Far-infrared absorption inCd1−xMnxSe: New impurity-band effects

Far-infrared magnetotransmission measurements on $n$-type ${\mathrm{Cd}}_{1-x}{\mathrm{Mn}}_x\mathrm{Se}$ imply the disappearance of optical excitation of $D^{+}{D}^{-}$ complexes as well as a red shift of the absorption associated with photon-induced hopping in magnetic field $B\approx 1$ T. In these relatively high donor-concentration samples ($a{N}_D^{\frac{1}{3}}\approx 0.15$), these effects can be consistently understood within a model of hydrogen moleculelike pairing of electrons localized on neighboring donors. At $B=0\mathrm{}$ the pair ground state is the spin singlet. The Zeeman effect of the triplet level (a few meV/T) causes the $S_z=-1\mathrm{}$ state to cross the singlet level and become the ground state. The model also explains large negative magnetoresistance observed in this material.