C. M. Gould

Experimental studies of solitons in superfluid 3He-A

Magnetic textures have been created and studied in the A phase of superfluid 3He at several pressures, in magnetic fields from 15 to 20 mT, by intense rf magnetic pulses which invert the magnetization of the fluid. To create this new state we find that the initial large pulse must tip the magnetization by at least 140‡. In this state the fluid evolves from a condition where the NMR frequency shift Ω-γH0is smaller than that in the unperturbed superfluid by a factor of 0.54, over a time scale of tens of seconds to a condition where the frequency shift is 0.70 times the unperturbed value. We have studied the fluids linear response to small-angle sampling pulses as well as the nonlinear response to large-angle pulses. By using two independent NMR coils we have seen this new state move through the fluid; we find that a lower limit on its propagation velocity is around one-half the spin wave velocity. All of our results are generally consistent with the model of solitons in the order parameter proposed by Maki and co-workers.

Is the A phase really “the A phase”?

The conventional wisdom regarding the identification of the microscopic order parameter of the A phase of superfluid 3He as the axial state is questioned. Examination of current experimental evidence reveals an inability to discriminate between the alternative models of the axial and axi-planar states. The theoretical basis for the conventional wisdom is shown to disagree with experiment.

Frequency pulling and heterodyning observed in voltage spectrum of the Fröhlich conductor NbSe3

Abstract The voltage power spectrum of NbSe 3 has been studied in the upper charge-density wave (CDW) state where only one CDW exists. In the non-Ohmic regime sharp spikes appear in the spectrum. The frequency of these spikes increases monotonically with current as in the lower CDW state. We study the “fine structure” of the spectrum and find evidence for “frequency pulling” between the modes. A linear coupled-mode treatment is used to model this effect. There is also evidence of heterodyning of an intrinsic (current independent) 2MHz mode. We briefly discuss these phenomena in terms of Frohlich sliding conductivity. Difficulties with the simple two-fluid model as well as models based on solid state turbulence are pointed out.

High‐field magnet for low‐temperature low‐field cryostats

A small superconducting magnet with a large highly confined field has been designed for ultralow‐temperature cryostats. The magnet provides a central field of 3 T, with a homogeneity of 0.1% over 7.3 mm and negligible hysteresis, while the field at a radial distance of 2 cm is zero. The confinement is accomplished by using the magnet wire itself to simulate the current distribution of a perfect type I superconducting shield.

Ferromagnetism of 3He films in the low-field limit.

We provide evidence for a finite-temperature ferromagnetic transition in two dimensions as H -->0 in thin films of 3He on graphite, a model system for the study of two-dimensional magnetism. We perform pulsed and cw NMR experiments at fields of 0.03-0.48 mT on 3He at areal densities of 20.5-24.2 atoms/nm(2). At these densities, the second layer of 3He has a strongly ferromagnetic tendency. With decreasing temperature, we find a rapid onset of magnetization that becomes independent of the applied field at temperatures in the vicinity of 1 mK. Both the dipolar field and the NMR linewidth grow rapidly as well, which is consistent with a large (order unity) polarization of the 3He spins.

Zero-sound determination of texture transitions in flowing superfluid3He-A

The conditions have been measured for which the texture of the order parameter of superfluid3He-A is uniform and aligned with an applied superflow. At finite superflows, a finite parallel magnetic field is required to disrupt the uniform aligned texture. The shape of the uniform textures phase boundary in avs-H plane is similar to theoretical predictions, for which the state just outside of the phase boundary is a helical texture. However, substantial disagreements with the theory remain.

Phase diagram of the A-B transition of superfluid3He

Abstract We have measured the A-B phase transition temperature of superfluid3He at pressures from zero to 29 bars, and in all magnetic fields up to the highest field limit of the B phase (∼0.59 Tesla). We have also devised a simple function which accurately describes the entire pressure-temperature-magnetic field phase diagram of the A-B transition. This function intrinsically satisfies all correct asymptotic behaviors in the low- and high-field limits. Using thermodynamic identities we can measure properties which previously could not be directly measured.

Breakdown of 3He-A superflow in low magnetic fields

Using a sensitive differential pressure gauge, we have measured the limits to dissipationless superflow in bulk superfluid 3He-A far from stabilizing walls. We report the first quantitative measurements of the breakdown of bulk superflow in the A phase. In addition to confirming the existence of a finite critical current in bulk uniform textures, we have discovered an unexpected tortuous texture in which there is an even larger critical current. These results indicate that the current theoretical models for determining textures are incomplete.

1 MHz bandwidth true NMR SQUID amplifier

We have developed an integrated dc SQUID magnetometer with additional positive feedback (APF) for low frequency true NMR applications. The APF scheme allows direct coupled read out from the SQUID to room temperature electronics and eliminates the need for the conventional modulation scheme, thereby greatly simplifying the flux-locked loop electronics. We have configured our SQUID system for the specific needs of sensitive NMR measurements which include large bandwidth and high slew rate. We have achieved a bandwidth of 1.2 MHz and a slew rate greater than 105Φ0/s for frequencies above 10 kHz.

Thermodynamic Magnetization of Normal and Superfluid 3He

We have measured the thermodynamic magnetization in the various phases of normal and superfluid3He. The A-B discontinuity differs from that inferred from nuclear-specific (NMR) experiments. We offer reasons why this apparent disagreement can be consistent with our current picture of super-fluid3He. One consequence of this measurement is an improved set of values for the phenomenological Landau-Ginzburg free energy β-parameters.