O. Buu

Quartz Tuning Fork Viscometers for Helium Liquids

Mechanical resonators, in the form of vibrating wires or torsional oscillators, have long been employed as sensors in liquid 3He and 3He–4He mixtures. The damping of resonators is due to the viscosity of the surrounding liquid which is a strong, well-known function of temperature for bulk Fermi liquids. It is therefore possible to use the viscous damping for thermometry in the millikelvin regime. An alternative sensor is the small quartz tuning fork which is driven by the piezoelectric effect and requires no external magnetic field. In this paper, we present measurements of the viscous damping of such a tuning fork when immersed in a 6.2% 3He–4He mixture, between 3 and 100 mK, and at zero and high (10 T) magnetic field. The measurements indicate that damping of the tuning fork resonance is dominated by the liquid helium properties and is insensitive to the applied magnetic field. The response of the tuning fork to the saturated helium mixture demonstrates that it could potentially be used for thermometry in any magnetic field. There is evidence of slip at the interface between the fork and the helium suggesting specular scattering from the smooth surface of the quartz. The fork is also able to detect the superfluid transition in pure liquid 3He.

Transverse Spin Diffusion in 3He-4He Mixtures—Part I

There is currently a debate on whether or not the transverse relaxation time is finite at low temperature in spin-polarized Fermi liquids. The experimental results so far have been contradictory. In this paper, the first in a series of two articles, we present NMR measurements carried out in 6.2% 3He-4He mixture at 11.3 T down to 3.5 mK. A careful analysis of our data yields an anisotropy temperature Ta = 6.1±1 mK, which is smaller than the values reported previously. The difference between this value and the earlier results is ascribed to restricted diffusion effects which were neglected in the previous analyses. The details of the analysis are given in the companion paper.

Non-linear effects from the dipolar demagnetising field in 3He at very high magnetic field

Abstract We present NMR measurements in 3He at 150 mK in a high magnetic field leading to non-linear effects caused by the dipolar demagnetisation field. We report the first observation of long-lived free induction decays and provide a theoretical interpretation.

An experiment to measure the thermal conductivity of polarized liquid 3He

Abstract The thermal conductivity of liquid 3He is expected to be sensitive to its nuclear polarization, as the Pauli principle restricts the phase space available for collisions. We demonstrate that a vibrating wire viscometer can be used both as a heater (through viscous heating) and a thermometer to measure this thermal conductivity. We report a preliminary experiment on the polarized liquid, which shows an increase of the conductivity upon polarization.

Polarization Dependence of the Specific Heat and the Origin of the Effective Mass in Liquid 3He

The enhanced effective mass of liquid 3He has been alternatively ascribed to the large spin fluctuations in this system (nearly ferromagnetic model) or to local correlations effects (nearly localized model). We recently measured that polarizing liquid 3He decreases its specific heat. We present our experiment, and discuss its implications in this theoretical context.

Polarization Dependence of the Viscosity of Liquid 3He: A Comparison Between Experiments and Theory

Our recent measurements of the viscosity of spin polarized liquid 3He revealed that, below 100mK, the relative enhancement of the viscosity is pressure independent and quadratic in polarization m up to m=70%. This simple behavior is remarkably close to that expected for a gas (i.e. isotropic scattering). For the dense liquid 3He, the so-called s-p approximation is believed to give a more realistic description of scattering. Using the formalism developed by Anderson, Pethick and Quader, we have computed the expected polarization dependence in this case, assuming polarization independent collision probabilities. Surprisingly, this leads to an initial linear decrease of the viscosity with polarization, which is not observed experimentally. This may imply that the present description of transport in liquid 3He is not complete.

Spin Diffusion Anisotropy in Liquid 3He

We report new experiments on the transverse and longitudinal spin dynamics of liquid 3He polarized by an 11.3T field. At the lowest temperatures probed, the transverse spin diffusion coefficient saturates with an apparent anisotropy temperature of Ta=12±2 mK. This temperature dependence is in accord with the general idea of spin-diffusion anisotropy but the value of Ta is half that expected from extrapolating earlier experiments at a lower field. Strong magnetostatic effects in the form of long-lived spin modes are also observed for small gradients and small tipping angles.

Spin-wave modes and spin diffusion anisotropy in a 3He-4He mixture at high B/T

We present nmr measurements in a 6.2% 3He-4He mixture carried out at 11.3 T with temperatures down to 3.5 mK, yielding a very high B/T ratio. We have measured the transverse spin diffusion with the spin-echo technique. We observe a saturation of the spin diffusion coefficient at low temperature corresponding to an anisotropy temperature Ta≃13mK. Using the same setup, we were able to excite Silin spin-wave modes in the experimental cell.

The Effect of the Demagnetizing Field on the NMR Spectra for Liquids Enclosed in a Cylinder

The NMR spectrum of a simple liquid in a cell of size L depends on D, the diffusion constant, G, the applied field gradient, and M0, the z-component of magnetization before tipping of the spins. For small tipping angles the shape of the spectrum depends on which of the corresponding frequency scales—ωD = D/L2,ωG = γ FL, and ωM = γ μ0M0—is the largest. We explore the evolution of the spectrum between the inhomogeneous broadening regime (ωG ≫ ωD,ωM) and the regime where the spin dynamics is dominated by magnetostatic modes (ωM ≫ ωD,ωG)for a liquid confined in a cylinder of length 2L, both for classical liquids, and for liquids which exhibit the Leggett–Rice effect.