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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