Experimental Studies of Transport in Reduced
A large fraction of current research in condensed matter is aimed at new materials. We mainly focus on the transport properties of reduced dimensionality conductors, in particular 2 dimensional electron gases (2DEGs) at heterojunctions (i.e. at the interface between two different semiconductors such as GaAs/GaAlAs) and in Si-MOSFETs. However we are also working on novel 3D materials; as an example we recently initiated a study of the transport properties of HgSe doped with Fe, a degenerate small-gap semiconductor.
We are particularly interested in thermoelectricity which gives, among other things, information about the coupling of the 2DEG with lattice vibrations (phonons). An example of power of this technique is our recent demonstration that, at low temperatures, the momentum relaxation time due to electron-phonon coupling can be measured with high precision using thermoelectricity, regardless of the mobility of the sample. Previous studies have been limited to very high mobility samples. We have also applied the technique to composite fermions, a new quasiparticle which exists only at very high magnetic fields in 2DEGs. Similar, but complementary, information is available from studies of the energy loss from hot electrons which is typically dominated by phonon emission. This gives the electron-phonon energy relaxation time which we have also studied extensively.
Much of our work is aimed at understanding the effects of electronic localization and the metal-insulator transition on thermoelectric transport. Some of our most recent work in this area involves the re-entrant metal insulator transition in SiGe hole gases in the quantum Hall regime. We are also interested in the behaviour of double quantum wells which have the interesting feature that each well partially screens the scattering potential from the other. The publications listed below and a review give an idea of the work that we have been pursuing over the last few years.
The experiments cover a wide range of temperature, and often involve the effects of magnetic fields. Our apparatus is suitable for the temperature range 0.3-200K with magnetic fields up to 8T, but experiments are also carried out at magnetic fields up to about 30T and at temperatures down to 0.1K at the High Field Magnet Lab at the University of Nijmegen, The Netherlands. We also collaborate extensively with various other organizations including the National Research Council, Ottawa.
R. Fletcher, M. Tsaousidou, P. T. Coleridge, Y. Feng and Z. R. Wasilewski, (2002) "Electron-phonon coupling and phonon drag thermopower of a very low mobility 2DEG", Physica E (Low dimensional systems and nanostructures) 12, 478 . Reprint
C. Possanzini, L. Ponomarenko, D. de Lang, A. de Visser, S. M. Olsthoorn, R. Fletcher, Y. Feng, P. T. Coleridge, R. L. Williams and J. C. Maan, (2002) "Scaling behaviour of Metal-Insulator transitions in a Si/SiGe 2-dimensional hole gas" , Physica E (Low dimensional systems and nanostructures) 12, 600-603. Reprint
R. Fletcher, V. M. Pudalov A. D. B. Radcliffe and C. Possanzini, (2001) "Critical behaviour of thermopower and conductivity at the metal-insulator transition in high-mobility MOSFETs" Semicond. Sci. and Tech. 16, 386-93.
R. Fletcher, Y. Feng, C. T. Foxon and J. J. Harris, (2000) "The electron-phonon interaction in a very low mobility GaAs/GaAlAs delta-doped gated quantum well" Phys. Rev. B 61, 2028-33.
R. Fletcher, V. M. Pudalov, M. Tsaousidou and P. N. Butcher, (2000) " The thermopower of a high mobility Si-MOSFET around the metal-insulator transition", Physica E (Low dimensional systems and nanostructures) 6 272-5.
R. Fletcher, (1999) " Magnetothermolectric effects in semiconductor systems", Invited Review for Semiconductor Science and Technology 14 R1-15. Preprint
B. Tieke, R. Fletcher, U. Zeitler, M. Heninn and J. C. Maan (1998) "Thermopower measurements of the coupling of phonons to electrons and composite fermions ", Phys. Rev. B 58 , 2017-25.
R. Fletcher, V. M. Pudalov and S. Cao (1998) " Diffusion thermopower of a silicon inversion layer at low magnetic fields ", Phys. Rev. B 57 , 7174-81.
A. Miele, R. Fletcher, E. Zaremba, Y. Feng, C. T. Foxon and J. J. Harris (1998) " Phonon drag thermopower and weak localization ", Phys. Rev. B 58, 13181-90.
Last revised Sept 2005. Now retired.