D. Candela

NMR experiments on a three-dimensional vibrofluidized granular medium.

A three-dimensional granular system fluidized by vertical container vibrations was studied using pulsed field gradient NMR coupled with one-dimensional magnetic resonance imaging. The system consisted of mustard seeds vibrated vertically at 50 Hz, and the number of layers N(l)<or=4 was sufficiently low to achieve a nearly time-independent granular fluid. Using NMR, the vertical profiles of density and granular temperature were directly measured, along with the distributions of vertical and horizontal grain velocities. The velocity distributions showed modest deviations from Maxwell-Boltzmann statistics, except for the vertical velocity distribution near the sample bottom, which was highly skewed and non-Gaussian. Data taken for three values of N(l) and two dimensionless accelerations Gamma=15,18 were fitted to a hydrodynamic theory, which successfully models the density and temperature profiles away from the vibrating container bottom. A temperature inversion near the free upper surface is observed, in agreement with predictions based on the hydrodynamic parameter micro which is nonzero only in inelastic systems.

Applications of controlled-flow laser-polarized xenon gas to porous and granular media study

We report initial NMR studies of continuous flow laser-polarized xenon gas, both in unrestricted tubing, and in a model porous media. The study uses Pulsed Gradient Spin Echo-based techniques in the gas-phase, with the aim of obtaining more sophisticated information than just translational self-diffusion coefficients. Pulsed Gradient Echo studies of continuous flow laser-polarized xenon gas in unrestricted tubing indicate clear diffraction minima resulting from a wide distribution of velocities in the flow field. The maximum velocity experienced in the flow can be calculated from this minimum, and is seen to agree with the information from the complete velocity spectrum, or motion propagator, as well as previously published images. The susceptibility of gas flows to parameters such as gas mixture content, and hence viscosity, are observed in experiments aimed at identifying clear structural features from echo attenuation plots of gas flow in porous media. Gas-phase NMR scattering, or position correlation flow-diffraction, previously clearly seen in the echo attenuation data from laser-polarized xenon flowing through a 2 mm glass bead pack is not so clear in experiments using a different gas mixture. A propagator analysis shows most gas in the sample remains close to static, while a small portion moves through a presumably near-unimpeded path at high velocities.

Interstitial gas and density segregation of vertically vibrated granular media

We report experimental studies of the effect of interstitial gas on mass-density segregation in a vertically vibrated mixture of equal-sized bronze and glass spheres. Sufficiently strong vibration in the presence of interstitial gas induces vertical segregation into sharply separated bronze and glass layers. We find that the segregated steady state (i.e., bronze or glass layer on top) is a sensitive function of gas pressure and viscosity, as well as vibration frequency and amplitude. In particular, we identify distinct regimes of behavior that characterize the change from bronze-on-top to glass-on-top steady state.

New evidence for zero-temperature relaxation in a spin-polarized Fermi liquid

Spin-echo experiments are reported for 3He-4He solutions under extremely high B/T conditions, B=14.75 T and T>or=1.73 mK. The 3He concentration x(3) was adjusted close to the value x(c) approximately 3.8% at which the spin-rotation parameter muM0 vanishes. In this way the transverse and longitudinal spin-diffusion coefficients D( perpendicular ),D( parallel ) were measured while keeping |muM(0)|<1. It is found that the temperature dependence of D( perpendicular ) deviates strongly from 1/T(2), with anisotropy temperature T(a)=4.26(+0.18)(-0.44) mK. This value is close to the theoretical prediction for dilute solutions and suggests that spin current relaxation remains finite as the temperature tends to zero.

NMR study of the dynamics of 3He impurities in the proposed supersolid state of solid 4He.

