Localization of electrons and excitations

Journal article, 2006

Electrons, electron holes, or excitations in finite or infinite 'multimer systems' may be localized or delocalized. In the theory of Hush, localization depends on the ratio Delta/lambda(Delta/2 = coupling lambda = reorganization energy). The latter theory has been extended to the infinite system [S. Larsson, A. Klimkans, Mol. Cryst. Liq. Cryst. 355 (2000) 217]. The metal/insulator transition often takes place abruptly as a function of,Delta/lambda. It is argued that localization in a system with un-filled bands cannot be determined on the basis of Mott-Hubbard U alone, but depends on the number of accessible valence states, reorganization energy lambda and coupling Delta (=2t). In fact U = 0 does not necessarily imply delocalization. The analysis here shows that there are many different situations for an insulator to metal transition. Charge transfer in doped NiO is characterized by Ni2+-Ni3+ exchange while charge transfer in pure NiO is characterized by a disproportionation 2Ni(2+)Ni(+)-> Ni3+. In spite of the great differences between these two cases, U has been applied without discrimination to both. The relevant localization parameters appear to be Delta and lambda in the first case, with only two oxidation states, and U,Delta and lambda in the second case with three oxidation states. The analysis is extended to insulator-metal transitions, giant magnetic resistance (GMR) and high T-c superconductivity (SC). lambda and Delta can be determined quite accurately in quantum mechanical calculations involving only one and two monomers, respectively. (c) 2006 Elsevier B.V. All rights reserved.