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

Menger sponge-like fractal body created by a novel template method

We have established experimental strategies on how to create a Menger sponge-like fractal body and how to control its fractal dimension. The essence was to utilize alkylketene dimer (AKD), which spontaneously forms super-water-repellent fractal surface. We prepared "fractal AKD particles" with fractal surface structure as templates of pores in fractal body. The fractal body was synthesized by filling the remained space between the packed template particles with a tetramethyl orthosilicate solution, solidifying it by the sol-gel process, and removing the template by calcinations. We have succeeded in systematically creating fractal bodies of silica with different cross-sectional fractal dimensions D(cs)=1.87, 1.84, and 1.80 using "fractal template particles" compressed under the ratio=1.0, 2.0, and 3.0, respectively. We also discussed the possibilities of their fractal geometries in comparison with mathematical models. We concluded that the created fractal bodies were close to a Menger sponge and its modified one. Our experimental strategy allows us to design fractality of porous materials.

NMR measurements of hyperpolarized He3 gas diffusion in high porosity silica aerogels

Hyperpolarized 3He is used to non-destructively probe by NMR the structure of custom-made and commercial silica aerogels (97% and 98.5% porous). Large spin-echo signals are obtained at room temperature and very low magnetic field (2 mT) even with small amounts of gas. Attenuation induced by applied field gradients results from the combined effects of gas diffusion and confinement by the porous medium on atomic motion. Nitrogen is used as a buffer gas to reach equivalent 3He pressures ranging from 5 mbars to 3.5 bars. The observed pressure dependence suggests a nonuniform structure of the aerogels on length scales up to tens of micrometers. A description by broad phenomenological distributions of mean free paths is proposed, and quantitatively discussed by comparison to numerical calculations. The investigated aerogel samples exhibit different effective diffusion characteristics despite comparable nominal porosities.

Defect production behind the shock wave front of an inhomogeneous quench

The creation of defects behind the half kink in the presence of external force distributions is considered. The influence of external potentials on kink production behind the shock wave front of an inhomogeneous quench is examined. It is shown that depending on the impurity strength and orientation its interaction with the front of the decaying false vacuum, even in homogenous systems, may lead to single or even multiple defect production in the vicinity of the impurity center.

Local mean-field study of capillary condensation in silica aerogels

We apply local mean-field (i.e., density functional) theory to a lattice model of a fluid in contact with a dilute, disordered gel network. The gel structure is described by a diffusion-limited cluster aggregation model. We focus on the influence of porosity on both the hysteretic and the equilibrium behavior of the fluid as one varies the chemical potential at low temperature. We show that the shape of the hysteresis loop changes from smooth to rectangular as the porosity increases and that this change is associated with disorder-induced out-of-equilibrium phase transitions that differ in adsorption and in desorption. Our results provide insight in the behavior of 4He in silica aerogels.

Kink production in the presence of random distributed impurities

Production of kinks during a quench in an overdamped regime of phi(4) model is investigated. An influence of random distributed impurities on defect production is studied.

Kink production in the presence of impurities

The production of kinks during a quench in an overdamped regime of phi(4) model is investigated. Expelling defects from regions of nonzero force coming from the impurity are predicted.

Modification of the superfluid 3He phase diagram by impurity scattering

The effect of impurity scattering on the phase diagram of pure superfluid 3He has been investigated by acoustic techniques near the bulk polycritical pressure. Impurities were introduced with a 98% porous silica aerogel. In zero applied field, the equilibrium phase is a B phase, consistent with predictions from isotropic scattering. The superfluid transition in a magnetic field at 25 bars is from normal to A phase and is independent of the magnetic field up to 2.9 kG. A first-order phase transition between A and B superfluids is observed with surprisingly strong supercooling in both zero and nonzero applied field.

Observation of a superfluid He-3 A- B phase transition in silica aerogel

New NMR studies of 3He in high-porosity aerogel reveal a phase transition from an A-like to a B-like phase on cooling. The evidence includes frequency shift and magnetic susceptibility data, and similar behavior is found in two quite different aerogel samples. The A-like phase is stable only very near to T(c) but can be supercooled to below 0.8T(c). This behavior has been seen clearly at 32- and 24-bar pressures, and the presence of negative frequency shifts suggests that an A-like phase exists near T(c) at pressures as low as 12 bars in a magnetic field of 28.4 mT.