Propagation of Second Sound and Heat Pulses in Solid Helium Crystals

Abstract

Although all chemists claim Onsager as one of their own, I have always considered him, and with justification to be a physicist. At Yale my earliest recollections as a graduate student included Lars as a regular member of afternoon physics teas and a frequent occupant of the first row in the Physics Club and seminar talks. He was disarmingly friendly, kind and always approachable which helped to reduce the large gap of understanding which often existed between him and some of us. Later as the experimental low temperature program at Yale began to pick up steam, Lars was a constant source of stimulating suggestions, encouragement and provocative ideas. In the late Fall of 1945 C. T. Lane, my brother Bill, and I became interested in a new type wave phenomenon predicted to exist in liquid He II by Tisza and Landau and called second sound. I remember Onsager’s exciting statement one day that you could detect it in the vapor. Lane finally got him to explain this statement and out of this grew the simple experiment in which second sound generated by a heater in the liquid was detected as first sound in the vapor by a small hearing aid microphone. Onsager had quickly recognized that the fluctuating temperature at the vapor liquid interface would be an efficient generator of pressure fluctuations in the vapor. Using a simple resonant technique the velocity was easily measured with precision¹ confirming and extending the earlier work of Peshkov² in Russia. Typically, this and subsequent work included important Onsagerian influences. The importance of his contributions in low temperature physics go well beyond the papers that bear his name.

This work was supported in part by the Office of Naval Research and the National Science Foundation.

The personal remarks at the beginning of the paper are those of the first author (H.A.F.) who for twenty years had the great pleasure of being in the Physics Department at Yale in a low temperature laboratory in close proximity to Lars Onsager’s office in Chemistry.