Systematics in the thermopower of electron-doped layered manganites

Abstract

We report on a comparative study of the transport properties of lightly La-doped layered CaO $({\mathrm{CaMnO}}_{3}{)}_{m}$ $(m=1,2,$ and $\ensuremath{\infty})$ and similar ${\mathrm{Mn}}^{3+}{/\mathrm{M}\mathrm{n}}^{4+}$ ratios. In contrast to the large and rather nonsystematic variations of the resistivity $\ensuremath{\rho}(T)$ for samples with the same doping and various m, the absolute thermopower $(S)$ exhibits remarkable systematics. In the paramagnetic (PM) state, S varies linearly with temperature and its extrapolation to $T=0$ has a finite intercept. The intercept and the slope of these lines vary systematically with $m.$ In particular, within the experimental error, the inverse slopes of the straight lines increase linearly with $1/m,$ which in its turn varies linearly with the average number of Mn-Mn near neighbors. The implications of this simple systematics are discussed using a model developed by Cutler and Mott [Phys. Rev. 181, 1336 (1968)] for transport by activated hopping of carriers obeying nondegenerate statistics. This model (or a possible extension into the semidegenerate regime) leads also to a simple, qualitative interpretation of $S(T)$ of these materials on crossing the N\'eel temperature ${(T}_{N})$ from the PM state and below ${T}_{N}.$