Abstract
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 28Si → 30Si, which should cause the temperature shift of the conductivity.
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In the present model a spin of lattice polaron is exchange coupled to local spins of Gd impurities, which are (and remain) disordered. The coupling makes the polaron energy levels shift more strongly with the field, as it would without the exchange coupling. Note that it has nothing to do with “magnetic” polaron, which can possibly be formed by a single charge carrier interacting with (anti)ferromagnetic background in dilute magnetic systems and producing local ferromagnetic regions, where it gets trapped. Because of the high carrier density in the a-Si/Gd films (~ 1021 cm-3) the magnetic polaron scenario does not apply.
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Bratkovsky, A.M. (2001). Giant Negative Magnetoresistance and Strong Electron-Lattice Coupling in Amorphous Semiconductors with Magnetic Impurities. In: Kaplan, M.D., Zimmerman, G.O. (eds) Vibronic Interactions: Jahn-Teller Effect in Crystals and Molecules. NATO Science Series, vol 39. Springer, Dordrecht. https://doi.org/10.1007/978-94-010-0985-0_14
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DOI: https://doi.org/10.1007/978-94-010-0985-0_14
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