observing such quasiinvariants in solid-like spin systems in Nuclear Magnetic Resonance (NMR) is not a strictly exact prediction. The aim of this work is to provide experimental evidence of several quasi-invariants, in the proton NMR of small spin clusters, like nematic liquid crystal molecules, in which the use of thermodynamic arguments is not justified. We explore the spin states prepared with the Jeener-Broekaert pulse sequence by analyzing the time-domain signals yielded by this sequence as a function of the preparation times, in a variety of dipolar networks, solids, and liquid crystals. We observe that the signals can be explained with two dipolar quasi-invariants only within a range of short preparation times, however at longer times liquid crystal signals show an echo-like behaviour whose description requires assuming more quasi-invariants. We study the multiple quantum coherence content of such signals on a basis orthogonal to the z-basis and see that such states involve a significant number of correlated spins. Therefore, we show that the NMR signals within the whole preparation timescale can only be reconstructed by assuming the occurrence of multiple quasi-invariants which we experimentally isolate.