Effect of particle size on the electron transport and magnetic properties of La0.7Ca0.3MnO3 has been investigated. While the ferromagnetic Tc, low field magnetic susceptibility, and insulator-metal transition are markedly affected by the particle size, the maximum magnetoresistance exhibited by the samples near Tc is not sensitive to the particle size. However, the magnetoresistance at 4.2 K increases with decrease in particle size, suggesting a substantial contribution by the grain boundaries. Preliminary measurements on La0.7Sr0.3MnO3 samples of different particle sizes also corroborate the above conclusions.

We have measured the resistances (and resistivities) of Ag and Cu nanowires of diameters ranging from 15 nm to 200 nm in the temperature range 4.2 K-300 K with the specific aim to assess the applicability of the Bloch-Grüneisen formula for electron phonon resistivity in these nanowires. The wires were grown within polymeric templates by electrodeposition. We find that in all the samples the resistance reaches a residual value at T=4.2 K and the temperature dependence of resistance can be fitted to the Bloch-Grüneisen formula in the entire temperature range with a well defined transport Debye temperature (ΘR). The value of Debye temperature obtained from the fits lie within 8% of the bulk value for Ag wires of diameter 15 nm while for Cu nanowires of the same diameter the Debye temperature is significantly lesser than the bulk value. The electron-phonon coupling constants (measured by α el−ph or αR) in the nanowires were found to have the same value as that of the bulk. The resistivities of the wires were seen to increase as the wire diameter was decreased. This increase in the resistivity of the wires may be attributed to surface scattering of conduction electrons. The specularity p was estimated to be about 0.5. The observed results allow us to obtain the resistivities exactly from the resistance and gives us a method of obtaining the exact numbers of wires within the measured array (grown within the template).

We report a systematic study of the electronic transport properties of the metallic perovskite oxide LaNiO 3−δ as a function of the oxygen stoichiometry δ (δ 0.14). The electrical resistivity, magnetoresistance, susceptibility, Hall effect and thermopower have been studied. All of the transport coefficients are dependent on the value of δ. The resistivity increases almost exponentially as δ increases. We relate this increase in ρ to the creation of Ni 2+ with square-planar coordination. We find that there is a distinct T 1.5 -contribution to the resistivity over the whole temperature range. The thermopower is negative, as expected for systems with electrons as the carrier, but the Hall coefficient is positive. We have given a qualitative and quantitative explanation for the different quantities observed and their systematic variation with the stoichiometry δ.

Abstract:We report that ZnO nanostructures synthesized by chemical route undergo a shape transition at ~ 20 nm from spherical to hexagonal morphology thereby changing the spectral components of the blue-green emission. Spherically shaped nanocrystals (size range 11 -18 nm) show emission in the range of 555-564 nm and the emission shifts to the longer wavelength as the size increases. On the other hand, rods and hexagonal platelets (size range 20-85 nm), which is the equilibrium morphology after the shape transition, show emission near 465-500 nm and it shifts to shorter wavelength as the size increases. The shape transition also leads to relaxation of microstrain in the system. Our analysis shows that the visible emission originates from a defect layer on the nanostructure surface which is affected by the shape transition. The change in the spectral component of the blue green emission on change of shape has been explained as arising from band bending due to depletion layer in smaller spherical particles which is absent in the larger particles with flat faces.