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G. M. Zhao
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In a recent paper, Nagaev cited the unpublished paper by Franck et al.to support his theoretical model for the mechanism of the giant isotope effect observed in La_{1-x}Ca_{x}MnO_{3+y} (x = 0.20, y > 0). His model suggests that the off-stoichiometric oxygen content depends strongly on the oxygen isotope mass, which leads to a giant oxygen-isotope effect. Here I show that his theoretical model is not consistent with any experimental results (even the results recently published by Franck et al.), and his estimate of polaronic bandwidth is wrong due to his misuse of polaronic theories.
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Oxygen isotope effects on the transport properties have been studied in high-quality epitaxial thin films of La_{0.75}Ca_{0.25}MnO_{3} and Nd_{0.7}Sr_{0.3}MnO_{3}. In the paramagnetic state, the resistivity can be well fitted by rho (T) = (A/sqrt{T})exp(E_{rho}/k_{B}T) with the parameters A and E_{a} depending strongly on the oxygen isotope mass. The resistivity below 80 K almost perfectly follows rho = rho_{o}+ Bomega_{s}/sinh^{2}(hbaromega_{s}/2k_{B}T) with hbaromega_{s}/k_{B} sim 100 K. Both rho_{o} and B increase by about 15(3)% upon raplacing $^{16}$O by $^{18}$O. The results provide quantitative constraints on the basic physics of manganites.
We report a study of oxygen isotope effects on low temperature specific heat, magnetization, and resistivity of La$_{1-x}$Ca$_{x}$MnO$_{3}$ and (La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$MnO$_{3}$. For the metallic compositions of La$_{1-x}$Ca$_{x}$MnO$_{3}$ and for charge-ordered La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ no change in low temperature specific heat has been detected with $^{16}$O -$^{18}$O exchange, while compounds of (La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$MnO$_{3}$ (0.4<y<0.6) show a significant change in low temperature properties. The low temperature specific heat indicates a presence of the charge-ordered phase even in compositions of (La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$MnO$_{3}$ which are metallic at low temperatures. We suggest that the changes induced by the oxygen isotope exchange are caused by an increase of the charge-ordered phase in $^{18}$O samples.
The Monte Carlo Ferromagnetic Ising model was used to study the electrical properties of manganese oxides due to the charge ordering phase occurring at doping, x = 0.5. The half-doped manganites have an insulator antiferromagnetic ground state. We calculated the internal energy, specific heat, resistivity and the magneto-resistance, MR, with parallel and anti-parallel applied magnetic fields. Our simulation reveals that the resistivity decreases exponentially and the electric current increases with increasing temperature according the free charge increase, to transport from an insulator to conductor phase. The magnetoresistance has negative small values with parallel magnetic field but has positive high values with unti-parallel magnetic field. The obtained semiconductor-metal transition behavior candidates the half-doped manganites to be very good semiconductors diode junctions.
In this paper, we discuss the most deleterious effect of charge state modification at the Mn site on the ground state of the CMR manganites.
Angle-resolved photoemission spectroscopy data for the bilayer manganite La1.2Sr1.8Mn2O7 show that, upon lowering the temperature below the Curie point, a coherent polaronic metallic groundstate emerges very rapidly with well defined quasiparticles which track remarkably well the electrical conductivity, consistent with macroscopic transport properties. Our data suggest that the mechanism leading to the insulator-to-metal transition in La1.2Sr1.8Mn2O7 can be regarded as a polaron coherence condensation process acting in concert with the Double Exchange interaction.
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