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

2. 2 MC Group Regensburg 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) nanoscale electrodes http://www.pa.msu.edu/cmp/csc/nanotube.html

3. 3 MC Group Regensburg Outline elastic transport k B T=0 Charge CNT-C 60 -CNT transport Spin FM-MWCNT-FM transport linear conductance Contact effects Structural modifications of the junction

4. 4 MC Group Regensburg 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 ~ H ab,S ab 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

5. 5 MC Group Regensburg CNT-C 60 -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 ? Rotate the molecule RG, G. Fagas, G. Cuniberti, F. Grossmann, K. Richter, and R. Schmidt, Phys. Rev. B65, 11341 (2002)

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

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

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

9. 9 MC Group Regensburg Evolution of the projected DOS with increasing (a->d) CNT-C 60 separation PDOS on CNT-caps PDOS on C 60

10. 10 MC Group Regensburg 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 ?

11. 11 MC Group Regensburg Switching behaviour

12. 12 MC Group Regensburg 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 l s ~ 200 nm-1  m Negative GMR ~ 30 % DOS

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

14. 14 MC Group Regensburg Conductance(E F,t in =0) ~ 2G 0 for full contacted MWCNT expected value for infinite metallic DWCNT ~ 4G 0 (t in =0) channel blocking ~ charge transfer+CNT-metal contact symmetry Paramagnetic case P=0 ~4e 2 /h=2G 0 ~2e 2 /h=1G 0 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)

15. 15 MC Group Regensburg full contact: GMR < 0 partial contact: GMR > 0 GMR weakly affected by t in Charge neutrality essential S. Krompiewski, RG and G. Cuniberti, cond-mat/0402359 Ferromagnetic case P=0.5

16. 16 MC Group Regensburg Elastic transport Inelastic transport: electron-vibron coupling Keldysh NEGF techniques Incommensurability ~structural disorder charge spin CNT-C 60 -CNT (capped) CNT-C 60 -CNT GMR in FM-DWCNT-FM junctions

17. 17 MC Group Regensburg 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. 11 35 ) F. Grossmann (TU-Dresden) R. Schmidt (TU-Dresden) A. Di Carlo (Tor Vergata, Rome, Talk Wed. 14 30 ) A. Pecchia (Tor Vergata, Rome, Poster) M. Gheorghe (Uni Regensburg, Poster) C. Böhme (Uni Marburg) MPIPKS+ADMOL Acknowledgements