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Superconductivity in moth balls: surprises in organic transistors

Financial support by. Superconductivity in moth balls: surprises in organic transistors. Jairo Sinova. April 10, 2002. Ref: J. Sinova et al , Phys. Rev. Lett. 87 , 226802 (2001). OUTLINE. Introduction to organic thin film transistors Experimental surprises

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Superconductivity in moth balls: surprises in organic transistors

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  1. Financial support by Superconductivity in moth balls:surprises in organic transistors Jairo Sinova April 10, 2002 Ref: J. Sinova et al, Phys. Rev. Lett. 87, 226802 (2001)

  2. OUTLINE • Introduction to organic thin film transistors • Experimental surprises • Quantum confinement in organic thin films • Superconductivity in organic materials: electron-phonon coupling • Comparison to experiments • Conclusion

  3. OUTLINE • Introduction to organic thin film transistors • Organic field effect transistors (FETs) • Future and present applications of plastic electronics • Materials used in organic field effect transistors and their properties • Experimental Surprises in the past year • 2-D electron transport in organic thin films • Superconductivity in organic materials: electron-phonon coupling • Comparison to experiments • Conclusion

  4. Vg >0 gate S D - - - - - - thin free charge carrier channel induced by electric field from gate Organic Field Effect Transistors • Density of carriers proportional to gate voltage: changes in VG have a dramatic change in channel conductance (important technologically) insulator semiconductor substrate J. H. Schön, S. Berg, Ch. Kloc, and B. Batlogg Science 2000 February 11; 287: 1022-1023 High mobility 2DEG: IQHE, FQHE, MIT, etc.

  5. Applications of plastic transistors: future and present LEDs plastic display Cheaper solar cells Printing plastic transistors and organic LEDs All plastic RAMS?

  6. MATERIALS USED IN ORGANIC FETs a a -6T -4T S S S S C60 S S S S S S Naphthalene: moth balls Anthracence Pentacene Tetracene The aromatic molecules: polyacenes ALSO: Tc=117 K!!

  7. Material Properties of the Polyacenes and organic semiconductors Energy levels of individual molecules Narrow bands in molecular crystal LUMO E HOMO D • Lower mobility than silicon • Soft and flexible (Van-der-Waals bonding) • Larger size molecules: richer vibration spectrum (polaron rich) • Narrow bands: low overlap of conducting orbitals (contrast with metals and silicon); low T • Heavier carrier masses • Polaron physics at higher T ~ 1.5 -3 eV (extended (delocalized) p-electrons)

  8. OUTLINE • Introduction to organic thin film transistors • Experimental Surprises in the past year • 2-D transport experiments in polyacene FETs • What are the key surprises? • Superconductivity: experimental finding • 2-D electron transport in organic thin films • Superconductivity in organic materials: electron-phonon coupling • Comparison to experiments • Conclusion

  9. 2DEG in Organic FETs: physical effects galore MF FQHE IQHE Gate gate insulator (Al2O3) MIT source and drain 2D Electron/Hole Gas FQHE SC MF IQHE MIT p / cm-2 1014 increasing voltage 1013 1012 1011 1010 experiments by Batlogg, et al; courtesy of Dr. A. Dodabalapour 109

  10. Gate-Induced Superconductivity in Polyacenes Increase of Tc with decreasing molecular size Similar behavior for oligothiophenes (a-4T, a-6T, and a-8T) J. H.Schön et al. Phys Rev. B 64, 035209 (2001). J. H. Schön et al. Nature 406, 702 (2000) courtesy of Dr. A. Dodabalapour

  11. Gate-Induced Superconductivity in Pentacene Electron-doping (~ 1014 cm-2) x 80 - 100 Å no bulk superconductivity courtesy of Dr. A. Dodabalapour J. H. Schön et al. Nature 406, 702 (2000)

  12. Electron-Phonon coupling strength spectrum experiments Conductance derivative spectrum of a pentacene-Pb tunnel junction Infrared absorption M. Lee, et al, PRL 86, 862 (2001)

  13. Questions and Puzzles • How can so many effects occur in one single sample? • How 2-d is the quantum confinement? • What electron-phonon coupling drives the superconductivity? • Is the FQHE regime highly interacting? • Is the vibrational spectrum affected by the injected electrons? • Is this behavior generic to all organic materials? . . .

  14. OUTLINE • Introduction to organic thin film transistors • Experimental Surprises in the past year • 2-D electron transport in organic thin films • Self-consistent calculation of the electronic structure • How two dimensional is the system? How many sub-bands are occupied? • Superconductivity in organic materials: electron-phonon coupling • Comparison to experiments • Conclusion

  15. VG=0 VG>0 1.3 eV 1.3 eV conduction band conduction band valence band valence band nm nm How confined are the carriers at the interface?:2D or not 2D VG Model calculation: organic semiconductor (anthracene) Au Al2O3 local density self consistent mean field calculation of the bands (continuous) Important parameters: dielectric constants, density of carriers, lattice constant, insulator-semiconductor gap difference.

