Transport properties of nanoparticles of complex oxides: likely presence of coulomb blockade at low temperature

Abstract

In this paper we report transport properties at low temperatures (3 K≤T≤300 K) in nanostructured functional oxides. Electrical resistivity in the nanoparticles of La_0.5Sr_0.5CoO₃ (LSCO), a ferromagnet, show a gradual change-over from a completely metallic state (in the bulk sample) to a completely insulating state (in the sample with the smallest particle size of ∼35 nm), while still remaining a ferromagnet, albeit with a lower T_C. In the LSCO nanoparticles (diameter <60 nm) there is a change-over in the temperature dependence of the resistivity at the lowest temperature (T<10 K) which we could identify as arising from the Coulomb blockade in the conducting grains which are separated by insulating grain boundaries that allow transport by tunneling. However, nanoparticles of the nonmagnetic material, LaNiO₃ (LNO) remain metallic (similar to the bulk sample) throughout the temperature range studied with the resistivity reaching a temperature independent value at the lowest temperature measured. We suggest that the essential difference between the electrical transport in these nanostructured materials arises due to the physical nature of the grain boundaries which is insulating in LSCO due to absence of long range spin order that is needed for the metallic state.