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

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

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

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

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

C 60 MATERIALS USED IN ORGANIC FETs Pentacene Tetracene AnthracenceNaphthalene: moth balls The aromatic molecules: polyacenes ALSO: S S S S S S  -6T S S S S  -4T T c =117 K!!

Material Properties of the Polyacenes and organic semiconductors Energy levels of individual molecules LUMO HOMO E  Narrow bands in molecular crystal (extended (delocalized)  -electrons) ~ eV 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

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

experiments by Batlogg, et al; courtesy of Dr. A. Dodabalapour MIT p / cm -2 2DEG in Organic FETs: physical effects galore 2D Electron/Hole Gas Gate source and drain gate insulator (Al 2 O 3 ) increasing voltage FQHE IQHE MF MIT SC

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

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

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

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?...

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

1.3 eV valence band conduction band V G =0 How confined are the carriers at the interface?:2D or not 2D Model calculation : local density self consistent mean field calculation of the bands (continuous) Important parameters: dielectric constants, density of carriers, lattice constant, insulator-semiconductor gap difference. VGVG organic semiconductor (anthracene ) Al 2 O 3 Au 1.3 eV valence band conduction band V G >0 nm

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

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 2D Electron/Hole Gas Gate source and drain gate insulator (Al 2 O 3 )

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

On the omission of the Holstein term A. Devos and M. Lannoo, PRB 58, 8236 (1998) non-degenerate LUMO/HOMO level no elec-phon coupling when screening is present This is NOT the case in fullerenes where the Holstein term is dominant and the SSH term is much smaller MoleculeDegen Unscreened Holstein Coupling (meV) Screened Holstein Coupling (meV) Anthracene L(1)1660 Tetracene L(1)1300 Pyrene L(1)1970 C 60 L(3)5247 C 28 H(3)80 C 20 H(4)183

3D Phonon Spectrum phonon spectrum dispersion calculation J. Sinova et al, PRL 87, (01) 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)

2D electron-3D phonon term

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

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, T c calculation –Agreement and predictions Conclusion/Final message

n 2d ~ /mol Calculation and experiment comparison M. Lee, et al, PRL 86, 862 (2001) calculation experiments J. H. Schön et al. Nature 406, 702 (2000) J. Sinova et al, PRL 87, (01) n 2d ~1/mo A A B B C C T c ~2 K

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

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 T c (~2K compared with ~3K in experiments) T c should increase with pressure (with t 0 ) in contrast with the fullerenes SC will disappear as p goes beyond half filling in single band FET organic semiconductors

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

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

FQHE IQHE MIT SC MF p / cm -3 2DEG in Organic FETs: physical effects galore 2D Electron/Hole Gas Gate source and drain gate insulator (Al 2 O 3 ) increasing voltage experiments by Batlogg, et al

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

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

Metal-Insulator-Transition in 2D Electron Density : 6   cm -2 Peak mobility : 2  10 4 cm 2 /Vs Critical Concentration : p c  3.2  cm -2 Strong El.-El. Interact. m* ~ 1.5 m e  eff ~ 6

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) h  lo = N h  c h  c = eB/m* 1/B N h  lo = N e/m* Measurement of Effective Mass as a Function of Temperature

Fermi Liquid behavior: excuse for BCS approach

H c2 and 

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