Polaron transport in the paramagnetic phase of electron-doped manganites

Abstract

The electrical resistivity, Hall coefficient, and thermopower as functions of temperature are reported for lightly electron-doped Ca1−xLaxMnO3(0 ≤ x ≤ 0.10). Unlike the case of hole-doped ferromagnetic manganites, the magnitude and temperature dependence of the Hall mobility (µH ) for these compounds is found to be inconsistent with small-polaron theory. The transport data are better described by the Feynman polaron theory and imply intermediate coupling (α ≃ 5.4) with a band effective mass, m * ∼ 4.3m0, and a polaron mass, mp ∼ 10m0.

Electron transport in CaMnO3 − δ at elevated temperatures: a mobility analysis

“…Another unusual feature is a decrease of Hall mobility in Ca 1− x La x MnO 3−δ (0≤х≤0.1) with the temperature increase at 130-300 K, which is particularly expressed in CaMnO 3− δ . Based on these observations, authors [19] arrived at a conclusion about large polaron charge carriers in CaMnO 3 − δ and its lightly electron-doped derivatives. The concentration of charge carriers in CaMnO 3−δ at room temperature does not exceed 0.001 per formula unit and is most possibly related with uncontrolled impurities [19].…”

“…However, the increase of temperature from near 130 K to about 700 K is accompanied with the increase of absolute values of thermopower [7,12,16], which is in apparent disagreement with (1). The Hall mobility in paramagnetic Ca 1−x La x MnO 3−δ is shown to increase with lanthanum content, i.e., with the concentration of charge carriers [19]. On the other hand, the increase in the amount of charge carriers in a small polaron conductor must be accompanied with the concomitant decrease in the amount of positions available for polaron jumps.…”

Impurity conduction and magnetic polarons in antiferromagnetic oxides

“…The latter behavior appears to be associated with FM polarons [7], whereas the former may be attributed to an additional FM contribution in CMO from Dzyaloshinsky-Moriya coupling (allowed by symmetry). These materials exhibit band-like, largepolaron transport with mobilities ∼ 1 cm 2 /V s at T ≥ T N [9,13], which contrasts with the small polaronic character of the paramagnetic phase of hole-doped manganites [14]. Figure 1 shows resistivity data (in zero magnetic field), plotted versus inverse temperature, illustrating the sensitivity of the charge transport in these materials to applied current (I) at low T where impurity conduction is predominant.…”

“…The data argue against the presence of self-trapped MPs in these compounds and we attribute the onset of a FM contribution to the low-T magnetization of SMO [8] to bound MPs which become stable only for k B T ≪ δ. The observations suggest that the tunable mobile carrier density characteristic of the present compounds might be observed and exploited in related compounds for novel studies of correlated electrons.The synthesis procedures for the single-crystal of CaMnO 3 (CMO) and polycrystalline specimen of SrMnO 3 (SMO) are described elsewhere [6,8,9,10]. The net electron densities determined from Hall measurements at room temperature were n H ≡ N D − N A ≃ 6 × 10 18 cm −3 (CMO) and 3 × 10 18 cm −3 (SMO).…”

“…The synthesis procedures for the single-crystal of CaMnO 3 (CMO) and polycrystalline specimen of SrMnO 3 (SMO) are described elsewhere [6,8,9,10]. The net electron densities determined from Hall measurements at room temperature were n H ≡ N D − N A ≃ 6 × 10 18 cm −3 (CMO) and 3 × 10 18 cm −3 (SMO).…”