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Register a new userNMR Time Reversal Experiments in Highly Polarised Liquid 3He-4He Mixtures
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Authors
Emmanuel Baudin
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Long-range magnetic interactions in highly magnetised liquids (laser-polarised 3He-4He dilute mixtures at 1 K in our experiment) introduce a significant non-linear and non-local contribution to the evolution of nuclear magnetisation that leads to instabilities during free precession. We recently demonstrated that a multi-echo NMR sequence, based on the magic sandwich pulse scheme developed for solid-state NMR, can be used to stabilise the magnetisation against the effect of distant dipolar fields. Here, we report investigations of echo attenuation in an applied field gradient that show the potential of this NMR sequence for spin diffusion measurements at high magnetisation densities.
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In a highly polarized liquid (laser-polarized 3He-4He mixtures in our experiment), dipolar magnetic interactions within the liquid introduce a significant nonlinear and nonlocal contribution to the Bloch equation that leads to instabilities during NMR evolution. We have launched a study of these instabilities using spin echo techniques. At high magnetizations, a simple 180 degree rf pulse fails to refocus magnetization, so we use a standard solid-state NMR pulse sequence: the magic sandwich. We report an experimental and numerical investigation of the effect of this sequence on unstable NMR evolution. Using a series of repeated magic sandwich sequences, the transverse magnetization lifetime can be increased by up to three orders of magnitude.
We discuss the stability of homonuclear and heteronuclear mixtures of 3He and 4He atoms in the metastable 2^3S_1 state (He*) and predict positions and widths of Feshbach resonances by using the Asymptotic Bound-state Model (ABM). All calculations are performed without fit parameters, using emph{ab-initio} calculations of molecular potentials. One promising very broad Feshbach resonance (Delta B=72.9^{+18.3}_{-19.3} mT) is found that allows for tuning of the inter-isotope scattering length.
The dynamics of 3He atoms in solid 4He have been investigated by measuring the NMR relaxation times T1, T2 in the region where a significant non-classical rotational inertia fraction (NCRIF) has been reported. For 3He concentrations x3 = 16 ppm and 24 ppm, changes are observed for both the spin-lattice relaxation time T1 and the spin-spin relaxation time T2 at the temperatures corresponding to the onset of NCRIF and, at lower temperatures, to the 3He-4He phase separation. The magnitudes of T1 and T2 at temperatures above the phase separation agree roughly with existing theory based on the tunneling of 3He impurities in the elastic strain field due to isotopic mismatch. However, a distinct peak in T1 and a less well-resolved feature in T2 are observed near the reported NCRIF onset temperature, in contrast to the temperature-independent relaxation times predicted by the tunneling theory.
Surface waves on both superfluid 3He and 4He were examined with the premise, that these inviscid media would represent ideal realizations for this fluid dynamics problem. The work on 3He is one of the first of its kind, but on 4He it was possible to produce much more complete set of data for meaningful comparison with theoretical models. Most measurements were performed at the zero temperature limit, meaning T < 100 mK for 4He and T ~ 100 {mu}K for 3He. Dozens of surface wave resonances, including up to 11 overtones, were observed and monitored as the liquid depth in the cell was varied. Despite of the wealth of data, perfect agreement with the constructed theoretical models could not be achieved.
We report on orientation of the order parameter in the 3He-A and 3He-B phases caused by aerogel anisotropy. In 3He-A we have observed relatively homogeneous NMR line with an anomalously large negative frequency shift. We can attribute this effect to an orientation of orbital momentum along the axis of density anisotropy. The similar orientation effect we have seen in 3He-B. We can measure the A-phase Leggett frequency, which shows the same energy gap suppression as in the B-phase. We observe a correlation of A - B transition temperature and NMR frequency shift.
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