| An experimental study for thermo-electric power (TEP) in a two-dimensional (2D) electron gas at low temperatures (T) has been completed. At zero magnetic field (B) and at T > 300 mK, the TEP of high-mobility samples has shown a temperature dependence in the form of a power law (T3--4), which indicates that the phonon-drag TEP is dominant and the diffusive TEP is negligible. Under this condition, TEP measurement can be applied to directly investigate electron-phonon interaction because impurity scattering is not directly relevant. In a small magnetic field (B < 0.3 T) and at T < 1 K, a new type of TEP oscillations has been observed. These oscillations result from the inter-Landau-Level (LL) resonance of electrons by acoustic phonons carrying a momentum equal to twice the Fermi wave number ( kF) at B = 0. For the first time, the inter-LL scattering is observed in TEP measurement. Numerical calculations show that both three-dimensional (3D) and two-dimensional (2D) interface phonons can contribute to this effect.; The TEP in the lowest LL has shown remarkable structure at fractional LL filling factors nu = 2/3 and 3/5. At both 2/3 and 3/5, the TEP evolve from a steep minimum at low T into a strong maximum at high T. Between 300 mK and 1.5 K the TEP at these filling factors are thermally activated.; Besides TEP measurement, magnetoresistance (MR) measurement is employed to study one-half state (nu = 1/2) at low temperatures (down to 50 mK) and ultrahigh magnetic fields (up to 42 T) in a square quantum well sample. Diagonal resistance (Rxx) exhibits a sharp, strongly temperature-dependent minimum centered at nu = 1/2, whereas concomitant Hall resistance (Rxy) does not develop into quantized plateau. The first derivative of Hall resistance with respect to magnetic field shows a sharp temperature-dependent minimum at nu = 1/2. These data deviate significantly from the characteristic transport features of composite Fermions (CFs) in single heterojunction samples. |
