1 mc group regensburg spin and charge transport in carbon-based molecular devices rafael gutierrez...
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Spin and Charge Transport in Carbon-based Molecular Devices
Rafael GutierrezMolecular Computing Group University of RegensburgGermany
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Carbon-based electronics
A. Rochefort et al, PRB 60,13824 (1999)
P. W. Chiu et al. Appl. Phys. Lett. 80,3811 (2002)
H. Watanabe et al.,Appl. Phys. Lett. 78, 2928 (2001)
nanoscaleelectrodes
http://www.pa.msu.edu/cmp/csc/nanotube.html
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Outline
elastic transport kBT=0
Charge CNT-C60-CNT transport
Spin FM-MWCNT-FM transport
linear conductance
Contact effects
Structural modifications of the junction
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Methodology
F. Grossmann, RG and R. Schmidt, ChemPhysChem 3, 101 (2002)
Density-functional (DF)-based tight-binding approach:• Expand eigenstates into valence LCAO basis• Extended Hückel-like Hamiltonian ~ Hab ,Sab via DFT
G. Seifert and H. Eschrig Z. Phys. Chem. 267, 529 (1986)D. Porezag et al. Phys. Rev. B 51, 12947 (1995)
Green function techniques
2-terminal Transmission T conductance g
Elastic scattering
Landauer
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CNT-C60-CNT junctionsMotivation:C. Joachim et al. Phys. Rev. B 58, 16407 (1998) ~ compression J. J. Palacios et al. Nanotechnology 12, 160 (2001) ~ charge transfer doping
Alternative way to modify the transmission ?
Rotate the molecule
RG, G. Fagas, G. Cuniberti, F. Grossmann, K. Richter, and R. Schmidt, Phys. Rev. B65, 11341 (2002)
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HOMO LUMO
• Structural optimization is essential• Strong mixing of CNT-states with C60 molecular orbitals
~ lifting of degeneracies
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• Strong orientational dependence of the
conductance!
• ~ Variations of 2-3 orders of magnitude
near EF
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Do the caps introduce something new ?
Molecular state?
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Evolution of the projected DOS with increasing (a->d) CNT-C60 separation
PDOS onCNT-caps
PDOS on C60
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Molecular state?
Metallisation via
”unconventional” MIGS
pentagonal defect
NO !
RG, G. Fagas,K. Richter, F. Grossmann and R. Schmidt, Europhys. Lett. 62, 90 (2003)
Do the caps introduce something new ?
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GeVHH 0
Switching behaviour
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GMR in FM-MWCNT-FM junctions
Experiments : K. Tsukagoshi et al. Nature 401, 572 (1999), B. Zhao et al. APL 80, 3141 (2002)
• Spin-coherent transport ls ~ 200 nm-1m
• Negative GMR ~ 30 %
P
APP
gggGMR
DOS
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Co(111) Co(111)
(2,2)@(6,6)
)(Im1 EGdEn r
Charge neutrality
A minimal model
• No mixing of up- and down-spin channels
• electrodes ~ single-band model
• MWCNT ~ -orbitals, inter-wall inter. tin = const.• (2,2)@(6,6) ~ perfect interface matching
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• Conductance(EF ,tin=0) ~ 2G0 for full contacted MWCNT
expected value for infinite metallic DWCNT ~ 4G0 (tin=0)• channel blocking ~ charge transfer+CNT-metal contact
symmetry
Paramagnetic case P=0
~4e2/h=2G0 ~2e2/h=1G0
see also e.g., S. Sanvito et al. Phys. Rev. Lett. 84, 1974 (2000);J. J. Palacios et al. Phys. Rev. Lett. 90, 106801 (2003)
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• full contact: GMR < 0• partial contact: GMR > 0 • GMR weakly affected by tin
• Charge neutrality essential
S. Krompiewski, RG and G. Cuniberti, cond-mat/0402359
Ferromagnetic case P=0.5
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Elastic transport
Inelastic transport: electron-vibron couplingKeldysh NEGF techniques
Incommensurability~structural disorder
charge spin
CNT-C60-CNT
(capped)CNT-C60-CNT
GMR in FM-DWCNT-FMjunctions
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G. Cuniberti (MC-Group,Uni Regensburg)G. Fagas (NMRC, Cork, Ireland, Poster)K. Richter (Uni Regensburg)S. Krompiewski (IfMP-Poznan, Poster)M. Hartung (Uni Regensburg, Poster)N. Ranjan (TU-Dresden, Poster)G. Seifert (TU Dresden, Talk Fri. 1135)F. Grossmann (TU-Dresden)R. Schmidt (TU-Dresden)A. Di Carlo (Tor Vergata, Rome, Talk Wed.
1430)A. Pecchia (Tor Vergata, Rome, Poster)M. Gheorghe (Uni Regensburg, Poster)C. Böhme (Uni Marburg)
MPIPKS+ADMOL
Acknowledgements