Comment on 'resistivity minimum behavior and weak magnetic disorder characteristics in La _2/3Ca _1/3MnO ₃ manganites'

In a recent paper (Chen et al. in J. Supercond. Nov. Magn. 22:465, 2009) Zhenping Chen et al. reported measurements of the electrical resistivity (ρ) and magnetization versus temperature (T) and magnetic field (H) on polycrystalline manganite compound La _2/3Ca _1/3MnO ₃. In particular, the ρ(T) dependence shows a shallow minimum at T _min∼20 K. Application of the external H up to 8 T leads to marked flattening of such minimum and to notable lowering of the T _min on ρ(T,H) curves. The authors attributed this low-temperature anomaly to competitive quantum Kondo effect and three-dimensional electron-electron interaction, using the results of numerical fittings of ρ versus T and H dependences in the interval of about 5-30 K. It is suggested in the comment that such a claim seems to be doubtful due to the fundamental inapplicability of this approach for analysis of low-temperature conductivity in polycrystalline manganites. It is noted that a natural cause for both the low-temperature minimum on ρ(T) curve and its evolution upon magnetic field application, is the well-known grain boundary effect. The numerous uncertainties and obscurities are also characteristic for the paper being commented upon.