Thermopower of a 2D electron gas in suspended AlGaAs/GaAs heterostructures

We present thermopower measurements on a high electron mobility two-dimensional electron gas (2DEG) in a thin suspended membrane.We show that the small dimension of the membrane substantially reduces the thermal conductivity compared to bulk material so that it is possible to establish a strong thermal gradient along the 2DEG even at a distance of few micrometers. We find that the zero-field thermopower is significantly affected by the micro patterning. In contrast to 2DEGs incorporated in a bulk material, the diffusion contribution to the thermopower stays dominant up to a temperature of 7 K until the phonon-drag becomes strong and governs the run of the thermopower. We also find that the coupling between electrons and phonons in the phonon-drag regime is due to screened deformation potentials, in contrast to piezoelectric coupling found with bulk phonons.Comment: 7 page

Magnetothermal Conductivity of Highly Oriented Pyrolytic Graphite in the Quantum Limit

We report on the magnetic field (0T$\le B \le 9$T) dependence of the longitudinal thermal conductivity $\kappa(T,B)$ of highly oriented pyrolytic graphite in the temperature range 5 K $\le T\le$ 20 K for fields parallel to the $c-$axis. We show that $\kappa(T,B)$ shows large oscillations in the high-field region (B > 2 T) where clear signs of the Quantum-Hall effect are observed in the Hall resistance. With the measured longitudinal electrical resistivity we show that the Wiedemann-Franz law is violated in the high-field regime.Comment: 4 Figures, to be published in Physical Review B (2003

Thermal conductivity and the Wiedemann-Franz law in the fractional quantum Hall effect regime around ν = 2

The range of validity of Wiedemann-Franz (WF) law is investigated for the quantum Hall effect regime around ν=12. The composite fermions (CFs) picture along with the appropriate transport theory enables us to use the integer quantum Hall effect and Shubnikovde Haas conductivity models to calculate the diagonal and non-diagonal components of the electrical and thermal diffusion conductivity tensors. The analysis shows that at ν=12 the system and CF components satisfy the Wiedemann-Franz law. The CF components satisfy the Wiedemann-Franz law for the whole range of the low effective magnetic fields while away from ν=12 the system components violate it. At high effective magnetic fields the CF components behave exactly as in the integer quantum Hall effect regime. The system's behavior at high effective magnetic fields violate the Wiedemann-Franz law for any value of the Landau level broadening. This along with the system's behavior at low fields around ν=12 demonstrates the fact that the difference in the physical mechanisms responsible for the Integer and the Fractional quantum Hall effect is reflected upon fundamental physical laws such as the Wiedemann-Franz law. © 2010 Elsevier Ltd

Thermoelectric transport of composite fermions at (formula presented) and (formula presented) A simple way of evaluating p

We propose a simple and generic way of evaluating p directly from the ratio of the experimental values of the diagonal components of the resistivity (formula presented) at filling factors (formula presented) and (formula presented) in the fractional quantum Hall regime. The p value determines the energy dependence of the scattering time, and the diffusion thermopower of the system. We use the idea of parallel conduction of two gases. One gas, composed of electrons, fully occupies one of the two spin levels of the lowest Landau level, and a second, composed of composite fermions, partially occupies the other spin level. The analysis is free of limitations connected with the specific scattering mechanisms and the nature of the carriers. The validity of the method is tested successfully, using the available experimental data, for electron and hole gases. © 2001 The American Physical Society

Systematic study of the wiedemann-franz law in the quantum-hall-effect regime

A systematic study of the Wiedemann-Franz law in the quantum-Hall-effect regime is presented. For this purpose the diffusion thermal conductivity tensor is calculated for a two-dimensional electron gas at low temperatures in a quantizing magnetic field. The range of validity of the Wiedemann-Franz law is investigated performing both analytical and numerical calculations. The analysis shows that for the diagonal component of the thermal conductivity, the Wiedemann-Franz law is violated with decreasing Landau level broadening. Responsible is the coefficient (Formula presented) and specifically the effect of the energy derivatives of the diagonal electrical conductivity and consequently the shape of the density of states. For the nondiagonal components we obtain smaller deviations. We give physical interpretations for the resulted behavior. © 1999 The American Physical Society

Electrical transport of composite fermions at ν= 3/2

The resistivity is calculated for a two-dimensional electron gas at low temperatures in the fractional quantum Hall effect regime at a filling factor ν = 3/2. The composite fermion picture enables us to use the integer quantum Hall effect and Shubnikov-de Haas (SdH) conductivity models for a quantitative comparison with experiment. We use the idea of parallel conduction of two gases. One gas, composed of electrons, fully occupies one of the two spin levels of the lowest Landau level, and a second, composed of composite fermions, partially occupies the other spin level. Two different formulas for the analysis of the SdH oscillations are used for the weak effective magnetic-field region and the large magnetic-field region, respectively, and satisfactory agreement with experiment is obtained. Comparison with the ν = 1/2 case is made. © 2001 The American Physical Society

Small polaron hopping transport along DNA molecules

We present a small polaron hopping model for interpreting the strong temperature (T) dependence of the electrical conductivity, σ, observed at high (h) temperatures along DNA molecules. The model takes into account the one-dimensional character of the system and the presence of disorder in the DNA double helix. Percolation-theoretical considerations lead to analytical expressions for the high temperature multiphonon-assisted small polaron hopping conductivity, the hopping distance and their temperature dependence. The experimental data for lambda phage DNA (λ-DNA) and poly(dA)-poly(dT) DNA follow nicely the theoretically predicted behaviour (ln σh ∼ T-2/3). Moreover, our model leads to realistic values of the maximum hopping distances, supporting the idea of multiphonon-assisted hopping of small polarons between next nearest neighbours of the DNA molecular 'wire'. The low temperature case is also investigated. © 2005 IOP Publishing Ltd