Phonons play two different roles in metals. At high temperatures, they scatter electrons, giving rise to electrical resistivity. At low temperatures, when a metal becomes a supeconductor, phonons bind pairs of electrons (Cooper Pairs), dropping the electrical resistance to zero. Phonons are a mathematical description of the vibrations of atoms in a crystal lattice. But are they real? That is, can they be detected outside the lattice in which they are supposedly created?
In the early 1970s, I devised an experiment to determine whether phonons were real. The experiment involved superconducting tunneling junctions on opposite faces of a crystal; one junction to generate high energy phonons and the other to detect high energy phonons. I could 'dial in' the energies for generating and detecting phonons simply by applying voltages across superconducting tunnel junctions. If the detector registered a current when the voltage across the generator passed a predicted value, it would prove that phonons were real.
The predictions and experiments were based on calculations performed using the BCS theory of superconductivity* and the discovery by Giaever** of electron tunneling in junctions, respectively.
*Bardeen, Cooper, and Schrieffer received the Nobel Prize in 1972 for the development of the theory of superconductivity.
**Giaever received the Nobel Prize in 1973 for his discovery of electron tunneling in superconductors.
I chose Sn as the generator junction and Pb as the detector junction. High energy phonons could only be detected in Pb junctions when the voltage across the Sn junction was greater than 3.9 mV. At this voltage, an incoming phonon can break apart a Cooper pair in the Pb junction, releasing electrons to tunnel, producing a signal indicating 'phonon detection.'
This dance depicts the interactions of electrons and phonons in metals in two acts. Act 1: the metal at high temperatures and the superconductor at low temperatures. Act 2: Generation and detection of phonons below and above 3.9 mV, the voltage delivering energy sufficient to break Cooper pairs.
Credit
Cast & Crew:
Irwin Singer and Ron Singer: electrons
Silvia Burstein-Hendi: lattice vibration and Cooper Pairer
Paula Giammarco: lattice vibration
Sara Lavan: lattice vibration
Mary Jo Smet: lattice vibration and low energy phonon
Melanie Tung: lattice vibration and high energy phonon
Videographers: Mike and Melinda McMullin
Audio: Joan Singer
Creator, Choreographer and video editor: Irwin Singer
Music: selections from Summer and Winter from The Four Seasons Concerto No.2, Antonio Vivaldi.
Acknowledgements:
Special thanks to Melanie, Silvia, Paula, Sara and Mary Jo from The Choreographers Collaboration Project, a modern dance company based in Alexandria, VA.
To The City of Alexandria for allowing me to shot the video in racquetball court #3 at Chinquapin Recreation Center.
To Maneesh Mishra from Prof. Izabela Szlufarska's group at U. Wisconsin, Madison, for atomistic simulations.
Excerpted from the PhD thesis of Irwin L. Singer, Indiana University, Department of Physics, 1976. "Generation and Detection of High-Energy Phonons by Superconducting Junctions.“
Peer-reviewed research article: I. L. Singer and W. E. Bron, Phys. Rev. B 14 (1976) 2832.
After his PhD studies. Dr. Singer went to the US Naval Research Lab as a post-doc in 1976 and has been there ever since working as a Tribologist in the Chemistry Division.
Submitted to the 2010 AAAS "Dance your PhD" Contest, 31 August 2010.
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