E. N. Smith

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.

Pulsed NMR studies of superfluid3He

We present a review of our pulsed NMR experiments in the A and B phases of superfluid3He, at pressures of 23 and 27 bar. We have measured the dipole frequency in3He-A, spin susceptibility in3He-B, and tip-angle-dependent frequency shifts in both phases. In general, we find excellent agreement with results by other workers. In3He-B, complex spin motions were observed involving long-lived free induction decays with a time-dependent precession frequency. Spin relaxation measurements for both3He-A and3He-B are discussed and compared with those of other workers. After driving the magnetization far from equilibrium, anomalous frequency shifts were found to persist in the A phase. We discuss a soliton-texture model by Bruinsma and Maki which accounts for these resonances.

Evidence for a metastable mode of superfluid 3HeA

Abstract A metastable mode of 3 HeA has been observed using pulsed NMR techniques. The frequency of the mode is related to the previous cw NMR observation of a transverse satellite resonance.

Modification of the He3 phase diagram by anisotropic disorder

Motivated by the recent prediction that uniaxially compressed aerogel can stabilize the anisotropic A phase over the isotropic B phase, we measure the pressure dependent superfluid fraction of (3)He entrained in 10% axially compressed, 98% porous aerogel. We observe that a broad region of the temperature-pressure phase diagram is occupied by the metastable A phase. The reappearance of the A phase on warming from the B phase, before superfluidity is extinguished at T(c), is in contrast to its absence in uncompressed aerogel. The phase diagram is modified from that of pure (3)He, with the disappearance of the polycritical point (PCP) and the appearance of a region of A phase extending below the PCP of bulk (3)He, even in zero applied magnetic field. The expected alignment of the A phase texture by compression is not observed.

Thermal and electrical conductance across copper joints below 100mK

Abstract We have measured the thermal conductance of copper and gold plated copper contact joints from 20mK to 100mK. We have measured the electrical conductance of the joints. For small area contacts the linear relationship of thermal conductivity with temperature was not followed, possibly due to strain near the contacts. We compared planar contact joints with tapered hole-cone joints of the type used in the Cornell microkelvin cryostat.

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.

Reduction of vibrational noise from continuously filled 1 K pots

We have examined the vibrational noise originating from 1.3 K pumped helium chambers (1 K pots) that are continuously filled from the 4.2 K bath through an impedance. The noise can be largely eliminated by either heating the pot or by heating the impedance directly. Noise in 1 K pots has detrimental consequences for the operation of audio frequency torsional oscillators and for high precision thermometry.

An Electronic Demagnetization Stage for the 0.5K to 1.8K Temperature Range

We have designed and built a demagnetization stage to operate between 1.8K and 0.5K, a somewhat unusual temperature range for adiabatic demagnetization refrigeration. The lower bound in temperature is dictated by the thermal performance of our 3He gas heat switch, the upper by NASA specifications. We intend this demagnetization stage to be the fast stage of a proposed two-stage continuously operating adiabatic demagnetization refrigerator (ADR) which will operate between 1.8K and 50 mK. Here we discuss thermal, mechanical, magnetic and chemical considerations which led to novel features in our salt pill design.

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.

A Geometry Dependent Thermal Resistance Between a Saturated Dilute 3He - 4He Solution and Sintered Silver Powder

We present direct measurements of the thermal resistance between a saturated dilute3He-4He solution and sintered silver powder between 5 and 150mK. Measurements obtained using different sinter geometries allow us to distinguish the contribution due to the thermal boundary resistance from that of the size-limited thermal resistance of the3He-4He solution within the pores of the sinter. The thermal boundary resistance per inverse unit volume is found to vary as T−3 and is insensitive to the sinter particle size.