Measurement of diffusion thermopower in the quantum Hall systems

We have measured diffusion thermopower in a two-dimensional electron gas at low temperature (T=40 mK) in the field range 0 < B < 3.4 T, by employing the current heating technique. A Hall bar device is designed for this purpose, which contains two crossing Hall bars, one for the measurement and the other used as a heater, and is equipped with a metallic front gate to control the resistivity of the areas to be heated. In the low magnetic field regime (B ≤ 1 T), we obtain the transverse thermopower S yx that quantitatively agrees with the S yx calculated from resistivities using the generalized Mott formula. In the quantum Hall regime (B ≥ 1T), we find that S yx signal appears only when both the measured and the heater area are in the resistive (inter-quantum Hall transition) region. Anomalous gate-voltage dependence is observed above ∼1.8 T, where spin-splitting in the measured area becomes apparent.