GIANT NEGATIVE MAGNETORESISTANCE AND STRONG ELECTRON-LATTICE COUPLING IN AMORPHOUS SEMICONDUCTORS WITH MAGNETIC IMPURITIES

摘要

Giant negative magnetoresistance (MR) has been recently observed in Si-Gd amorphous alloys at compositions close to metal-insulator transition. The negative MR in a-Si/Gd is accompanied by massive spectral weight transfer in optical conductivity with magnetic field, which is absent in a-Si/Y. However, the weight transfer with temperature was observed in both systems. The theory of this phenomenon is suggested, which takes into account a strong carrier-lattice coupling leading to formation of local singlet pairs. The breakdown of the pairs by the temperature and exchange interaction with Gd spins provides a mechanism for the observed behavior. This behavior is compared with earlier observations of large negative MR in a-Ge/Cr and InO_x amorphous alloys, where a similar explanation applies. The leading role of lattice polaron effect can be checked by measuring isotope effect upon substitution ~(28)Si → ~(30)Si, which should cause the temperature shift of the conductivity.