Showing papers by "John B Ketterson published in 1972"
29 citations
TL;DR: In this article, a valve actuated hydraulically by liquid He and sealed repeatedly with a leak rate of less than 10−10 cm3/sec of liquid He II.
Abstract: In order to seal off an experimental cell containing liquid He at temperatures down to 20 mK and at pressures up to 25 atm we have developed a valve that is actuated hydraulically by liquid He and which seals repeatedly with a leak rate of less than 10−10 cm3/sec of liquid He II.
27 citations
22 citations
TL;DR: In this article, the authors measured the temperature dependence of the velocity and the attenuation of sound in the range 60-700 mK and confirmed the maximum in the sound velocity observed by Abraham and Osborne, which is in good agreement with that of Abel, Anderson and Wheatley but differs from that of Betts, Keen, and Wilks.
Abstract: We have measured the temperature dependence of the velocity and the attenuation of sound in the range 60–700 mK. The maximum in the sound velocity observed by Abraham and Osborne is confirmed. The viscosity deduced from the attenuation measurements is in good agreement with that of Abel, Anderson, and Wheatley but differs from that of Betts, Keen, and Wilks. The sound velocity as a function of pressure at 150 mK was also measured and from this the low-temperature pressure dependence of the density was determined.
21 citations
TL;DR: In this article, the authors report data on the temperature dependence of the attenuation from 0.1 to 1.0 K at frequencies of 15, 45, 105, and 256 MHz for pressures between 0 and 24.7 atm.
Abstract: In an attempt to help resolve previously observed discrepancies between the theory for ultrasonic attenuation and the data at the vapor pressure, we have extended our measurements to higher pressures. We report data on the temperature dependence of the attenuation from 0.1 to 1.0 K at frequencies of 15, 45, 105, and 256 MHz for pressures between 0 and 24.7 atm. At low pressures or temperatures, the data resemble the theoretical predictions, but the measured attenuation is about twice that predicted by most theories. At higher pressures and for temperatures above about 0.3 K, the attenuation shows a sharp reduction below the value extrapolated from low temperatures according to previous theories. It has recently been suggested that this unexpected behavior is due to a restriction on the number of thermal phonons that are able to play a part in the attenuation process at higher temperatures and pressures.
19 citations
TL;DR: In this paper, an improved fit to the inelastic neutron scattering data for the excitation spectrum was achieved with this expression; α1 was positive in the resultant fit. But this was only for the long-wavelength portion of the 4He elementary spectrum, where α2 is traditionally taken to be negative.
Abstract: For many years it has been assumed that the long-wavelength portion of the 4He elementary excitation spectrum could be described by
$$\varepsilon \left( k \right)=c\hbar k\left( 1+{{\alpha }_{2}}{{k}^{2}}+... \right)\varepsilon \left( k \right)=c\hbar k\left( 1+{{\alpha }_{2}}{{k}^{2}}+... \right)$$
(1)
where e is the energy of the excitation, k is its wave number, and c is the sound velocity; α2 is traditionally taken to be negative. Recently, however, Barucchi et al.1 predicted that the excitation spectrum could be described by
$$\varepsilon \left( k \right)=c\hbar k\left( 1+{{\alpha }_{1}}k+{{\alpha }_{2}}{{k}^{2}}+... \right)$$
(2)
and Molinari and Regge2 suggested that an improved fit to the inelastic neutron scattering data’ for the excitation spectrum could be achieved with this expression; α1 was positive in the resultant fit.†
18 citations
12 citations
TL;DR: In this article, the Fourier series representation was extended to include the representation of the relaxation time over the Fermi surface and the inversion of the Dingle-Robinson temperature data.
Abstract: We extend our previous Fourier-series-representation schemes to include the representation of the relaxation time over the Fermi surface and to perform the inversion of the Dingle-Robinson temperature data. The scheme is applied to recent results of Miller, Poulsen, and Springford on Au: Cu alloys.
9 citations
TL;DR: In this paper, the authors determined the thermal expansion of liquid 4He from 0.1 to 0.7 K by comparing the temperature dependence of the constant density and the constant pressure sound velocities.
2 citations
TL;DR: In this paper, a phenomenological model was developed which assumes that the density of states N(E) ∝ (e2 + Δ)−1/4 whwrw Δ−1 is a lifetime.
Abstract: Susceptibility measurements on the Kondo alloy La:Ce show that X = cF(T,H) for 1.3 K ≤ T ≤ 300 K, c ≤ 15 at %, H ≤ 20 kG where F is independent of concentration c. At high temperatures (T > 30 K), F(T,O), like other Kondo systems, obeys a Curie‐Weiss law 1/(T + 27). At low temperatures (0.6 ≤ T ≤ 30 K), f (T,0) ∝ T−1/2. To test the sensitivity of this result to local environment and the effect of spin‐orbit scattering, measurements were made on Ce .01ThxLa.99−x (x ≤ 0.4) alloys. These measurements show that the Ce contribution to the susceptibility, cF(T,0), is unchanged by the addition of Th. The magnetic field dependence of X does not fit a Brillouin function. Generalizing Anderson's suggestion, a phenomenological model is developed which assumes that the density of states N(E) ∝ (e2 + Δ)−1/4 whwrw Δ−1 is a lifetime. The calculated numerical results from this model for F(T,H) are in good agreement with the experimental results. Qualitatively F ∝ (T2 + μ2H2 + Δ)−δ where δ = 1/4. The Hirschkoff et al. data on Cu:Fe also fits this form with δ = l/3 and Δ∝c2 . The value δ = 1/3 does not fit our data. The importance of crystalline fields and the value of TK will be discussed. Other work on this system is briefly reviewed.
2 citations
TL;DR: In this article, the authors measured the temperature dependence of sound velocity in liquid liquid from 0.07 to 0.7 K for frequencies from 12 to 105 MHz and found that the velocity shift increases with frequency as predicted by theory.
Abstract: We have measured the temperature dependence of the sound velocity in liquid
4
He from 0.07 to 0.7 K for frequencies from 12 to 105 MHz. While the data show an approximateT
4 temperature dependence, the frequency ordering of the curves is observed to undergo an inversion at about 0.2 K. Above this temperature we confirm that the velocity shift decreases with frequency, while below this temperature we now find that the velocity shift increases with frequency as predicted by theory.
TL;DR: In this article, the thermal link consists of several thousand small diameter copper wires which are capable of making good thermal contact to liquid helium while minimizing the effects of eddy current heating.
Abstract: Construction of a thermal feedthrough which is useful at ultralow temperatures and high magnetic fields is described. The thermal link consists of several thousand small diameter copper wires which are capable of making good thermal contact to liquid helium while minimizing the effects of eddy current heating.