ELSEVIER Physica B 210 (1995) 311 333 PHYSICA[ Measurements on the vortex sheet in rotating superfluid 3He-A fJ. Parts*, V.M.H. Ruutu, J.H. Koivuniemi, M. Krusius, E.V. Thuneberg, G.E. Volovik 1 Low Temperature Laboratory, Helsinld University of Technology, 02150 Espoo, Finland Abstract When a cylindrical container filled with superfluid 3He-A is rotated around its symmetry axis, several different configurations of quantized vorticity are possible: which of them will be preferred depends on the specifics how the rotating state is formed. The most unusual is the vortex sheet, a domain wall in the order parameter texture into which vortex lines are confined. This metastable structure has the lowest critical velocity of formation if a domain wall with the appropriate orientation is already present in the container. In this case the vortex sheet becomes the preferred rotating state which provides the solid-body rotation of the superfluid component on an averaged scale. Its presence can be identified from the cw NMR spectrum which samples the order parameter texture. Here the experimental properties of the vortex sheet are reviewed, as deduced from NMR measurements. 1. Introduction Superfluid 3He-A is currently the best experimentally accessible example of a superfluid or a superconductor, where the topology of various types of objects of the order parameter field can be investigated. The most eas- ily studied objects are vortices. They can be created in large numbers simply by rotating the liquid because they constitute the equilibrium state of the rotating superfluid. Many different vortex structures exist in 3He-A: they can be "singular" or "continuous", "dipole locked" or "dipole unlocked", and "singly quantized" or "multiply quan- tized". Their structure can be studied with NMR spectro- scopy or from the transmission properties of ultra sound, which both give information on the order parameter texture. *Corresponding author. 1Permanent address: Landau Institute for Theoretical Physics, 117334 Moscow, Russian Federation. A striking new form of vorticity in 3He-A is the vortex sheet (Fig. 1). It is a combined object consisting of a do- main wall into which special types of vortex lines are stacked as a linear array. The vortices do not easily escape from the domain wall, and in order to distribute them as uniformly as possible in a rotating container, the sheet is arranged in equidistant folds. The sheet has higher energy than separated vortex lines at low angular velocity (t2 < 3 rad/s). Nevertheless, it appears as a meta- stable object. If a suitably oriented domain wall already exists in the container, vortex quanta are more easily created at its contact lines with the container wall and are added to the sheet, rather than nucleated in any form of separate vortex lines. In this report we focus on the experimental properties of the vortex sheet, as they emerge from our NMR measurements. Indications for its existence have been seen earlier [-1], but only now these observations have been interpreted and correlated with a new structure of vorticity I-2]. Theoretically, the existence of the vortex sheet seems obvious, and its experimental identification 0921-4526/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0921-4526(94)01 1 16-8