Evidence for vorticity in superfluid 3He-A

We studied textural changes due to rotation in a cylindrical vessel in an axial magnetic field with NMR techniques. Vortices were expected to change the NMR line, but the effect was assumed to be small, increasing slowly with Ω, the angular velocity of rotation. We found, however, astonishingly large and regular changes in the NMR spectra as a function of Ω. This regularity, and especially the linear dependence on angular velocity, strongly suggested the existence of vortices in rotating He3-A.
Our experiments on rotating superfluid He3-A showed a change in the NMR line shape as a result of rotation: The peak amplitude decreases in proportion to f(T)g(Ω), where Ω is the angular velocity of rotation; at the same time the line broadens. Near Tc, f(T) is a linear function of reduced temperature  1−T/Tc. At small velocities g(Ω)∝Ω. The linear dependence on angular velocity reflects the linear increase in the number of quantized vortices.
 
Usually, the He3 sample was cooled at rest deep to the inside of the A phase. During the ensuing warmup the cryostat was successively rotated and stopped while the NMR spectra were recorded. In some of our experiments the He3 sample was cooled to the A phase while rotating. Our experimental cell is a long (L = 30 mm) and narrow (diam = 5 mm) cylinder.
FIGURE The first observation of superfluid vortices in rotating He3-A; the broadening of the nuclear magnetic resonance spectra is an indication of soft vortex cores on the order of 10 microns in radius. In this recorded time sequence of NMR absorption peaks: t1, start rotation; t2, final velocity reached; t3, stop the motor; t4, cryostat stops. The field homogeneity was 8.7 x 10-5, angular velocity = 0.84 rad/s, and 1 —T/Tc, = 0.152. Each division on the horizontal axis is 1 min.

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