Phase Transitions in Disordered Systems: I. Exciton/electron-Hole Liquid/plasma System in Germanium and Ii. Amorphous Ferromagnets and Spin Glasses

In Part I of this thesis, we present experimental evidence demonstrating that a metal-insulator (M-I) phase transition can occur separately from the liquid-gas (L-G) transition in the exciton/electron-hole (e-h) gas/e-h liquid system in stressed Ge. Using a strain well to confine the photoexcited carriers, we analyzed the spectral content and spatial distribution of e-h recombination luminescence. We observe a line of first-order transitions between the exciton gas and e-h gas occurring up to 7 K which we associate with the M-I transition. This is well above the temperature of the L-G critical point which we found to be 4.5 (+OR-) 0.5 K with a critical density of 3 (+OR-) 1 x 10('16) cm. Spectroscopic evidence is also presented for a triple point which we estimate to be about 4 K. At this temperature, we were able to fit our measured luminescence spectra at particular photoexcitation powers only by including a theoretical line shape for the e-h gas with a density of 2.0 (+OR-) .5 x 10('16) cm('-3). A simple expansion of the free energy of the e-h system is presented from which we calculate a critical temperature for the M-I transition of 5.4 K which, possible because of quantum effects, is substantially below our measured value. Lifetime measurements of the different phases of the e-h system are also made. In particular, we have measured an extremely long lifetime of 1.5 ms for strain-confined excitons below 3.2 K. This compares favorably with our predicted radiative lifetime of 2.0 ms in stressed Ge. Above 3.2 K, a rapid decrease in the exciton lifetime is observed with increasing temperature, concurrent with an exponential decrease in the observed luminescence intensity. Three models for thermally-activated loss of strain-confined excitons are considered as possible explanations.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

1981

Author dc:creator

• Schowalter, Leo John

• Physics, Condensed Matter

Language dc:language

eng

Identifier

(UMI)AAI8114478

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/77202