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Spin and Charge Transport in Carbon-based Molecular Devices

Spin and Charge Transport in Carbon-based Molecular Devices. Rafael Gutierrez Molecular Computing Group University of Regensburg Germany. P. W. Chiu et al. Appl. Phys. Lett. 80,3811 (2002). nanoscale electrodes. H. Watanabe et al., Appl. Phys. Lett. 78, 2928 (2001).

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Spin and Charge Transport in Carbon-based Molecular Devices

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  1. Spin and Charge Transport in Carbon-based Molecular Devices Rafael Gutierrez Molecular Computing Group University of Regensburg Germany MC Group Regensburg

  2. P. W. Chiu et al. Appl. Phys. Lett. 80,3811 (2002) nanoscale electrodes H. Watanabe et al., Appl. Phys. Lett. 78, 2928 (2001) A. Rochefort et al, PRB 60,13824 (1999) Carbon-based electronics MC Group Regensburg http://www.pa.msu.edu/cmp/csc/nanotube.html

  3. Charge CNT-C60-CNT transport Spin FM-MWCNT-FM transport elastic transport kBT=0 linear conductance Contact effects Structural modifications of the junction Outline MC Group Regensburg

  4. Density-functional (DF)-based tight-binding approach: • Expand eigenstates into valence LCAO basis • Extended Hückel-like Hamiltonian ~ Hab ,Sabvia DFT • G. Seifert and H. Eschrig Z. Phys. Chem. 267, 529 (1986) • D. Porezag et al. Phys. Rev. B 51, 12947 (1995) Landauer 2-terminal Transmission T conductance g Elastic scattering Methodology Green function techniques F. Grossmann, RG and R. Schmidt, ChemPhysChem 3, 101 (2002) MC Group Regensburg

  5. Rotate the molecule CNT-C60-CNT junctions Motivation: 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 ? RG, G. Fagas, G. Cuniberti, F. Grossmann, K. Richter, and R. Schmidt, Phys. Rev. B65, 11341 (2002) MC Group Regensburg

  6. HOMO LUMO • Structural optimization is essential • Strong mixing of CNT-states with C60 molecular orbitals~ lifting of degeneracies MC Group Regensburg

  7. Strong orientational dependence of the conductance! • ~ Variations of 2-3 orders of magnitude near EF MC Group Regensburg

  8. Molecular state? Do the caps introduce something new ? MC Group Regensburg

  9. PDOS on CNT-caps PDOS on C60 Evolution of the projected DOS with increasing (a->d) CNT-C60 separation MC Group Regensburg

  10. Do the caps introduce something new ? pentagonal defect Molecular state? Metallisation via ”unconventional” MIGS NO ! RG, G. Fagas,K. Richter, F. Grossmann and R. Schmidt, Europhys. Lett. 62, 90 (2003) MC Group Regensburg

  11. Switching behaviour MC Group Regensburg

  12. DOS Experiments: K. Tsukagoshi et al. Nature 401, 572 (1999), B. Zhao et al. APL 80, 3141 (2002) • Spin-coherent transport ls ~ 200 nm-1mm • Negative GMR ~ 30 % GMR in FM-MWCNT-FM junctions MC Group Regensburg

  13. (2,2)@(6,6) Co(111) Co(111) • No mixing of up- and down-spin channels • electrodes ~ single-band model • MWCNT ~ p-orbitals, inter-wall inter. tin = const. • (2,2)@(6,6) ~ perfect interface matching Charge neutrality A minimal model MC Group Regensburg

  14. ~4e2/h=2G0 ~2e2/h=1G0 Paramagnetic case P=0 • Conductance(EF ,tin=0) ~ 2G0 for full contacted MWCNTexpected value for infinite metallic DWCNT ~ 4G0 (tin=0) • channel blocking ~ charge transfer+CNT-metal contact symmetry 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) MC Group Regensburg

  15. Ferromagnetic case P=0.5 • 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 MC Group Regensburg

  16. Elastic transport charge spin CNT-C60-CNT GMR in FM-DWCNT-FM junctions (capped) CNT-C60-CNT Incommensurability ~structural disorder Inelastic transport: electron-vibron coupling Keldysh NEGF techniques MC Group Regensburg

  17. Acknowledgements 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 MC Group Regensburg

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