NMR Dynamics with Large Dipolar Interactions in Liquid Helium-3 Films

In a growing number of experimental situations NMR dynamics is found to be deeply affected by strong dipolar couplings. We have developed models based on approximations of dipolar interactions and applied them to a thin film situation. Weakly damped, robust eigenmodes are found as solutions of the simplified Bloch equations in 2D systems. 3D numerical simulations also reveal the existence of spectral clustering (collapse of NMR spectra into sets of narrow lines) at low tip angles. Systematic pulsed NMR experiments previously performed in thin films of laser-polarised helium-3 have been re-analysed. The resulting spectra consist of several well-resolved lines with the expected frequencies. The effect of applied field gradients on the characteristics of the modes has been studied. The results demonstrate the robustness of spectral clustering in highly polarised thin films.