Heat conductivity of La_1−xSr_xMnO₃ surface layers

Abstract

Using the transient thermoelectric effect (TTE), we have measured the thermoelectric power S(T) and the heat conductivity κ(T) of La_1−xSr_xMnO₃ ceramic surface layers with nominal compositions x=0.075, 0.1, 0.15, 0.2 and 0.3 in the temperature range 50–340 K. Except for a shift in the characteristic temperatures, the surface layer S(T) is still similar to the bulk thermopower while the heat conductivity κ(T) is markedly different, even changing the slope at intermediate temperatures. As S is less structure sensitive than κ, we relate these differences to an increased number density of defects near the surface. In particular, a stoichiometry gradient close to the surface and oxygen vacancy related (two level tunneling-) modes are indicated. In addition, new S and κ data are presented for lower doping, i.e. x=0.075 and 0.1.