Polarization anomalies at filling factors nu=3, 5, and 7: Evidence for Skyrmions at nu >= 1

We have investigated the ground state spin configuration of a two-dimension al electron system (2DES) by studying the circular polarization of the magn etophotoluminescence (PL) from GaAs-AlxGa1-x. As single heterojunctions. At the filling factor v = 1 we observe a maximum in the polarization of the P L as expected within the Skyrmion framework. In addition, we observe minima in the polarization of fully occupied Landau levels at odd-integer filling factors v > I. These minima become more pronounced as the temperature of t he 2DES is raised by a few degrees Kelvin, but disappear at higher temperat ures. This temperature dependence suggests that they are many-body in origi n and we interpret them as evidence for Skyrmion formation at odd-integer v > 1. [S0163-1829(99)51632-9].