26 research outputs found
Evidence of several dipolar quasi-invariants in Liquid Crystals
In a closed quantum system of N coupled spins with magnetic quantum number I,
there are about (2I + 1)^N constants of motion. However, the possibility of
observing such quasi-invariant (QI) states 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 QI, in the proton NMR
of small spin clusters, besides those already known Zeeman, and dipolar orders
(strong and weak). 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. We observe that the signals can be explained with two dipolar QIs
only within a range of short preparation times. At longer times the time-domain
signals have an echo-like behaviour. We study their multiple quantum coherence
content on a basis orthogonal to the z-basis and see that such states involve a
significant number of correlated spins. Then we show that the whole preparation
time-scale can only be reconstructed by assuming the occurrence of multiple QI
which we isolate experimentally
Quasi-equilibrium and quantum correlation in an open spin-pair system
Quasi-equilibrium states that can be prepared in solids through Nuclear
Magnetic Resonance (NMR) techniques are out-of-equilibrium states that slowly
relax towards thermodynamic equilibrium with the lattice. In this work, we use
the quantum discord dynamics as a witness of the quantum correlation in this
kind of state. The studied system is a dipole interacting spin pair whose
initial state is prepared with the NMR Jeener-Broekaert pulse sequence,
starting from equilibrium at high temperature and high external magnetic field.
It then evolves as an open quantum system within two different dynamic
scenarios: adiabatic decoherence driven by the coupling of the pairs to a
common phonon field, described within a non-markovian approach; and
spin-lattice relaxation represented by a markovian master equation, and driven
by thermal fluctuations. In this way, the studied model is endowed with the
dynamics of a realistic solid sample. The quantum discord rapidly increases
during the preparation of the initial state, escalating several orders of
magnitude compared with thermal equilibrium at room temperature. Despite the
vanishing of coherences during decoherence, the quantum discord oscillates
around this high value and undergoes a minor attenuation, holding the same
order of magnitude as the initial state. Finally, the quantum discord
dissipates within a time scale shorter than but comparable to spin-lattice
relaxation.Comment: 19 pages, 5 figure
Quantum irreversible decoherence behaviour in open quantum systems with few degrees of freedom. Application to 1H NMR reversion experiments in nematic liquid crystals
An experimental study of NMR spin decoherence in nematic liquid crystals (LC)
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 (IQD) in
LC, 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 IQD 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 IQD. In order to contrast experiment and theory, the theory
was adapted to obtain the IQD function corresponding to the MREV8 reversion
experiments. Non-idealities of the experimental setting are analysed in detail
within this framework and their effects on the observed signal decay are
numerically estimated. It is found that, though 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 behavior of the IQD. As
unique characteristic of the IQD, 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 IQD associated with a quantum open
spin system in LC. 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 equilibriu
Quasi-equilibrium states in thermotropic liquid crystals studied by multiple quantum NMR
We study the nature of the quasiinvariants in nematic 5CB 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 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.Comment: 7 figures. http://www.famaf.unc.edu.ar/series/AFis2005.ht
NMR proton spin dynamics in thermotropic liquid crystals subject to multipulse excitation
Previous experiments of NMR spin-lattice relaxation times as a function of the Larmor frequency, as measured with the field-cycling technique (FC), were shown to be very useful to disentangle the various molecular motions, both local and collective, that dominate the relaxation in different time scales in liquid crystals. However, there are many examples where the known theoretical models that represent the molecular relaxation mechanisms cannot be fitted to the experimental trend in the region of low fields, making it difficult to obtain reliable values for the spectral densities involved, especially for the cooperative motions which dominate at low frequencies. In some cases, these anomalies are loosely ascribed to "local-field" effects but, to our knowledge, there is not a detailed explanation about the origin of these problems nor the range of frequencies where they should be expected. With the aim of isolating the dipolar effects from the influence of molecular dynamics, and taking into account the previous results in solids, in this work we investigate the response of the proton spin system of thermotropic liquid crystals 4-pentyl-4(')-cyanobiphenyl (5CB) and 4-octyl-4(')-cyanobiphenyl (8CB) in nematic and smectic A phases, due to the NMR multipulse sequence 90ν°-(Ď-θx-Ď)N. The nuclear magnetization presents an early transient period characterized by strong oscillations, after which a quasistationary state is attained. Subsequently, this state relaxes towards internal equilibrium over a time much longer than the transverse relaxation time T2. As occurs in solids, the decay time of the quasistationary state T2e presents a minimum when the pulse width θx and the offset of the radiofrequency are set to satisfy resonance conditions (spin-lock). When measured as a function of the pulse spacing Ď in "on-resonance" experiments, T2e shows the behavior expected for cross relaxation between the effective Zeeman and dipolar reservoirs, in accordance with the thermodynamic theory previously developed for solids. Particularly, for values of Ď comparable with T2, the relaxation rate follows a power law T2e â Ď-2, in all the observed cases, for the resonance conditions θx=Ď/3 and equivalent frequency Ďe=Ď/3Ď. When Ď is similar to or greater than typical dipolar periods, the relaxation rate becomes constant and for Ď much shorter than T2, the thermodynamic reservoirs get decoupled. These experiments confirm that the thermodynamic picture is valid also in liquid crystals and the cross relaxation between the reservoirs can be detected without interference with spin-lattice relaxation effects. Accordingly, this technique can be used to estimate the frequency range, where cross-relaxation effects can be expected when Zeeman and dipolar reservoirs are put in thermal contact with each other and with the lattice, as in FC experiments. In particular, the present results allow us to associate the anomalies observed in low-field spin-lattice relaxation with nonadiabatic energy exchange between the reservoirs