N. Mulders

Low Velocity Quantum Reflection of Bose-Einstein Condensates

We study how interactions affect the quantum reflection of Bose-Einstein condensates. A patterned silicon surface with a square array of pillars resulted in high reflection probabilities. For incident velocities greater than 2.5 mm/s, our observations agreed with single-particle theory. At velocities below 2.5 mm/s, the measured reflection probability saturated near 60% rather than increasing towards unity as predicted by the accepted theoretical model. We extend the theory of quantum reflection to account for the mean-field interactions of a condensate which suppresses quantum reflection at low velocity. The reflected condensates show collective excitations as recently predicted.

Orbital glass and spin glass states of 3He-A in aerogel

AbstractGlass states of superfluid A-like phase of 3He in aerogel induced by random orientations of aerogel strands are investigated theoretically and experimentally. In anisotropic aerogel with stretching deformation two glass phases are observed. Both phases represent the anisotropic glass of the orbital ferromagnetic vector Ηthe orbital glass (OG). The phases differ by the spin structure: the spin nematic vector

Phase diagram of the superfluid phases of 3 He in 98% aerogel

\hat d

Torsional oscillator and synchrotron X-Ray experiments on solid 4He in aerogel.

can be either in the ordered spin nematic (SN) state or in the disordered spin-glass (SG) state. The first phase (OG-SN) is formed under conventional cooling from normal 3He. The second phase (OG-SG) is metastable, being obtained by cooling through the superfluid transition temperature, when large enough resonant continuous radio-frequency excitation is applied. NMR signature of different phases allows us to measure the parameter of the global anisotropy of the orbital glass induced by deformation.

A-B transition of superfluid {sup 3}He in aerogel and the effect of anisotropic scattering

An important theme that threads through many areas of current interest in condensed matter physics is the effect of randomness and disorder. Prior to the 1960s, disorder and impurities were often viewed as unavoidable nuisances that masked the true behavior of ideal systems. We have since learned that disorder itself can bring forth fascinating and often unexpected new phenomena in condensed phases of matter. (See the December 1988 special issue of PHYSICS TODAY, dedicated to disordered solids.)

Scaling of the superfluid fraction and T(c) of 3He in aerogel

The phase diagram of the superfluid phases of 3 He in 98% aerogel was determined in the range of pressure from 15 to 33 bars and for fields up to 3 kG using high-frequency sound. The superfluid transition in aerogel at 33.4 bars is field independent from 0 to 5 kG and shows no evidence of an A 1 -A 2 splitting. The first-order transition between the A and B phases is suppressed by a magnetic field, and exhibits strong supercooling at high pressures. We show that the equilibrium phase in zero applied field is the B phase with at most a region of A phase ≤20 μK just below T c at a pressure of 33.4 bars. This is in contrast to pure 3 He which has a large stable region of A phase and a polycritical point. The quadratic coefficient for magnetic-field suppression of the AB transition, g a (β), was obtained. The pressure dependence of g a (β) is markedly different from that to the pure superfluid, g 0 (β), which diverges at a polycritical pressure of 21 bars. We compare our results with calculations from the homogeneous scattering model for g a (β), defined in a Ginzburg-Landau theory in terms of strong-coupling parameters β. We find qualitatively good agreement with the experiment if the strong-coupling corrections are rescaled from known values of the βs for pure 3 He, reduced by the suppression of the superfluid transition temperature. The calculations indicate that the polycritical pressure in the aerogel system is displaced well above the melting pressure and out of experimental reach. We cannot account for the puzzling supercooling of the aerogel AB transition in zero applied field within the framework of known nucleation scenarios.

Acoustic properties of silica aerogels between 400 mK and 400 K

The properties of liquid 3He in a low-density aerogel preliminarily covered with a few monolayers of 4He were studied by pulsed and nonlinear CW NMR techniques. It was found that an NMR frequency shift from the Larmor value exhibits a sharp increase at a magnetization tilting angle exceeding 104°. Nonlinear CW NMR signals related to the formation of a macroscopic region featuring homogeneous precession of the magnetization (homogeneous precession domain) were observed. The experimental results confirm that the low-temperature superfluid 3He phase in the aerogel is analogous to the B-phase in bulk 3He and indicate that the spin supercurrents play an important role in the spin dynamics of superfluid 3He in aerogel.

Nuclear magnetic relaxation of 3He in contact with an aerogel above the Fermi temperature

X-ray diffraction experiments show that solid 4He grown in aerogel is highly polycrystalline, with an hcp crystal structure (as in bulk) and a crystallite size of approximately 100 nm. In contrast to the expectation that the highly disordered solid will have a large supersolid fraction, torsional oscillator measurements show a behavior that is strikingly similar to high purity crystals grown from the superfluid phase. The low temperature supersolid fraction is only approximately 3 x 10(-4), and the onset temperature is approximately 100 mK.