Superconductivity in moth balls: surprises in organic transistors

1
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
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?
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MATERIALS USED IN ORGANIC FETs
The aromatic molecules polyacenes
ALSO
Tc117 K!!
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Material Properties of the Polyacenes and organic
semiconductors

• 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)
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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
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
J. H. Schön et al. Nature 406, 702 (2000)
J. H. Schön et al. Phys Rev. B 64, 035209 (2001).
courtesy of Dr. A. Dodabalapour
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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)
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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)
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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?

.
.
.
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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
How confined are the carriers at the
interface?2D or not 2D
VG
Model calculation
organic semiconductor (anthracene)
Al2O3
Au
local density self consistent mean field
calculation of the bands (continuous)
Important parameters dielectric constants,
density of carriers, lattice constant, insulator-
semiconductor gap difference.
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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

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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
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Modeling electron-phonon coupling in anthracene
after the LDA/Hartree calculation this reduces to
A
B
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On the omission of the Holstein term
Molecule Degen Unscreened Holstein Coupling (meV) Screened Holstein Coupling (meV)
Anthracene L(1) 166 0
Tetracene L(1) 130 0
Pyrene L(1) 197 0
C60 L(3) 52 47
C28 H(3) 80 80
C20 H(4) 183 183
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
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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)
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2D electron-3D phonon term
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Calculation of
orbital overlap
n.n. distance
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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

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Calculation and experiment comparison
experiments
calculation
A
C
B
n2d1/mo
n2d0.2-0.7/mol
M. Lee, et al, PRL 86, 862 (2001)
C
A
B
J. H. Schön et al. Nature 406, 702 (2000)
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DOS and SC relations injected carrier density
trends

• Rounded by disorder
• SC will go away if p increases beyond half filling

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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

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UPDATE FROM MM 02 C. Kloc
Not same material but similar SC physics
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A Final Message From The Prophetic Mr.McGuire
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2DEG in Organic FETs physical effects galore
p / cm-3
MF
1014
increasing voltage
FQHE
1013
SC
1012
1011
1010
IQHE
109
experiments by Batlogg, et al
MIT
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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
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Anisotropy (Pentacene)
3D Band Transport High T ( 400 K) Crossover
to Hopping
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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
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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
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Fermi Liquid behavior excuse for BCS approach
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Hc2 and ?
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Modeling electron-phonon coupling in anthracene
after the LDA/Hartree calculation this reduces to
A
B
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