Thermopower in Pr ₁-x Ca_xMnO₃

Abstract

We report measurements of thermopower in Pr_1-xCa_xMnO₃ (0.33 ≤ x ≤ 0.48). A clear anomaly has been observed at the charge ordering temperature T_co for x = 0.33, 0.34, 0.47 and 0.48. Negative thermopower below T_co shows that the carriers are of electron-type in the charge-ordering state of x = 0.47 and 0.48. This indicates that the hopping of e_g electrons at the Mn⁴⁺ sublattice, i.e., extra Mn³⁺ ions, dominates the transport properties in the charge-ordering phase. Thermopower below T_co changes from negative to positive around x -- 0.35 as the e_g-electron number (1-x) increases, which implies a spatial ordering of extra Mn³⁺ ions. The ordering of extra Mn³⁺ ions possibly stabilizes the 1:1 commensurate charge ordering with the wave vector of (100) with x far from the commensurate composition, x = 0.50. Thermopower is found to be significantly dependent on magnetic field. The result should be closely related to the stability of the charge ordering in a magnetic field.