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Sign Change of Thermopower at Low Temperatures in Nd0.7(Ca,Sr,Ba)0.3MnO3

The temperature dependence of thermopower and resistivity for the series of Nd0.7(Ca,Sr,Ba)0.3MnO3 with a fixed average A-site cation radius ⟨rA⟩ have been measured. Infrared spectra revealed that the size differences between various A-site ions cause large local distortions of MnO6 octahedra. The thermopower is enhanced above metal-insulator transition temperature Tm and is reduced below Tm by increasing amount of A-site cation disorder. The sign change of thermopower induced by A-site cation disorder has been observed at low temperatures. The observed thermopower behaviors have been explained by Mott's formula and the effect of the spin disorder scattering.