Experimental results on the high-field thermopower of a two-dimensional electron gas in a GaAs-${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Al}}_{\mathrm{x}}$As heterojunction

We report experimental results on the thermopower $S_{\mathrm{xx}}$ and Nernst-Ettingshausen coefficient $S_{\mathrm{yx}}$ for a GaAs-${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Al}}_{\mathrm{x}}$As heterojunction both at zero field and at high magnetic fields. Although the high-field data are qualitatively in agreement with the theoretical models based on diffusion effects, the experimental magnitudes are much larger than those predicted, with the discrepancy in the high-field limit (ν=(1/2)) being about two orders of magnitude. It seems probable that phonon drag is responsible, but detailed predictions are not yet available. Although $S_{\mathrm{xx}}$ resolves spin splitting with a clarity similar to that seen in ${\rho }_{\mathrm{xx}}$, the fractional quantum Hall effect (FQHE) is not visible in $S_{\mathrm{xx}}$ at ν=(1/3) even though it is clearly seen in ${\rho }_{\mathrm{xx}}$. The FQHE is observed in $S_{\mathrm{yx}}$ but we argue that this is not as significant as its absence in $S_{\mathrm{xx}}$.