Detailed measurements of the longitudinal thermopower of two-dimensional electrons in the first excited Landau level are reported. Clear signatures of numerous fractional quantized Hall states, including those at ν = 5/2 and 7/3, are observed in the magnetic field and temperature dependence of the thermopower. An abrupt collapse of the thermopower is observed below about T = 40 mK at those filling factors where re-entrant insulating electronic states have been observed in conventional resistive transport studies. The thermopower observed at ν = 5/2 is discussed in the context of recent theories which incorporate non-abelian quasiparticle exchange statistics.

The longitudinal thermopower of ultrahigh mobility two-dimensional ͑2D͒ electrons has been measured at both zero magnetic field and at high fields in the compressible metallic state at filling factor =3/ 2 and the incompressible fractional quantized Hall state at =5/ 2. At zero field our results demonstrate that the thermopower is dominated by electron diffusion for temperatures below about T = 150 mK. A diffusion-dominated thermopower is also observed at =3/ 2 and allows us to extract an estimate of the composite fermion effective mass. At =5/ 2 both the temperature and magnetic field dependence of the observed thermopower clearly signal the presence of the energy gap of this fractional quantized Hall state. We find that the thermopower in the vicinity of =5/ 2 exceeds that recently predicted under the assumption that the entropy of the 2D system is dominated by non-Abelian quasiparticle exchange statistics.

A simple hot electron thermocouple is realized in a two-dimensional electron system (2DES) and used to measure the diffusion thermopower of the 2DES at zero magnetic field. This hot electron technique, which requires no micron-scale patterning of the 2DES, is much less sensitive than conventional methods to phonon-drag effects. Our thermopower results are in good agreement with the Mott formula for diffusion thermopower for temperatures up to T ∼ 2 K.PACS numbers: 73.50. Lw, 73.63.Hs, 72.20.Pa The thermoelectric properties of low-dimensional electronic systems provide information about carrier transport that is complementary to that obtained from ordinary charge transport. For example, in an ordinary Drude metal the electrical conductivity σ is simply proportional to the momentum scattering time τ . In contrast, the diffusion thermopower S d depends upon both τ and its energy derivative dτ /dE [1]. Additional motivation for measuring thermopower comes from its close connection to the entropy per particle in the low dimensional system. While this connection has been long appreciated for non-interacting electrons [2], it has also been found to hold in strongly interacting, disorder-free cases at high magnetic field, notably the half-filled lowest Landau level [3]. Very recently it has been suggested that thermopower may even reflect the excess entropy associated with non-abelian quasiparticle exchange statistics [4]. Finally, beyond these very fundamental motivations there is also the simple fact that thermopower, harnessed in a humble thermocouple device, provides a very effective way to measure temperature.In semiconductor-based two-dimensional electron systems experimental access to the diffusion thermopower and the important information it contains has been limited by the parasitic effects of phonons [5,6]. In a typical experiment the needed temperature gradient is established by applying heat to one end of a bar-shaped sample while the other end is thermally "grounded". The overwhelming majority of the applied heat is transported by phonons. The resulting phonon wind exerts a drag force on the electron gas which leads to a thermoelectric voltage independent of that arising from the diffusion thermopower of the electrons themselves. This phonon-drag thermopower, S ph , can exceed S d by more than an order of magnitude. Only by going to very low temperatures (T 0.2 K) can the diffusion component of the thermopower be observed in such experiments [7].In this paper we report new measurements of the thermopower S of a two-dimensional electron system (2DES) in a GaAs/AlGaAs heterostructure. A simple hot electron technique is employed which greatly reduces the con- tribution of phonon drag to the measured thermopower [8]. As a result we are able to show that the magnitude and the temperature and density dependences of S are in good agreement with the Mott formula [9] for the diffusion thermopower S d for temperatures up to about T ∼ 2 K, an order of magnitude higher than in previous measurements [7]. For T 3 K we...