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Title. I. molecule. lead 1. lead 2. V. Transport Through Single Molecules: Resonant Transmission, Rectification, Spin Filtering, and Tunneling Magnetoresistance. Harold U. Baranger, Duke University with Rui Liu, San-Huang Ke, and Weitao Yang

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  1. Title I molecule lead 1 lead 2 V Transport Through Single Molecules: Resonant Transmission, Rectification, Spin Filtering, and Tunneling Magnetoresistance Harold U. Baranger, Duke University with Rui Liu, San-Huang Ke, and Weitao Yang Thanks to Michael Fuhrer and Larry Sita, U. Maryland Conductance? I-V curve? e-e interactions? Vibrations? Devices?

  2. How to contact a molecule?? How to contact?? Comp. Tech. • self-assembled alkanethiol monolayers-PRB 48, 1711 (1993) • mechanically controllable break junction –Science 278,252 (1997) • electromigration junction – APL 75, 301 (1999)

  3. Examples: Experiments on Conjugated Molecules Expt. Examples Reed & Tour groups, Science 278, 252 (97) Comp. Tech. Rawlett, et al. APL 81, 3043 (02) Reichert, et al. (Karlsruhe) APL 82, 4137 (03) Organic molecules: gap of order 1 V

  4. m1 m2 • 1. Transmission of incident flux: • Single-particle electron states • Energy of relevant states are in window determined by eV about Fermi energy • Consider flux impinging on molecule from lead 1 • How much gets transmitted? I molecule lead 1 lead 2 V • 2. Electronic structure from Density Functional Theory in local approx. • Use Kohn-Sham theory to get self-consistent equilibrium density & structure Reliable! lots of experience in quantum chemistry • Use Kohn-Sham single-particle states for transmission – NOT JUSTIFIED! • For non-equilibrium, get self-consistent density matrix by filling states coming from lead 1 to m1 and states coming from lead 2 to m2 Theoretical Approach: Two Main Ingredients Method (1) Comp. Tech.

  5. molecule lead 1 lead 2 extended molecule Computational Methods Method (2) • Semi-infinite leads at constant m (no voltagle drop); no spin polarization • Extended molecule: include large amount of leads in the “molecule” • First-principles DFT theory using SIESTA program(Double-zeta plus polarization basis set, optimized Troullier-Martins pseudopotentials, PBE version of GGA functional for exchange-correlation) • Transmission from Green function built from Kohn-Sham orbitals Comp. Tech. San-Huang Ke, H.U.Baranger, and W. Yang, PRB (2004) Datta group, PRB (2001); Ratner group, Chem. Phys. (2002); Guo group, PRB (2003)

  6. Simple case: 1 Carbon ring + S to bond to Au Benzene: T Comp. Tech. [San-Huang Ke]

  7. Additional Au makes a difference: T(E) resonance and NDR! Benzene: I-V Comp. Tech. transmission resonance at Fermi energy negative differential resistance [San-Huang Ke]

  8. Image resonant state Benzene: ldos Comp. Tech. Surface of constant local density of states: (ie. one contour of a 3d contour plot) [San-Huang Ke]

  9. Metallocenes: Organometallic Sandwich Complexes Metallocenes Comp. Tech. M=Fe: ferrocene 6 electrons in levels in box S=0 M=Co: cobaltocene 7 electrons in levels in box S=1/2 [Rob Toreki, Organometallic HyperTextBook]

  10. Experiment: I-V of a phenyl-ethynyl-ferrocene complex Fcene: data 1 !!! Comp. Tech. [Getty, Engtrakul, Wang, Fuhrer, and Sita; U. Maryland]

  11. Experiment: I-V of Ferrocene-OPE compared to OPE Fcene: data 2 !!! Comp. Tech. [Getty, Engtrakul, Wang, Fuhrer, and Sita; U. Maryland]

  12. Conductance of Ferrocene-OPE: Calculation Fcene: calc Comp. Tech. resonance at the Fermi energy [Rui Liu]

  13. Conductance of Fc-OPE: Experiment & Calculation Fcene: calc 2 Comp. Tech.

  14. But what about the OPE control? – It ALSO conducts… Fcene: control • This embarassing result is an example of a long-standing problem in the theory of transport through molecules: theory says fully conjugated molecules should conduct, while experiment shows that they do not. • Not clear why: • because of using the Kohn- Sham wave-functions to find the transmission? • local exchange-correlation? (derivative discontinuity) • … • The fact that the Fc molecule avoids this embarassment is an important clue… Comp. Tech.

  15. increasing energy Cobaltocene: One more electron in a nice place… Cobaltocene lowest energy bonding state: Comp. Tech. e2g e2g a1g’ e1u

  16. Cobaltocene Rectifier Rectifier: I-V Rectifier: Conducts under forward bias, but not under reverse bias Comp. Tech. [Rui Liu]

  17. Transmission Resonances in Cobaltocene Rectifier Rectifier: T, ldos Density of states projected on molecule Transmission (E,V) Comp. Tech. Resonance A (HOMO at V=0): Resonance B (LUMO at V=0): [Rui Liu]

  18. Comp. Tech. Potential Drop and Charge Distribution in Rectifier Rectifier: potential [Rui Liu]

  19. Spin active molecule Use Cobaltocene’s Spin: Molecular Spintronics Spin active molecule Goal: Move spin active parts from leads into molecules Comp. Tech. Cobaltocene spin filter: Apply B field to align spin of cobaltocene; Current is spin polarized

  20. Spintronic Switch in a Molecule with 2 Cobaltocenes diCo: energetics diCo diCo-2C Comp. Tech. Energetics of the singlet-tripletsplitting • Ground state => S=0 (super-exchange*). • The more insulating the spacer, the smaller the energy difference. • B field needed to excite molecule from S=0 to S=1 depends on spacer • *The term used for the indirect exchange coupling of unpaired spins via orbitals having paired spins.

  21. diCo diCo-2C Transmission of di-Cobaltocene Molecules: A Good Switch and Spin-Valve! diCo: T Comp. Tech. [Rui Liu]

  22. Conclusions Conclusion Lessons we have learned on molecular electronics and spintronics: • Contact atomic structure does matter! additional Au caused a dramatic increase of conductance • Ferrocene containing molecule has good conductance! first case of agreement between theory and experiment • Cobaltocene has a very nice additional electron: * resonance near the Fermi energy of Au * unpaired spin to use for spintronics • Rectifier, spin filter, and spintronic switch using cobaltocene Comp. Tech. Credits: Rui Liu, San-Huang Ke, Weitao Yang, and HUB Expt: Michael Fuhrer, Larry Sita, and their team

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