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Using NMR techniques we monitor the structure and number of vortices in a cylindrical container, rotating at constant angular velocity Ω, before and after the passage of the A-B interface. The initial A phase contains the equilibrium number of vortices. The final B phase is found to have a deficit of vortices which depends nonmonotonically on the measured time t AB of the A→B transition. We argue that a vortex layer is formed in front of the moving A-B interface. The measured vortex deficit can be understood by analyzing the stability of the layer

Insight in the dynamics of quantized vortex lines has been obtained by performing NMR measurements in uniformly rotating superfluid [sup 3]He-[ital B]. Compared to superfluid [sup 4]He, in [sup 3]He-[ital B] the viscosity of the normal component is three orders of magnitude larger and the pinning of vortices is weaker. The collective modes governing the hydrodynamic response of an array of vortex lines are highly overdamped. Two distinct modes are identified: (1) a relatively fast mutual-friction-resisted mode, which controls the redistribution of the vortex density on a time scale of a few seconds, and (2) an exponentially relaxing slow mode with a time constant of a few minutes, which governs asymptotically the approach to equilibrium at nearly constant vortex density. Measurement of the fast motion allows us to extract the dissipative mutual friction. The slow mode is dominated by the elastic tension along the vortex line and by weak pinning at the top and bottom surfaces of the rotating container. With increasing vortex density the surface pinning becomes a collective process, in which coherently moving groups of vortices are pinned in unison. Collective pinning is a result of the shear elasticity related to the crystalline order of the vortex lattice.more » Our measurements on the slow mode indicate that the crystalline correlation extends over many lattice spacings.« less

Exponentially damped relaxation with a time constant (1 ÷ 3) min after perturbation of steady rotation has been discovered in rotating 3He-B, using NMR techniques. The effect represents an overdamped collective mode of the vortex array, governed by vortex tension and pinning. Elastic-shear deformation of the array is found to influence pinning, but does not contribute in the bulk liquid. The presence of shear elasticity provides the first evidence for crystalline order of a vortex array in 3He.

In rotating 3He-B the critical flow velocity for the nucleation of singular vortex lines is high and allows the existence of different rotating states: vortex-free counterflow, the equilibrium vortex state, and metastable states with a well-defined vortex cluster containing less than the equilibrium number of vortices. These states can be distinguished and characterized using CW NMR techniques. We derive an estimate for the magnetic field induced anistropy of the superfluid energy gap from an analysis of the spatial distribution of the order parameter texture in the vortex-free state. When vortices are introduced, their number in the vortex cluster can be counted using a simple experimental procedure. This allows us to measure the critical rotation velocity for the nucleation of vortices, which proves to correspond to the maximum flow velocity in the cylindrical NMR cell.

We present experimental proof that in rotating {sup 3}He-{ital B} the vortex-core transition temperature {ital T}{sub {ital V}} separates axisymmetric vortices above {ital T}{sub {ital V}} from vortices with spontaneously broken axial symmetry below {ital T}{sub {ital V}}. The nonaxisymmetry is observed in the presence of coherent spin precession as a new soft Goldstone mode, manifested as oscillations and spiral twisting of the core anisotropy axis. These are driven by the precessing spin via spin-orbit coupling and lead to magnetic relaxation from viscous losses, which depend on vortex pinning.

We have studied the B⇌A phase transitions in a rotating container. The initial state contains the equilibrium number of vortices, and after the A-B interface has traversed the container, the vortex state was studied using NMR. We find that the A-phase state resulting from a B→A transition contains the equilibrium number of continuous vortices. Possible explanations are discussed.

The phase-shifted NMR absorption response of an array of vortices to a harmonic modulation of the rotation velocity is discussed. The measured phase lag between the rotation drive and its response can be explained by a torque, which acts on the nonaxisymmetric vortex core cross-section and derives contributions from both the vortex displacement and velocity. The result supports the earlier conclusion that the NMR absorption signal can be employed for studying vortex motion in rotating3He-B.

A unique property of 3 He-B is vortex-free rotation, due to a high nucleation threshold of the singular vortices. In contrast to other superfluids, rotational states consisting of a vortex cluster surrounded by vortex-free counterflow form spontaneously and can also be prepared with a fixed number of vortices. The clusters can be analyzed by means of NMR measurements with high precision. These techniques have been exploited to study rotational states in the presence of different phase boundaries.

NMR absorption by vortices in the homogeneously precessing spin domain is discussed in terms of a torque, which arises from spin-orbit coupling between the coherently precessing spins and the nonuniformity in the orbital order parameter in the vortex core. A new soft mode of the nonaxisymmetric vortex core has been observed to contribute to spin relaxation.

First experiments suggesting the presence of singular vortices in rotating /sup 3/He-A are reported. Our studies on the interaction between negative ions and vortex textures verify the existence of the continuous vortices seen earlier by NMR, but evidence for a new vortex state with high ion mobility along the core is also found. The nucleation of the new vortex depends on the external magnetic field and the rotation history of the sample.

First experiments suggesting the presence of singular vortices in rotating 3He–A are reported. We have studied the interaction between negative ions and vortices by utilizing the focusing of ions by vortices. Our results verify the existence of continuous vortices seen earlier by NMR, but evidence for a new vortex state with high ion mobility along the core was also found. The nucleation of the new vortex state depends on the external magnetic field and the rotation history of the sample.