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

Sven Larsson

doi:10.1007/s10948-016-3666-0

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.

Highlights

The slow advent of a consistent theory for cuprate superconductivity (SC) has to a large extent been caused by a failure to realize the importance of structural dependence on the number of electrons at a site in a crystal with localized electronic structure
As will be discussed below, we find conductivity due to electron pair hopping of the following type with activation energy in the 0.5 eV region: Cu(I) + Cu(III) → Cu(III) + Cu(I)
Ucross), the symmetry leading to the selection rule is obviously broken since Cu(I)/Cu(III) states are mixed into the Cu(II)/Cu(II) state

Summary

Introduction

The slow advent of a consistent theory for cuprate superconductivity (SC) has to a large extent been caused by a failure to realize the importance of structural dependence on the number of electrons at a site in a crystal with localized electronic structure. Attractive Cu(III) sites appear after doping (x > 0), and the Cu(II)/Cu(III) transitions in the upper-left corner in Fig. 1 become possible [(1)] This explains the transfer of spectral density to energies below 1 eV. MMCT transitions appear at lower energies (Fig. 1, lower right corner) due to the nearby hole This decrease is seen in the spectrum of La2−xSrxCuO4 [9]. As will be discussed below, we find conductivity due to electron pair hopping of the following type with activation energy in the 0.5 eV region: Cu(I) + Cu(III) → Cu(III) + Cu(I) This mechanism contributes in the SC region, and in the under- and overdoped regions outside the SC region. The same model may be used as will be commented on later

Electron Transfer Between Sites

Mobility in the Marcus Model

One-Electron Exchange in the Underdoped Case

Possible Explanations of Raman and MIR Absorption for Cuprates

Discussion