Inseob Hahn

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.

Thermodynamic magnetization discontinuity between the A and B phases of superfluid3he

We present data for the magnitude of the thermodynamic magnetization discontinuity at the equilibrium A-B transition in superfluid3He, ΔMAB, over its entire pressure-temperature-magnetic field phase diagram. The data was taken by measuring flux changes in a SQUID pick-up coil, which was calibrated by a novel first-principles technique. We compare our results with those of NMR experiments, and find that the two types of measurements yield different values, and this discrepancy is not the result of thermometry or field errors.

Unresolved problems in the superfluid phases of3He

The early studies of the superfluid phases of3He were marked by often spectacular agreements between theoretical predictions and experimental results. Nevertheless, several experiments have failed to achieve either agreement with the prevailing theory or consistency with each other — and improved experiments do not remove these discrepancies. We will discuss two recent experiments at U.S.C. which indicate that there remain fundamental questions beyond experimental uncertainties. One of these is the stability of superflow in a complex texture in the A-phase. Experimental evidence indicates that superflow may actually be stabilized by the existence of a texture. The other is the long-standing discrepancy between static (SQUID) and dynamic (NMR) measurements of the magnetization discontinuity at the A-B transition. Our recent experiments using a first principles measurement of ΔM support the continued existence of the discrepancy.

Magnetization differences at the A-B transition of superfluid3He

Abstract We report results from a new SQUID-based measurement of the total magnetization change of superfluid 3 He at the A-B phase transition, ΔM AB total . We used a simple first-principles calibration technique. At low fields ∼0.03 T, our data agree with past measurements, verifying the long-standing discrepancy between ΔM AB total and ΔM AB nuclear , the magnetization change which is measured by NMR. Use of a self-shielded solenoid allowed the first measurement of ΔM AB total over the full range of magnetic field. We found the magnetization discrepancy to persist through the whole phase diagram.

Radially shielded superconducting magnets for use in cryostats

Three small superconducting magnets with active and passive shields that produce confined magnetic fields in low temperature cryostats have been designed and built. The design of these magnets uses both on and off-axis field calculations which are fed back to an optimization algorithm. These design principles and construction techniques are discussed.

Magnetic field dependence of the superfluid 3He AB transition

Abstract We are measuring the boundary between the superfluid A and B phases of 3He as a function of magnetic field and pressure. The results yield new information on Landau-Ginzburg parameters, microscopic quasiparticle scattering amplitudes, superfluid specific heats, and potentially other normal liquid properties through comparison with models of the superfluid.

Digital bridge for ultralow temperature thermometry

Abstract A transparent replacement for the usual set of precision ratio transformers within ultralow temperature thermometry systems is described. This new device uses digital electronics to both lower costs and to provide local intelligence for greater flexibility. The device has been used in our experiments on superfluid 3He without the difficulties usually attendant with interfacing computers to ultralow temperature apparatuses.