Study of CADMIUM(,1-X)MANGANESE(,X)SELENIUM: Electronic Properties at Intermediate Donor Concentrations

Far-infrared (FIR) optical as well as electrical transport measurements have been performed in n-type Cd(,1 -x)Mn(,x)Se single crystals in magnetic fields up to 10T. Several previously observed and new phenomena were studied in detail. Specifically, FIR magnetotransmission data 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 (TURN) 1T at 4.3K. In these relatively high donor concentration samples (aN(,D)('1/3) (DBLTURN) 0.15), these effects can be consistently understood within a model of hydrogen molecule-like pairing of electrons localized on neighboring donors. At B = 0 the pair ground state is the spin singlet. The Zeeman effect of the triplet level (a few meV/Tesla) causes the S(,z) = -1 state to cross the singlet level and become the ground state. The model also explains large negative magnetoresistance observed in this material. Electron cyclotron resonance has been observed in the same samples at helium temperatures where practically all electrons are bound on donors. The number of electrons participating in the cyclotron resonance absorption of FIR radiation increases rapidly by an order of magnitude as the resonance magnetic field is raised from 1.5 to 5T. The origin of the implied extended electronic states is attributed to an Anderson-type delocalization transition in clusters of donors of higher than average concentration and volume greater than 4(pi)/3d (magnetic-length)('3), resulting from random fluctuations of local donor density. In addition, we report FIR measurements of electric -dipole spin resonance of band electrons. Material constants, such as electron effective mass, spin-orbit interaction constant, effective Weiss temperature of the Mn('2+) spin system, were determimed.