Low-field diffusion magnetothermopower of a high-mobility two-dimensional electron gas

The low-magnetic field-diffusion thermopower of a high-mobility GaAs heterostructure has been measured directly on an electrostatically defined micron-scale Hall-bar structure (4 mumx8 mum) at low temperature (T=1.6 K) in the field regime (Bless than or equal to1.2 T) where the formation of edge states does not influence the measurements. The sample design allowed the determination of the field dependence of the thermopower both parallel and perpendicular to the temperature gradient, denoted respectively by S(xx) (longitudinal thermopower) and S(yx) (the Nernst-Ettinghausen coefficient). The experimental data show clear oscillations in S(xx) and S(yx) due to the formation of Landau levels and reveal that S(yx)approximate to120S(xx) at a magnetic field of 1 T, which agrees well with the theoretical prediction that the ratio of these tensor components is dependent on the carrier mobility: S(yx)/S(xx)=2omega(c)tau.