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formed in the zero temperature limit if the order parameter distribution is driven far out of equilibrium, for instance by the ultra-fast motion of the hyper-cooled AB interface or by the annihilation of different order-parameter structures?
NMR MEASUREMENT OF VORTICES AT THE LOWEST TEMPERATURES
The pattern of vortices and macroscopic counterflow in superfluid 3He is formed by the orbital part of the order parameter, which is coupled by weak spin-orbit interaction to the nuclear magnetic spin of the Cooper pair. Different NMR methods have been developed to probe the order-parameter texture. At best the resolution is sufficient to discern a change by one individual vortex line. Generally one monitors at low excitation level the local oscillator response which reflects the thermal equilibrium situation. Such a response becomes increasingly insensitive towards low temperatures. To continue using NMR detection of vortices at the lowest temperatures in the collisionless regime, an important goal becomes the search for better NMR methods, with improved resolution and sensitivity for the measurement of vortices and other topological defects in the B phase below 0.4 Tc. Here non-local collective resonances are the most attractive alternative.
Fig. 2. Macroscopic quantum oscillator: coherent spin wave resonances in the B-phase flare-out texture close to the top end plate in the rotating cylinder. The transverse component M⊥ of the coherently precessing total magnetization is plotted as a function of the frequency shift from the Larmor value f0. The series of NMR absorption lines correspond to harmonic oscillator eigenvalues for the horizontal potential well (closely spaced lines) and the axial well (separation into different series).
In the ballistic temperature regime a new coherent resonance mode appears, known as the Persistent Induction Mode, which corresponds to spin wave resonances in a potential well formed by a slowly changing distribution of order parameter orientations in the centre of the flare-out texture of a cylindrical sample, when monitored with continuous wave excitation (Fig. 2). At carefully adjusted low excitation level and precision magnetic field sweep the mode can be enhanced to dominate the NMR absorption. We have explored the use of this mode for the detection of vortices and vortex-free flow in rotation in collaboration with Yuriy Bunkov. The method has proven useful in certain applications and is competitive in sensitivity with our most frequently used method based on an analysis of the NMR absorption line shape. For better comparison, the method of line-shape analysis has now also been evaluated down to below 0.2 Tc. The reports on these measurements are in preparation.
Annual Report 2007