Conductivity in Cuprates Arises from Two Different Sources: One-Electron Exchange and Disproportionation

Abstract

Simulation of the resistivity in the normal state of doped La2−xSrxCuO4 has been performed using a hopping model based on Marcus theory. The results are in substantial agreement with experimental results. At oxidative doping, Cu(III) sites are formed and electron mobility possible due to hopping: Cu(III)Cu(II) → Cu(II)Cu(III) (one-electron exchange). In the underdoped, non-metallic region, the resistivity (ρ) decreases from almost insulation at T = 0 to a minimum at about T = 100 K. ρ then increases more than linearly with T (∼T3/2) in the region 100 < T < 500 K. A photo-induced metal-metal (MM) charge transfer transition at 2 eV 2Cu(II) + h ν→ Cu(I) + Cu(III) is responsible for the strong absorption in the visible spectrum of La2CuO4. The down-shift of spectral density with doping (x) in La2−xSrxCuO4 depends on the appearance of Cu(III) sites which makes optical as well as thermal one-electron exchange transitions possible with lower energy. Disproportionation occurs spontaneously for x > 0.06, opening up for electron pair formation. Configuration interaction between two-electron states of low chemical potential, but strong vibrational coupling, gives rise to the superconductor and pseudogaps. Data from photo-induced conductivity and absorption spectra are used in the simulation, which gives results in good agreement with experiments. Possible explanations for Raman and MIR absorption suggest themselves.