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We report here the NMR observation of optically induced DNP for II-VI semiconductors with hexagonal structure. Two undoped, n-type CdS single crystals with largely different resistivities were investigated. Optical enhancement of the 113Cd spin polarization was only observed in the higher resistivity sample for T <= 2 K. At this temperature, the optically induced spin polarization was negative when compared to the thermal polarization as shown in the figure on the left. The polarization was negative irrespective of the helicity of the pumping light.
The figure on the right shows a plot of the nuclear spin polarization
in the limit of the nuclear spin relaxation time due to interaction with
electrons being much shorter
than the nuclear spin-lattice relaxation time. The polarization is shown
as a function of the electronic polarization P0
and the ratio of the electron lifetime 
and the electronic spin
relaxation time s for CdS at 2 K. The nuclear spin polarization
is only weakly modulated by
P0 and negative.
Implying a scalar electron-nucleus hyperfine coupling
and since the gyromagnetic ratio of 113Cd is negative, an emissive
113Cd signal is expected, in agreement with the experiments.
T. Pietraß and M. Tomaselli, Phys. Rev. B 59, 1986-1989 (1999).
