SPECULATIONS ABOUT GREY TIN.
Abstract
Groves and Paul have, on the basis of experimental evidence, given strong arguments that in grey tin the valence and conduction bands touch at k = o. This suggests that at sufficiently low temperatures this phase may undergo an excitonic transition. The critical temperature may be estimated from the binding energy E sub B of an exciton: kT sub c approximately equals E sub B = (mu/m)/epsilon squared Rydbergs, where mu is an effective reduced mass and epsilon is an effective dielectric constant. From the work of Groves and Paul and of Ewald and co-workers it is possible to estimate (mu/m) at about 0.02. If one accepts the estimate epsilon = 24 given by Lindquist and Ewald one finds T sub C approximately equals 5 degrees K. From group theoretical considerations one finds that the new phase would belong to one of three space groups of lower symmetry and in one case would be weakly ferromagnetic. These conclusions are however highly tentative because of uncertainty about the dielectric constant epsilon and the suppression of the transition by small concentrations of impurities. Experimental studies of the dielectric constant epsilon(omega) would be highly desirable and interesting. (Author)
Document Details
- Document Type
- Technical Report
- Publication Date
- Apr 01, 1968
- Accession Number
- AD0667837
Entities
People
- D. Sherrington
- W. Kohn
Organizations
- University of California, San Diego