nanoelectronics - Institut d'Études Scientifiques de Cargèse (IESC)

Tuesday
Anomalous Thermopower in a Low-Density Two-Dimensional Electron System:
Metallic Dependence, Giant Magnitude and Oscillatory density-dependence
Vijay Narayan 1 , M. Pepper 2 , J. Griffiths 1 , H. Beere 1 , F. Sfigakis 1 , G. Jones 1 , D. Ritchie 1 and
A. Ghosh 3
1 Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3
0HE, United Kingdom
2 Department of Electronic and Electrical Engineering, University College London, Torrington
Place, London WC1E 7JE, United Kingdom
3 Department of Physics, Indian Institute of Science, Bangalore 560012, India
We present thermopower S and electrical resistivity ρ 2DES measurements in a low-density
(~ 10 14 m -2 ) two-dimensional electron system (2DES). We are interested in the regime where
ρ 2DES >> h/e 2 and consequently the 2DES is expected to be strongly localised. Remarkably,
however, we observe several aspects to the 2DES behaviour that are not consistent with
insulating behaviour. First, the temperature-dependence of S is manifestly metallic, S
growing linearly as a function of temperature up to ≈ 0.7 K. Second, the magnitude of S
exceeds the Mott value valid for non-interacting metallic 2DESs at similar carrier densities by
over two orders of magnitude. And third, we observe a seeming decoupling between ρ 2DES
and S in their density-dependence whereby strong oscillations and even sign changes are
observed in the latter which are completely absent in the former. We explore the role of the
many-body Coulomb potential in these observations and the scope offered by this system
towards sub-Kelvin Peltier refrigeration.
[1] Vijay Narayan, M. Pepper, J. Griffiths, H. Beere, F. Sfigakis, G. Jones, D. Ritchie and
A. Ghosh, Unconventional metallicity and giant thermopower in a strongly interacting twodimensional
electron system, Phys. Rev. B 86, 125406 (2012)
[2] Vijay Narayan, M. Pepper, J. Griffiths, H. Beere, F. Sfigakis, G. Jones, D. Ritchie and
A. Ghosh, Evidence of Novel Quasiparticles in a Strongly Interacting Two-Dimensional
Electron System: Giant Thermopower and Metallic Behaviour, J. Low Temp. Phys
(accepted), DOI 10.1007/s10909-012-0718-0
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Delay-time and thermopower distributions at the spectrum edges of a chaotic
scatterer
A. Abbout (1,2) , G. Fleury (1) , J.-L. Pichard (1) and K. Muttalib (3)
(1)
Service de Physique de l'Etat Condensé CNRS URA 2464, CEA Saclay, 91191 Gif-sur-
Yvette, France
(2)
Laboratoire CRISMAT, CNRS UMR 6508, 6 boulevard Maréchal Juin, F-14050, Caen,
France
(3)
Department of Physics, University of Florida, Gainesville, FL 32611-8440, USA
We study chaotic scattering outside the wide band limit, as the Fermi energy E F approaches
the band edges E B of a one-dimensional lattice embedding a scattering region of M sites [1].
The Hamiltonian H M of this region is taken from the Cauchy orthogonal ensemble. The
scattering is chaotic at E F if the average level density per site of H M at E F describes a semicircle
as E F varies inside the conduction band. The edges of this semi-circle coincide with the
band edges E B . We show that the delay-time and thermopower distributions differ near the
edges from the universal expressions valid in the bulk [2]. To obtain the asymptotic universal
forms of these edge distributions, one must keep constant the energy distance E F – E B
measured in unit of the same energy scale (proportional to M -1/3 ) which is used for rescaling