J. S. Korhonen

SINGLE-VORTEX NUCLEATION IN ROTATING SUPERFLUID 3HE-B

Nucleation of vortices in units of one quantum has been observed with c.w. NMR in a rotating cylinder filled with 3He-B. During acceleration a new vortex is created each time the counterflow velocity at the perimeter reaches a critical value vc(T, p). The measured vc resembles the calculated velocity vcb(T, p) of the bulk superflow instability, but is smaller by a factor with power law dependence on the superfluid coherence length. This indicates that a nucleation event takes place whenever the flow exceeds vcb locally at the nucleation site and the nucleation barrier vanishes.

Gap anisotropy, vortex cluster, and nucleation of vortices in rotating 3He-B

Abstract 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.

Slow Mode and Pinning in a Vortex Array of Rotating 3He-B

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.

Interaction of vortices with the homogeneously precessing resonance mode in superfluid 3He‐B

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Studies with coherently precessing magnetization on counterflow and vortices in rotating3He-B

The homogeneously precessing domain (HPD) is a coherent dynamic order parameter state of 3He-B, which originates from the stiffness of the spin part of the order parameter. We have investigated the HPD state in the presence of vortex-free counterflow as a function of the rotation velocity ω. The energy difference between the orbital textures across the domain boundary leads to a shift of the boundary, such that the precessing domain shrinks ∝ω2. In the equilibrium vortex state no such shift is observed. By monitoring the shift vs ω we have studied the B phase superfluid density anisotropy in a magnetic field, the nucleation of quantized vortex lines at the side wall of the rotating container, and the spatial distribution of counterflow and vortices, when the number of vortices is less than in equilibrium.

Quantized vortices at a moving A-B interface in superfluid3He

Abstract 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.

Vortex clusters in superfluid 3He-B

Abstract 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.

Evidence for Soft Vortex Cores in 3He-B at High Magnetic Fields

Negative ion mobility in rotating 3He-B at magnetic fields H 0.1 T has been measured along the common direction of H and the rotation axis Ω. When Ω > 0, we observed an increase of mobility, which originates from a reorientation of the anisotropic energy gap. The increase can be understood quantitatively only if soft vortex cores, predicted by Sonin, are assumed. Their size was found to be on the order of the magnetic Healing length ξH.

Coupling of NMR absorption to the motion of nonaxisymmetric vortices in rotating3He-B

Abstract 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.

Combined ion and nmr measurements on rotating 3HeA

Abstract We have constructed an experimental chamber permitting simultaneous observation of both the transverse cw-NMR spectrum and the time-of-flight of negative ions from a rotating sample of vortices in 3He. Our measurements in the A-phase confirm that the vortex state, causing the large delay and strong deformation of the ion pulses drifting parallel to the rotation axis, is also responsible for the satellite peak corresponding to continuos vortices in the NMR spectrum. Unfortunately, singular vortices, whose distinct ion response was detected earlier, could not be observed in the present setup.