X(Z)-X(XY) CROSSOVER IN A 2-DIMENSIONAL ELECTRON-GAS IN ALAS

We have performed magnetoresistance measurements up to 20 T on high-mo bility two-dimensional electron gases in single AlAs quantum wells wit h well widths ranging from 40 to 80 Angstrom. We observe a sharp trans ition fr om a two-fold valley degenerate X(xy) occupied state in a 60 Angstrom well to a non-degenerate X(z) occupied state in a 40 Angstrom well. These two states are distinguished by their different effective masses, which have been determined using the temperature dependence o f the Shubnikov-de Haas effect. Using self-consistent calculations, we can explain this transition in relative occupation assuming an X-spli tting energy of 24 meV. Furthermore, for the X(xy) electrons, the effe ct of the large ratio of spin to Landau splitting is shown by the occu rrence of strong magnetoresistance minima at filling factors 2, 6 and 10 in the Hall effect.