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We report the results of thermoelectric power ͑TEP͒ of a Cr doped La 0.5 Pb 0.5 Mn 1Ϫx Cr x O 3 (xϭ0 -0.45) system measured both in the presence and the absence of magnetic field (Bϭ1.5 T). The small field dependence of the Seebeck coefficient is observed around the metal-semiconductor transition ͑MST͒ temperature (T p ) of the samples. The field dependence of TEP is stronger in the undoped sample. It is noticed from the TEP data that the small polaron hopping conduction mechanism is valid for all these samples ͑for TϾT p ). The polaron radius r p is found to decrease with increase of magnetic field. Low-temperature ͑below T p ) fielddependent and field-independent TEP data can be fitted with SϭS 0 ϩS 3/2 T 3/2 ϩS 4 T 4 suggesting that the electron-magnon scattering strongly affects the low-temperature ͑ferromagnetic phase͒ TEP data of the manganites. Activation energy gradually increases with increasing Cr concentration both in the presence and the absence of magnetic field. Field-dependent thermopower also indicates the importance of spin fluctuations affecting the phonon scattering. Power factor (S 2 /) estimated from the Seebeck coefficient ͑S͒ and resistivity () ͑at zero and 1.5 T field͒ showed a minimum around the MST temperature similar to the field-dependent thermal conductivity data.

Comprehensive low-temperature specific heat data C(T,H) of Na

A fine Au powder, with a mean particle diameter of 4 nm, has been successfully fabricated. The crystalline structure of the 4 nm Au nanoparticles remains in fcc symmetry. No structural changes were found between 15 and 450 K. A crossover from a positive thermal expansion at low temperatures to a negative thermal expansion at high temperatures was observed in the fcc cell parameter at about 125 K. Anomalies associated with the crossover were also observed in the magnetic response and the heat capacity measurements. The observations can be reasonably well interpreted by accounting for the effects of the valence electron potential on the equilibrium lattice separations, with a weakly temperature dependent level spacing.

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