Thermoelectric properties of GaAs-${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Al}}_{\mathrm{x}}$As heterojunctions at high magnetic fields

Detailed results are presented on both the thermoelectric and resistive coefficients of four high-mobility GaAs-${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Al}}_{\mathrm{x}}$As heterojunctions at fields up to 20 T. In the presence of a field both the thermopower and Nernst-Ettingshausen (NE) data behave qualitatively as expected from the predictions of the diffusion model but the magnitudes of both coefficients are always too large; in particular, in the quantum limit at ν=(1/2), the thermopower has a magnitude of -50 mV $K^{\mathrm{-}1}$ instead of the expected -120 μV ${\mathrm{K}}^{\mathrm{-}1}$. The zero-field thermopower is also found to be too large and have the wrong temperature dependence to be ascribed to diffusion. It is possible that phonon drag is responsible for these anomalies though the NE coefficient shows a variation with field which is characteristic of diffusion and it is difficult to reconcile this with phonon drag. The fractional quantum Hall effect (FQHE) is visible in the NE coefficient but not in the thermopower; this makes it possible to trace the sensitivity to the FQHE to a particular component of the thermoelectric tensor.