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

Abstract

Simulation of the resistivity in the normal state of doped La 2−x Sr x CuO 4 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 (∼ T 3/2 ) in the region 100 < T < 500 K. A photo-induced metal-metal (MM) charge transfer transition at 2 eV 2Cu(II) + hν → Cu(I) + Cu(III) is responsible for the strong absorption in the visible spectrum of La 2 CuO 4 . The down-shift of spectral density with doping (x) in La 2−x Sr x CuO 4 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.

Spatially Resolved Electron Transport through Anode‐Respiring Geobacter sulfurreducens Biofilms: Controls and Constraints

“…Two factors, activation loss (φ act ) and ohmic resistance loss (φ om ), contribute in different ways to this potential loss, preventing G. sulfurreducens from experiencing the poised anode potential (φ anode ) throughout the biofilm. Activation losses incur in the charge transfer of electrochemical reactions, [14,63] such as the transfer of electrons on the conductive biofilms (Cyt red * * Cyt ox + e À ), and from the biofilm onto the electrode (Rxn 2). The ohmic loss results from the electronic resistance on biofilms and at the electrode surface.…”

Correlations between Spectra and Resistivity in Transition Metal Oxides

“…A paper by Schneemeyer et al 35 shows that "Metallic behavior and the possibility of superconductivity are favored As shown in earlier papers, 18,24 it is possible to derive an equation for the resistivity by applying a weak electric field and considering the difference in current in field direction and opposite to field direction. The result is i k j j j j y { z z z z i k j j j j j y…”

“…Hence there has to be an intermediate state (Figure 3). 24 The latter interacts with both the precursor state and the successor state. Cu(II)/Cu(II) is simply the state correspond-ing to the antiferromagnetically coupled ground state.…”

“…The Marcus model has previously been used by the present author as a hopping model to calculate the resistivity in transition metal oxides (TMOs). 24 The model may also be used to correlate resistivity and photoinduced, optical conductivity spectra. 27 It will be clear that the band model should only be used in the itinerant limit of delocalized electrons.…”

“…It is also important to keep the difference between vertical U (U vert ) and adiabatic U (U ad ) in mind. 24 Experimental scientists have been successful in performing "doping" whereby D and λ become zero. For example in LaVO 3 some amount x/2 of La 2 O 3 is replaced by xSrO to give La 1−x Sr x VO 3 .…”