Bose-Einstein Condensation of Excitons and Biexcitons and Coherent Nonlinear Optics with Excitons , Sviatoslav A. Moskalenko and David W. Snoke , Cambridge U. Press, New York, 2000. $85.00 (415 pp.). ISBN 0-521-58099-4
Bose-Einstein Condensation of Excitons and Biexcitons and Coherent Nonlinear Optics with Excitons , by Sviatoslav A. Moskalenko and David W. Snoke, is a most useful text by two physicists each of whom has made substantial contributions to the field of Bose-Einstein condensation (BEC) with excitons, a subject attracting increasing interest at present. My own awareness of this subject goes back to 1993, when I read a paper reporting evidence of BEC in an excitonic gas; the paper caused quite a stir. I was on sabbatical at that time working at NIST, in Gaithersburg, Maryland, and so had time to read it properly. Snoke was a coauthor on the paper, which described a significant development clearly, and a heated debate arose about what had been observed and the potential for future experiments.
Excitons are weakly interacting composite bosons, and one should, therefore, be able to observe BEC in its pure form (BEC is seen in its pure form only in weakly interacting systems) using a gas of excitons. For this and other reasons, there had been an effort to produce a sufficiently dense and cold excitonic gas with which to observe BEC.
That was an exciting time for research in BEC in general, with breakthroughs in the production of atomic condensates soon to occur. (This closely related field has grown even more rapidly in the intervening years.) There had also been a very lively meeting on BEC in Trento, Italy, in 1993, where the challenges and opportunities for studies of degenerate, weakly interacting gases had been discussed. Moskalenko and Snoke state, in the introduction to their book, that they had met at this meeting and decided then to write a systematic text on the subject of BEC in excitons and biexcitons. They have succeeded in an admirable fashion, producing an excellent text for graduate students and experienced workers in the field.
The book contains a thorough introduction to all aspects of condensed matter physics, combined with much of the formal theory required to understand a wide range of experiments. Potential readers should, however, equip themselves with a decent background in general quantum field theory in order to make best use of the text. The authors provide accounts of the more specialized theory needed for excitons and their dynamics in the presence of laser driving and relaxation.
The complex relaxation processes seen in exciton systems are described in detail using the Keldysh formalism for nonequilibrium systems. Knowledge of the Keldysh formalism is essential for workers in the field and yet is spread across the literature. The book is, therefore, especially welcome, being a most useful single source for this material as well as a well-written explanation of many important points for the field. The book has a good and rarely encountered balance between formal theory and analysis of specific cases in which experimental data are available.
Excitons behave as composite bosons only at sufficiently low densities. As the density of an excitonic gas is increased, the physics we see becomes that of an electron-hole plasma, an important subject in its own right. The book gives a good account of this link along with a survey of the relevant literature, which helps greatly in placing the subject properly in the context of condensed matter research as a whole.
One of the most interesting phenomena that the text describes is the production of coherent pulses of excitons. One can study the propagation of such pulses and even look at their nonlinear optics. The nonlinear interaction can be used to squeeze the field of the coherent excitons, just as light fields are squeezed by a nonlinear interaction. The ways in which such coherent beams of excitons can be used in other areas of science and technology are a matter of intense study at present.