High sensitivity nuclear quadrupole resonance approach for detection of modulation wave motion in incommensurate systems

Interlayer molecular exchange in an anticlinically ordered chiral liquid crystal

The interlayer molecular exchange has been determined in the antiferroelectric smectic-C(*)(A) phase of alphad(2) deuterated 4-(1-methylheptyloxycarbonyl)phenyl4(')-octyloxybiphenyl-4-carboxylat e via quadrupolar deuteron NMR self-diffusion in the spatially varying electric field gradient produced by the anticlinic smectic layer structure. The interlayer self-diffusion coefficient is here by two orders of magnitude smaller than in synclinically ordered smectic phases. The results support the entropic suppression model of the origin of anticlinic smectic ordering. The applied technique could possibly allow for a new insight into the local structure of the intermediate "clock" phases.

What are the conditions for exponential time-cubed echo decays?

Diffusion of precessing spins through a constant field gradient is well-known to produce two distinctive features: an exp(-bt(3)) decay of the echo amplitude in response to two pulses and a much slower decay of the Carr-Purcell echo train. These features will appear whenever the spin frequency is described by a continuous random-walk. The present work shows that this may also occur in the presence of motions with long correlation times tau(c)-continuous Gaussian frequency noise with an exponential autocorrelation has the correct properties over time durations smaller than tau(c). Thus, time-cubed echo decays will occur in situations other than physical diffusion. The decay rate of the Carr-Purcell echo train is shown to vary with the pulse spacing tau whenever the correlation time tau(c) is long; the slower Carr-Purcell decay compared to the two-pulse echo decay is not unique to diffusion. Simulations are presented that display time-cubed decays. The simulations confirm two important criteria: the echo time must be less than tau(c) and the frequency noise must consist of nearly continuous variations, as opposed to step-like changes. These criteria define the range of physical parameters for which time-cubed decays will be observable.