The dynamics of (3)He atoms in solid (4)He have been investigated by measuring the NMR relaxation times T(1) and T(2) in the region where a significant nonclassical rotational inertia fraction has been reported. For (3)He concentrations x(3)=16 and 24 ppm, changes are observed for both the spin-lattice relaxation time T(1) and the spin-spin relaxation time T(2) at the temperatures corresponding to the onset of the nonclassical rotational inertia fraction and, at lower temperatures, to the (3)He-(4)He phase separation. The magnitudes of T(1) and T(2) at temperatures above the phase separation agree roughly with existing theory based on the tunneling of (3)He impurities in the elastic strain field due to isotopic mismatch. However, a distinct peak in T(1) and a less well-resolved feature in T(2) are observed near the reported nonclassical rotational inertia fraction onset temperature, in contrast to the temperature-independent relaxation times predicted by the tunneling theory.

Spin polarization of xenon films at low-temperature induced by 3He

Abstract We have measured the 129 Xe spin-lattice relaxation time T 1 for xenon films adsorbed on silica gel in an 8 T magnetic field at dilution refrigerator temperatures, both with and without 3 He filling the sample cell. Without 3 He , T 1 increases rapidly as the temperature is lowered. With 3 He , T 1 is considerably shortened, and is consistent with temperature-independent quantum relaxation. Using this technique, it is possible to brute-force polarize large quantities of xenon in high B / T conditions.

Nuclear Magnetism of Normal 3He and 3He-4He Mixtures in Aerogel

We report NMR experiments at 8 T on3He and3He-4He mixtures filling the pores of 95% porous aerogel, for temperatures T ≥ 6 mK. Magnetization measurements of pure3He reveal a localized layer approximately one monolayer thick. The longitudinal relaxation includes a component logarithmic in time, which is apparently associated with a fraction of the localized3He atoms. When the localized3He is displaced by adding4He the logarithmic relaxation disappears and T1for the dominant exponential relaxation increases. Measurements of the spin diffusion coefficient with the aerogel filled with dilute solution in equilibrium with bulk phase-separated mixture provide an unambiguous determination of the spin mean free path,ℓs = 58 nm

NMR investigation of the low-temperature dynamics of solid He-4 doped with He-3 impurities

The lattice dynamics of solid He-4 has been explored using pulsed NMR methods to study the motion of He-3 impurities in the temperature range (0.05-0.20 K) where experiments have revealed anomalies attributed to superflow or unexpected viscoelastic properties of the solid He-4 lattice. We report the results of measurements of the nuclear spin-lattice and spin-spin relaxation times that measure the fluctuation spectrum at high and low frequencies, respectively, of the He-3 motion that results from quantum tunneling in the He-4 matrix. The measurements were made for He-3 concentrations 16 < x(3) < 2000 ppm. For He-3 concentrations x(3) = 16 and 24 ppm, large changes are observed for both the spin-lattice relaxation time T-1 and the spin-spin relaxation time T-2 at temperatures close to those for which the anomalies are observed in measurements of torsional oscillator responses and the shear modulus. These changes in the NMR relaxation rates were not observed for higher He-3 concentrations.

Giant Viscosity Enhancement in a Spin-Polarized Fermi Liquid

The viscosity is measured for a Fermi liquid, a dilute 3He-4He mixture, under extremely high magnetic field/temperature conditions (B<or=14.8 T, T>or=1.5 mK). The spin-splitting energy microB is substantially greater than the Fermi energy kBTF; as a consequence the polarization tends to unity and s-wave quasiparticle scattering is suppressed for T<<TF. Using a novel composite vibrating-wire viscometer an enhancement of the viscosity is observed by a factor of more than 500 over its low-field value. Good agreement is found between the measured viscosity and theoretical predictions based upon a t-matrix formalism.

Transport experiments on dilute, spin-polarized Fermi fluids

A series of experiments has been carried out on very dilute3He-4He mixtures and on pure3He liquid at very high field/temperature ratios (H≤8 T, T≥4.3mK). In this regime these systems display a strong Leggett-Rice effect, weakly damped paramagnetic spin waves, and significant polarization effects on spin and momentum transport. The dilute mixture experiments used NMR to observe standing spin waves, and vibrating-wire viscometry to measure momentum transport. A very satisfactory agreement is found with the recent kinetic-equation calculations of Jeon and Mullin. The pure3He experiments may provide the first evidence for field-induced relaxation of transverse spin currents.