L. Pollack

Determination of the electric field gradient and relaxation time measurements in scandium metal at very low temperature

We present the results of measurements on a single crystal sample of scandium metal at temperatures down to 100 ΜK using nuclear quadrupole resonance (NQR). We find two regimes in the relaxation curves: an initial fast relaxation, followed by a slower relaxation consistent with the three exponential recovery expected for an I = 7/2 system in zero external magnetic field. The Korringa constant for this longer time relaxation in our sample is 90 +- 9 msec K−1. By observing deviations in the ratio of the intensities of adjacent nuclear spin transitions at the lowest attainable temperatures, we have been able to make a determination of the sign of the total electric field gradient present in the crystal. We find that the lowest energy state of the nuclear spin system corresponds to mI = +-7/2. A combination of these deviations and pulse NQR allows us to use this system as an absolute thermometer in the ΜKelvin regime.

Nqr experiments on Sc-Y alloys at very low temperatures

We have performed Nuclear Quadrupole Resonance (NQR) experiments on an alloy of scandium and yttrium. We find that the characteristic resonance frequencies in zero external field are 50 percent larger than in pure scandium. This increase in frequency makes the Sc-Y alloy a better candidate for absolute thermometry below 500 µK. However, the spin-lattice relaxation time of this alloy is more than an order of magnitude longer than in the pure scandium. In addition, we have observed an abrupt increase in the Korringa constant for temperatures below 5mK.

Determination of the sign of the electric field gradient in indium by nuclear resonance

We have studied the temperature dependence of two of the four magnetic-field perturbed NQR lines in indium. Our results indicate that the sign of the internal electric field gradient (EFG) is positive, in accord with the recent heat capacity measurements of the ISSP group and one recent theoretical prediction. We also discuss the implications of the sign of the EFG on the use of indium as an absolute thermometer below 1 mK.

The new cornell copper demagnetization stage

The persistent failure of a previous welded copper demagnetization stage to reach temperatures significantly below 100 microkelvin caused us to replace the original structure. The new stage has been machined from a single ingot of moderately high purity (4 9s) polycrystalline copper, with a much more rigid cross-section. Heat treatment increased the RRR of the copper to 4000. The cryostat is now capable of maintaining temperatures below 50 microkelvin for over a week. However there appear to be gradients between the thermometer and the copper nuclei in the main field, which may be a factor of 5 or more colder. Much of the improved performance comes from the identification and removal of an existing heat leak. The vibrational heating is dramatically smaller than that of the old stage, inferred from the magnetic field dependence of the heat leak.

Superfluidity of Pure 3He and Mixtures of 3He and 4He in Aerogel

We describe new torsional oscillator experiments on3He confined in 98.2% open aerogel. In one, we monitored the superfluid fraction of pure3He at T << Tcwhile we gradually changed the sample pressure. The resulting change in density alters ξ0of the superfluid3He relative to the distribution of the length scales (correlations) of silica in the aerogel. We observed a T = 0 normal-to-super fluid transition at a pressure of about 6.5 bar, in marked contrast to the bulk where liquid3He is superfluid at all pressures. In the second experiment, we measured the temperature dependence of the3He ρsat a pressure of 21.6 bar with different amounts of4He present in the cell. Adding 2-3%4He slightly increases both Tcand ρs. We found that for4He concentrations between 2% and 34%, the3He Tcincreases by a very small amount. However, ρs, which for pure3He in aerogel at 0.5Tcis no more than 11%, falls by another factor of 7. This behavior (constant Tc, reduced ρs) is similar to that observed in granular superconducting films where the long-range order is controlled by phase coherence between adjacent grains.

Field dependence of cross relaxation in scandium (I=7/2)

Abstract We are studying the magnetic field dependence of the cross relaxation mechanism that couples the equal energy transitions ±5/2↔±7/2 and ±5/2↔±1/2 through a second order dipolar interaction. Application of a small magnetic field, ≈ 10 Gauss, changes the characteristic shape of the magnetization recovery curves. We attribute this change to the destruction of the cross relaxation since the condition of exact energy equality is no longer satisfied in the presence of such a magnetic field.

An experiment to measure the effect of magnetic fields on the superfluid fraction and transition temperature of3He in aerogel

We present preliminary results from a torsional oscillator experiment designed to measure the superfluid fraction and transition temperature of3He in aerogel in magnetic fields up to 0.4 T.