Liquid 3He In Aerogel Research Articles

Effect of critical fluctuations on the spin transport in liquid He3

The contribution of pair fluctuations to the spin current in liquid 3He in aerogel near the critical temperature of transition to the superfluid state is calculated.

Drag Force on a High Porosity Aerogel in Liquid 3He

We discuss the drag force exerted on a high porosity aerogel in liquid 3He and its effect on sound propagation. The drag force is governed by distinct laws depending on the Knudsen number Kn, the ratio of the quasiparticle mean free path to a characteristic linear size of a body immersed in liquid. In the liquid 3He-aerogel system, the characteristic length scale is aerogel strand diameter and both of the Knudsen limit (Kn ≫ 1) and the hydrodynamic limit (Kn ≪ 1) can be achieved in the normal phase by varying temperature. We show that the crossover between the two limits can be found as a drastic change in the temperature dependence of sound attenuation in the normal phase of liquid 3He in aerogel.

Low temperature adsorption of 3He on silica aerogel surface and its influence on 3He spin kinetics

Significant influence of the aerogel surface heterogeneity on the processes of 3He nuclear magnetic relaxation at temperatures 1.5 − 4.2 K is discovered. This influence appears, for instance, in differences of the 3He T1 relaxation times for small portion of 3He, adsorbed at different temperatures. Binding energy data of 3He and distributions of this energy in two types of aerogel were obtained experimentally. Adsorbed 3He molecules with binding energies 60 – 250 K play supreme role in processes of nuclear magnetic relaxation of gaseous and liquid 3He in aerogel.

Theory of heat transport of normal liquid 3He in aerogel

The introduction of liquid 3He into a silica aerogel provides us with a model system in which to study the effects of disorder on the properties of a strongly correlated Fermi liquid. The transport of heat, mass and spin exhibits cross-over behavior from a high temperature regime, where inelastic scattering dominates, to a low temperature regime dominated by elastic scattering off the aerogel. We report exact and approximate solutions to the Boltzmann–Landau transport equation for the thermal conductivity of liquid 3He, including elastic scattering of quasi-particles by the aerogel and inelastic quasi-particle collisions. These results provide quantitative predictions on the transport properties of liquid 3He in aerogels over a wide range of pressures, temperatures and aerogel densities. In particular, we obtain a scaling function, F(T/T⋆), for the normalized thermal conductivity, , in terms of a reduced temperature, T/T⋆, where T⋆ is a cross-over temperature defined by the elastic and inelastic collision rates. Theoretical results are compared with the available experimental data for the thermal conductivity.

Ultrasound attenuation measurements in the A-like phase of superfluid 3He in aerogel

Liquid 3He impregnated in high porosity aerogel shows many interesting features: significant suppression of superfluid transition and substantial modification of the the A-like to B-like transition. Ultrasound has been found to be a sensitive tool to measure these changes. Recently, the ultrasound attenuation measurements done in the B-like phase provided strong evidence for gapless superfluidity in liquid 3He in aerogel. We conducted high frequency sound propagation measurements at 6.22 MHz in the A-like phase of superfluid 3He induced by magnetic fields up to 4.44 kG. The magnetic field is applied perpendicular to the direction of sound propagation to see the noticeable change in attenuation between the A-like and the B-like phases. We measured the sound attenuation in the A-like phase to be larger than in the B-like phase, and this observation enabled us to identify the A-like to B-like phase transition as a function of magnetic field and pressure.

Effect of correlated disorder on the temperature of unconventional cooper pairing: 3He in aerogel

Using liquid 3He in aerogel as an example, it is shown that correlations in positions of impurities affect the temperature $T_c$ of transition of Fermi liquid in an unconventional superfluid or superconductive state. The effect is significant if the correlation length for impurities is greater than the coherence length in the superfluid or superconductive state $\xi_0$. For 3He in aerogel the suppression of $T_c$ is expressed in terms of the structure factor of aerogel. With the account of the fractal structure of aerogel a simple expression is obtained for the decrease of $T_c$ from its clean value. This expression is in a satisfactory agreement with experimental data.

Superfluid 3He phases in aerogel

Within a phenomenological approach a criterion for a choice of forms of the order parameter of the superfluid phases of 3He in aerogel in a vicinity of the transition temperature is derived. The order parameter of bulk B-phase of 3He meets this criterion and that of the bulk A-phase doesn`t. A class of order parameters of Equal Spin Pairing (ESP) type meeting the derived criterion is discussed. Order parameters belonging to this class are proposed as candidates for the observed A-like superfluid phase of liquid 3He in aerogel. Effect of magnetic field on the order parameters of this class is considered.

Robust Superfluid Phases of3He in Aerogel

Within a phenomenological approach possible forms of the order parameter of the superfluid phases of 3He in the vicinity of the transition temperature are discussed. The effect of aerogel is described by a random tensor field interacting with the orbital part of the order parameter. With respect to their interaction with the random tensor field a group of “robust” order parameters which can maintain long-range order in the presence of the random field is specified. Robust order parameters, corresponding to Equal Spin Pairing (ESP) states are found and proposed as candidates for the observed A-like superfluid phase of liquid 3He in aerogel.

Transverse sound in liquid-3He-aerogel system.

Transverse sound in normal liquid 3He in aerogel is studied on the basis of the Landau transport equation taking into account the simultaneous oscillation of the aerogel. We show that the nature of transverse sound is strongly modified if aerogel is immersed in the liquid. Scattering of 3He quasiparticles by the aerogel molecules causes friction between the liquid and the aerogel, giving rise to coupled motion of the two systems. There exists a low-attenuation transverse sound mode in this coupled system. The high-temperature behavior of its attenuation alpha is given by alpha proportional, variant const+T-2, which is in contrast to alpha proportional, variant T in pure liquid 3He in the hydrodynamic regime.

Thermal Conductivity of Normal Liquid 3He in Aerogel

We report thermal conductivity measurements for normal phase liquid 3He inside 98 and 95% aerogels at dilution refrigerator temperatures (5 to 50 mK). In our measurements we use cylindrical samples of aerogel which have been fabricated inside quartz tubes. Each tube is enclosed at one end by a small chamber to which a known heat input is applied at a measured temperature. The other end is in an open volume of helium which is heat sunk to the refrigerator. Vibrating wire thermometry is used to measure the helium temperatures. The thermal conductivity measurements show clearly the effects of quasiparticle-“impurity” scattering, from which values of the mean free path are derived. These are 92 nm for the 98% aerogel and 56 nm for the 95% sample.

Flow of normal 3He in aerogel

We consider the flow of normal 3He in aerogel in the temperature regime T c⩽ T⪡ T F. A recent phenomenological treatment that predicted the transition from Drude to Hagen–Poiseuille flow at low T and low aerogel concentration, was restricted to small Knudsen numbers λ ∗/d ( λ ∗= quasiparticle mean free path of 3He in aerogel, d= sample dimension), the so-called slip regime. In this contribution we lift this restriction and present a variational calculation, valid at arbitrary Knudsen numbers. We calculate the slip length of liquid 3He in aerogel and predict the temperature dependence of the flow resistance in this general situation.

Transport properties of normal liquid 3He in aerogel

The impregnation of aerogel by 3He provides a new opportunity to study the effects of disorder on the properties of liquid 3He. The transport coefficients exhibit cross-over behaviour from the high-temperature regime, where inelastic 3He– 3He scattering dominates, to a low-temperature regime dominated by elastic scattering of 3He off the aerogel. Within the homogenous scattering model, analytical expressions for the thermal conductivity and viscosity can be derived and are shown in graphical form for 3He in aerogel.