Viscosity, ion mobility, and the λ transition

A model is presented of the λ transition in superfluid helium in which fluctuations near the transition are approximated by distinct regions of normal fluid and superfluid. The macroscopic viscosity of such a medium is computed. The ion mobility is also computed, taking into account a region of normal fluid around the ion induced by electrostriction. The results are, for the viscosity, ηλ-η∼t^0.67 and for the mobility μ-μλ∼t^0.92 both in excellent agreement with recent experiments. The model suggests that the λ transition itself is the point at which superfluid regions become macroscopically connected

Superfluidity Without Superflow in Unsaturated Helium Films

It is shown experimentally that the superfluid fraction ρs/ρ is continuous and finite at the point at which superflow vanishes in unsaturated helium films. It follows that there is a region of superfluidity without superflow. In addition it is shown that the behavior of the partial molar entropy may account for the disappearance of superflow without requiring that ρs/ρ vanish

Theory and Observation of Displacement Phenomena in Coadsorbed Films

Displacement refers to a surprising phenomenon observed in a number of adsorption systems. At low temperatures, some relatively inert gases, such as krypton (Kr) or methane ( CH4), are found to displace molecules from preadsorbed monolayer films of a more condensable species, such as carbon tetrachloride ( CCl4), from a graphite substrate. We present a simple thermodynamic model to explain displacement and make a prediction applicable to both first-order and continuous displacement processes. We also present measurements on CH4/CCl4 and Kr/CCl4 coadsorbed films from 77–112 K that verify our prediction and yield the CCl4 film spreading pressure

Coadsorption phase diagram for Kr/CCl4 on graphite

We present the results of an extensive calorimetric study of krypton coadsorbed on graphite precoated with a saturated monolayer of carbon tetrachloride. Combining the heat capacity data with film equation of state measurements from a previous study [W. J. Weber and D. L. Goodstein, Phys. Rev. Lett. 83, 3888 (1999)] permits construction of the Kr/CCl4 coadsorption phase diagram between 77 and 130 K. Kr succeeds in displacing the CCl4 from the surface, by a continuous process which results, at lower temperatures, in a film indistinguishable from that of pure Kr adsorbed on graphite. At higher temperatures, a new first-order phase transition, unique to the coadsorption system, is observed and likely indicates a transition to a mixed Kr/CCl4 film. Finally, measurements at higher Kr coverages reveal evidence for a high temperature extension of the reentrant layering phenomena previously observed for Kr on graphite

Comment on some proposed mechanisms for attenuation of third sound

Two explanations have been proposed for an apparent discrepancy between theoretical prediction and experimental measurement of third-sound attenuation. One of these proposes a new "macroscopic quantum uncertainly principle," and the other proceeds via nonlinear, anharmonic effects due to zero-point fluctuations. We argue that neither suggestion is acceptable

The origins of nuclear astrophysics at Caltech

Shortly before the start of World War II, several theoretical physicists, including Hans Bethe and Carl von Weizsacker, advanced the idea that the sun derives it energy from nuclear reactions within its core. C. C. Lauritsen and William Fowler, nuclear physicists at Caltech's Kellogg Laboratory, were among the first experimentalists to appreciate the application of nuclear physics to stellar interiors. Post-war strategies for studying nuclear processes in the stars included an innovative series of unofficial, weekly seminars with Mt. Wilson astronomers at director Ira Bowen's house, the testing of Bethe's carbon cycle in Kellogg, and the collaboration with a diverse group of scientists ranging from cosmologist Fred Hoyle to astronomers Margaret and Geoffrey Burbidge. The events leading up to the publication of the 1957 paper by Fowler, Hoyle, Burbidge, and Burbidge, in The Reviews of Modern Physics, now regarded as a watershed in the history of nuclear astrophysics, are discussed. For his work in low-energy nuclear astrophysics, Fowler won the 1983 Nobel Prize in physics

Richard P. Feynman, Teacher

One of the principal purposes of this article is to consider Dick Feynman in his role as teacher. Let me not keep you in suspense about my conclusion. I think Dick was a truly great teacher, perhaps the greatest of his era and ours. That's not to say he was always completely successful, as he himself emphasized in his preface to The Feynman Lectures on Physics. I would contend that these lectures often failed at the level of their superficial intent: If his purpose in giving them was to prepare classes of adolescent boys to solve examination problems in physics, he may not have succeeded particularly well; if his purpose in creating those three red volumes was to provide effective introductory college textbooks, he may not have succeeded, either. If, however, his purpose was to illustrate, by example, how to think and reason about physics, then, by all indications, he was brilliantly successful. Perhaps this is why the books are genuine and lasting classics of the scientific literature and why his lectures left an enduring trace on those fortunate enough to have heard or read them. His achievement as a teacher—and as an inspiration and model for other teachers—was based on nothing less than seeing all of physics with fresh new eyes

Criticality and superfluidity in liquid ^4He under nonequilibrium conditions

We review a striking array of recent experiments and their theoretical interpretations on the superfluid transition in ^4He in the presence of a heat flux Q. We define and evaluate a new set of critical point exponents. The statics and dynamics of the superfluid-normal interface are discussed, with special attention to the role of gravity. If Q is in the same direction as gravity, a self-organized state can arise, in which the entire sample has a uniform reduced temperature, on either the normal or superfluid side of the transition. Finally, we review recent theory and experiment regarding the heat capacity at constant Q. The excitement that surrounds this field arises from the fact that advanced thermometry and the future availability of a microgravity experimental platform aboard the International Space Station will soon open to experimental exploration decades of reduced temperature that were previously inaccessible