Magnetic vortices in rotating 3He-B

In rotating superfluid 3He-B, the order parameter distribution was found to be influenced by an orientational free energy term that was linear both in the magnetic field and rotation velocity (i.e. it changed sign on the reversal of either the angular velocity or the magnetic field). The magnitude of this free energy term changed abruptly at the vortex core transition. These findings clearly indicate that the B-phase vortices have an intrinsic magnetic moment [PRL 51, 1362 (1983)].

FIGURE The lowest three spin-wave resonance frequencies measured at 29.3 bars in the stationary state (right-hand side) and during rotation at Ω = 1.40 rad/s (left-hand side): open circles, vectors H parallel with Ω; closed circles, H antiparallel with Ω; solid lines are the calculated eigenfrequencies. The inset illustrates the measured NMR absorption spectrum at T/Tc = 0.51.

This work reported first measurements on gyromagnetism in superfluid He3-B. The physical effect is quite unique and has no counterpart in superfluid He. In contrast to usual gyromagnetic features, the effect in He3-B is inherently related to the peculiar symmetry break in the p-wave paired condensate.

Gyromagnetism is observed in our measurements via the NMR frequency shift, which depends on the relative directions of the angular velocity of rotation Ω and the applied magnetic field H. The gyromagnetic contribution to the frequency shift is interpreted as arising from the spontaneous magnetic moment of vortices. This would imply that at least a fraction of the vortex core consists of liquid He in a nonunitary pairing state, significantly different from the isotropic Balian-Werthamer state.

In the rotating state there occurs a jump at T = 0.6Tc, in the magnitude of the frequency shift, which implies a discontinuity in the vortex free energy. This phenomenon represents a first-order phase transition in the structure of the vortex core since it displays the properties expected of a transition associated with each individual vortex, but not of the vortex lattice as a whole.

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