  16. OUTLINE • Introduction to organic thin film transistors • Experimental Surprises in the past year • 2-D electron transport in organic thin films • Superconductivity in organic materials: electron-phonon coupling • General BCS superconductivity • Model: what type of electron-phonon to consider? • Vibrational spectrum calculation • Comparison to experiments • Conclusion

  17. Superconductivity: B-C-S • In normal superconductors electrons form pairs (Cooper pairs) • Phonon assisted, carriers have opposite spins • Cooper pairs follow B-E statistics and a ‘condensation’ leads to SC SC in organic (polyacenes) materials • 2D electrons-3D phonons • non-standard e-ph coupling • Rich vibrational spectra Gate gate insulator (Al2O3) source and drain 2D Electron/Hole Gas

  18. Su-Schrieffer-Heeger coupling Modeling electron-phonon coupling in anthracene after the LDA/Hartree calculation this reduces to A B

  19. non-degenerate LUMO/HOMO level no elec-phon coupling when screening is present On the omission of the Holstein term A. Devos and M. Lannoo, PRB 58, 8236 (1998) This is NOT the case in fullerenes where the Holstein term is dominant and the SSH term is much smaller

  20. 3D Phonon Spectrum phonon spectrum dispersion calculation Atom-Atom potential model using the Williams’ parameters to obtain the secular equation Taddei, et al., J. Chem. Phys. 58, 966 (73) Dorner et al., J. Phys. C 15, 2353 (82) J. Sinova et al, PRL 87, 226802 (01)

  21. 2D electron-3D phonon term

  22. assume t is proportional to orbital overlap • obtain orbitals using the Hückel approximation Calculation of orbital overlap n.n. distance

  23. OUTLINE • Introduction to organic thin film transistors • Experimental Surprises in the past year • 2-D electron transport in organic thin films • Superconductivity in organic materials: electron-phonon coupling • Comparison to experiments • Electron-phonon coupling calculation, Tc calculation • Agreement and predictions • Conclusion/Final message

  24. Tc~2 K J. Sinova et al, PRL 87, 226802 (01) Calculation and experiment comparison experiments calculation A C B n2d~1/mo n2d~0.2-0.7/mol M. Lee, et al, PRL 86, 862 (2001) C A B J. H. Schön et al. Nature 406, 702 (2000)

  25. DOS and SC relations: injected carrier density trends • Rounded by disorder • SC will go away if p increases beyond half filling

  26. Model Calculation Results and Predictions • Shows the sharp onset of SC with gate voltage • Agreement with peaks observed in • absorption/tunneling experiments • Correct order of Tc (~2K compared with ~3K • in experiments) • Tc should increase with pressure (with t0) in • contrast with the fullerenes • SC will disappear as p goes beyond half filling • in single band FET organic semiconductors

  27. UPDATE FROM MM 02: C. Kloc Not same material but similar SC physics

  28. A Final Message From The Prophetic Mr.McGuire Mr. McGuire was right: there is a future in plastics work done in collaboration with Alvaro S. Nuñez John Schliemann Allan H. MacDonald Tomas Jungwirth

  29. 2DEG in Organic FETs: physical effects galore Gate gate insulator (Al2O3) source and drain 2D Electron/Hole Gas p / cm-3 MF 1014 increasing voltage FQHE 1013 SC 1012 1011 1010 IQHE 109 experiments by Batlogg, et al MIT

  30. Mermin-Wagner Theorem: an academic exercise in a MF regime No true long range order in 2-D Thermal and quantum fluctuations destroy it In a mean field regime these fluctuations are very small and superfluid-stiffness very large

  31. Anisotropy (Pentacene) 3D Band Transport High T (~ 400 K) : Crossover to Hopping

  32. Metal-Insulator-Transition in 2D Electron Density : 61010 - 51011 cm-2 Peak mobility : 2104 cm2/Vs Critical Concentration : pc 3.21011 cm-2 Strong El.-El. Interact. m* ~ 1.5 me eeff ~ 6

  33. hlo = N hc hc = eB/m* 1/BN hlo = N e/m* Magneto-Phonon Effect  m*(T) Resonant Scattering of Charge Carriers between Landau-Levels by LO-Phonons V. L. Gurevich and Y. A. Firsov, Zh. Eksp. Teor. Fiz. 40, 198 (1961) (Sov. Phys.JETP 13, 137 (1961)). R. A. Stradling and R. A. Wood,` J. Phys. C1, 1711 (1968) Measurement of Effective Mass as a Function of Temperature

  34. Fermi Liquid behavior: excuse for BCS approach

  35. Hc2 and 

  36. Su-Schrieffer-Heeger electron-phonon coupling Assume crystal screening : omission of the Holstein term Modeling electron-phonon coupling in anthracene after the LDA/Hartree calculation this reduces to A B

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