Professor

Department of Physics
The University of Texas at Austin

For more information

• Dr. MacDonald's biography
• Dr. MacDonald's research group
• About The University of Texas at Austin and its Department of Physics

About the presentation

"Bose Condensation from Einstein to Our Time"
In 1924 Satyendra Nath Bose wrote to Einstein from India asking for his help in getting a paper published. Bose had already sent it to the Philosophical Magazine, where it had been turned down. (Einstein translated the paper into German, and it was subsequently published in Zeitschrift für Physik.) Because of this interaction Einstein temporarily turned away from his quest for a unified theory of gravitation and electromagnetism and started work on the quantum theory of radiation. Thus was born the concept of "Bose-Einstein" statistics for "bosons", quanta that (like the photon particles of quantized light and unlike electrons) carry an integer value of intrinsic angular momentum (spin). For bosons, there is no limit to the number of particles that can simultaneously occupy any one quantum state. Einstein noted that if the number of such particles is conserved, even totally non-interacting particles should undergo a change of behaviour at low enough temperatures - Bose-Einstein condensation. In Bose-condensed systems most particles occupy the same quantum state, elevating quantum behavior from the microscopic world to the macroscopic world.

Bose-Einstein condensation (BEC) occurs in Helium liquids where it is responsible for superfluidity and in many metals where pairs of electrons (Cooper pairs) act as bosons and condense, giving rise to superconductivity. More recently Bose-Einstein condensation was observed in vapors of weakly interacting alkali metal atoms giving rise to a new window on the quantum behavior of many-interacting boson particles. MacDonald discussed yet another example of Bose-Einstein condensation which has been discussed in the solid state physics literature for more than 40 years, but has been realized experimentally only very recently. The bosons in this case are pairs formed from an electron in one semiconductor quantum well layer and a hole (a missing electron) in a second semiconductor quantum well layer that combine to form a bound state known as an exciton. Many of the properties of excitonic BECs follow from the fact that exciton number is not quite perfectly conserved, as Einstein had assumed. MacDonald discussed recent surprising experimental results on the properties of semiconductor bilayer exciton BECs and efforts to develop a successful theory.