The measurement of the hole density in the ferromagnetic semiconductor
Ga1−xMnxAs is notoriously difficult using standard transport
techniques due to the dominance of the anomalous Hall effect. Here, we report
the first spectroscopic measurement of the hole density in four
Ga1−xMnxAs samples (x=0,0.038,0.061,0.083) at room temperature
using Raman scattering intensity analysis of the coupled plasmon-LO-phonon mode
and the unscreened LO phonon. The unscreened LO phonon frequency linearly
decreases as the Mn concentration increases up to 8.3%. The hole density
determined from the Raman scattering shows a monotonic increase with increasing
x for x≤0.083, exhibiting a direct correlation to the observed Tc.
The optical technique reported here provides an unambiguous means of
determining the hole density in this important new class of ``spintronic''
semiconductor materials.Comment: two-column format 5 pages, 4 figures, to appear in Physical Review