CYCLOTRON-RESONANCE TO 100 MK OF A GAAS HETEROJUNCTION IN THE ULTRA-QUANTUM LIMIT

A helium dilution refrigerator has been modified to enable cyclotron r esonance measurements to 100 mK on low-density, 2D electron systems in the ultra-quantum limit. Previous cyclotron resonance work to 300 mK indicates the presence of a phase boundary at a filling factor of nu(c ) almost-equal-to 1/10, separating gas-like behavior at nu < nu(c), an d a new mixed-spin liquid at nu greater-than-or-equal-to nu(c). The pr esent experiment probes the ground state of these phases, as the tempe rature is lowered to the Wigner solid regime. We find that, even at 10 0 mK, the system is spin polarized only at low values of nu.