Anisotropic spin diffusion and multiple spin echoes in3He-4He solutions |
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Authors: | R J Ragan W J Mullin |
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Affiliation: | (1) Department of Physics and Astronomy, University of Massachusetts, 01003 Amherst, MA |
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Abstract: | We propose a 1-t1- 2 pulse-NMR experiment to detect the spin-diffusion anisotropy, =D -D , in degenerate spin-polarized3He-4He mixtures, where D and d are the transverse and longitudinal spin diffusion coefficients. In such an experiment the nonlinearity of the dynamics produces multiple spin echoes (MSE). At the
3He concentration x3 4% the spinrotation parameter vanishes ( M 0), so that the nonlinearity of the equations of motion is entirely due to the anisotropy. In this situation, detection of MSE amounts to observation of D. For slight anisotropy, i.e. D/D 0.25, we use a perturbation scheme similiar to that developed by Einzel et al. (in that case, for small M and small demagnetizing field) to calculate the second and third echo heights. For larger anisotropy we numerically calculate the echo heights. We find that for D/D = 0.5 the heights are 2 % of the first echo, and should be detectable. The ( 1, 2) tip-angle dependence of the D echoes is different from that of the M and demagnetization echoes, and furthermore, they occur at right angles to these echoes (in spin space). Thus, even when small spin-rotation and demagnetization effects are present, the 1-t1- 2 experiment provides a sensitive means of detecting the anisotropy. |
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