Superflow-Stabilized Nonlinear NMR in Rotating 3He-B

Nonlinear spin precession has been observed in 3He-B in large counterflow of the normal and superfluid fractions. The new precessing state is stabilized at high rf excitation level and displays frequency-locked precession over a large range of frequency shifts, with the magnetization at its equilibrium value. Comparison to analytical and numerical calculation indicates that in this state the orbital angular momentum L of the Cooper pairs is oriented transverse to the external magnetic field in a ``non-Leggett'' configuration with broken spin-orbit coupling. The resonance shift depends on the tipping angle theta of the magnetization as omega - omega_L = (Omega_B^2 / 2 omega_L)(cos(theta) - 1/5). The phase diagram of the precessing modes with arbitrary orientation of L is constructed.Comment: Revtex file, 5 pages, 4 figures, version submitted to Phys. Rev. Let

Superconducting Nb-film LC resonator

Sputtered Nb thin-film LC resonators for low frequencies at 0.5 MHz have been fabricated and tested in the temperature range 0.05--1 K in magnetic fields up to 30 mT. Their Q value increases towards decreasing temperature as sqrt(T) and reaches 10^3 at 0.05 K. As a function of magnetic field Q is unstable and displays variations up to 50%, which are reproducible from one field sweep to the next. These instabilities are attributed to dielectric losses in the plasma deposited SiO_2 insulation layer, since the thin-film coil alone reaches a Q > 10^5 at 0.05 K.Comment: 6 pages, 7 figures, submitted to Review of Scientific Instrument

Vortex lines or sheets - what is formed in dynamic drives?

In isotropic macroscopic quantum systems vortex lines can be formed while in anisotropic systems also vortex sheets are possible. Based on measurements of superfluid 3He-A, we present the principles which select between these two competing forms of quantized vorticity: sheets displace lines if the frequency of the external field exceeds a critical limit. The resulting topologically stable state consists of multiple vortex sheets and has much faster dynamics than the state with vortex lines.Comment: RevTex, 5 pages, sumbitted to Phys. Rev. Let

Coexistence of Single and Double-Quantum Vortex Lines

We discuss the configurations in which singly and doubly quantized vortex lines may coexist in a rotating superfluid. General principles of energy minimization lead to the conclusion that in equilibrium the two vortex species segregate within a cylindrical vortex cluster in two coaxial domains where the singly quantized lines are in the outer annular region. This is confirmed with simulation calculations on discrete vortex lines. Experimentally the coexistence can be studied in rotating superfluid $^3$He-A. With cw NMR techniques we find the radial distribution of the two vortex species to depend on how the cluster is prepared: (i) By cooling through $T_c$ in rotation, coexistence in the minimum energy configuration is confirmed. (ii) A glassy agglomerate is formed if one starts with an equilibrium cluster of single-quantum vortex lines and adds to it sequentially double-quantum lines, by increasing the rotation velocity in the superfluid state. This proves that the energy barriers, which separate different cluster configurations, are too high for metastabilities to anneal.Comment: 12 pages, 11 figures; Changed content, 15 pages, 14 figure

Double-quantum vortex in superfluid He-3-A

Linear defects are generic in continuous media(1). In quantum systems they appear as topological line defects which are associated with a circulating persistent current. In relativistic quantum field theories they are known as cosmic strings(2), in superconductors as quantized flux lines(3), and in superfluids(3,4) and low-density Bose-Einstein condensates(5) as quantized vortex lines. A conventional quantized vortex Line consists of a central core around which the phase of the order parameter winds by 2 pi n, while within the core the order parameter vanishes or is depleted from the bulk value. Usually vortices are singly quantized (that is, have n = 1). But it has been theoretically predicted that, in superfluid He-3-A, vortex lines are possible that have n = 2 and continuous structure, so that the orientation of the multicomponent order parameter changes smoothly throughout the vortex while the amplitude remains constant. Here we report direct proof, based on high-resolution nuclear magnetic resonance measurements, that the most common vortex line in He-3-A has n = 2. One vortex line after another is observed to form in a regular periodic process, similar to a phase-slip in the Josephson effect

Shear Flow and Kelvin-Helmholtz Instability in Superfluids

The first realization of instabilities in the shear flow between two superfluids is examined. The interface separating the A and B phases of superfluid 3He is magnetically stabilized. With uniform rotation we create a state with discontinuous tangential velocities at the interface, supported by the difference in quantized vorticity in the two phases. This state remains stable and nondissipative to high relative velocities, but finally undergoes an instability when an interfacial mode is excited and some vortices cross the phase boundary. The measured properties of the instability are consistent with the classic Kelvin-Helmholtz theory when modified for two-fluid hydrodynamics