Scanning Tunneling Spectrosopy of single magnetic adatoms and complexes at surfaces

1. Scanning Tunneling Spectrosopy of single magnetic adatoms and complexes at surfaces Peter Wahl Max-Planck-Institute for Solid State Research Stuttgart • STS of adsorbates • Spin detection via the Kondo effect • Scaling behavior of single Kondo impurities • Chemical analysis by STM • The Kondo effect of molecules

2. Experimental • Low temperature STM operating at 4K • Up to 5T magnetic field • UHV sample preparation • In-situ sample transfer

3. Scanning Tunneling Spectroscopy • Energy Resolution governed by the temperature of the tip • dI/dV~LDOS for U<<Φ • Spectra contain contributions from sample and tip

4. Background subtraction on off Example: CO/Cu(100)

5. Spin detection by STS • Spin-polarized STS • Spin-Flip Spectroscopy • Spin detection via the Kondo effect

6. The Kondo Effect The spin of magnetic impurities is screened by the conduction electrons. 1934: Resistivity minimum in dilute magnetic alloys1 1968: Kondos explanation by spin-flip scattering2 1 W.J. de Haas, J. de Boer and G.J. van den Berg, Physica1, 1115 (1934) 2 J. Kondo, Phys. Rev.169, 437 (1968) A.C. Hewson, The Kondo Problem to Heavy Fermions (1993)

7. The Kondo Effect Anderson Model1 e- Impurity Jz   J+,-  1 P.W. Anderson, Phys. Rev.124, 41 (1961)

8. The Renaissance of the Kondo Effect 1998: Single spin in a quantum dot STS on Co/Au(111) Vg 5 nm 0 -1.0 1.0 Vds (mV) dI/dV (a. u.) -60 -40 -20 0 20 40 60 Bias (mV) V. Madhavan et al., Science280, 567 (1998)J. Li et al., Phys. Rev. Lett. 80, 2893 (1998) D. Goldhaber-Gordon et al.,Nature391, 156 (1998) S. M. Cronenwett et al., Science281, 540 (1998)

9. STS on Cobalt Adatoms 10 nm width = 2 kBTK Co/Ag(111) Friedel oscillationsin surface state LDOS Phys. Rev. B65, 121406 (2002)

10. Lineshape q=0 q=1 Fano lineshape q=100 indirect direct M. Plihal and J.W. Gadzuk, Phys. Rev. B63, 085404 (2001)

11. STS on Cobalt Adatoms What determines TK ? 1 Phys. Rev. Lett.88, 096804 (2002); 2 H.C. Manoharan, C.P. Lutz, and D.M. Eigler, Nature403, 512 (2000); 3 Phys. Rev. B65, 121406 (2002); 4 Phys. Rev. Lett.93, 176603 (2004); 5 N. Knorr, PhD Thesis, Lausanne (2002); 6 V. Madhavan et al., Science280, 567 (1998)

12. Monolayer systems Co/Cu(111) Co/1 ML Ag/Cu(111) Co/Ag(111) TK=54 K TK=92 K TK=92 K  Kondo effect is dominated by the local environment.

13. Model U  2.8eV Δ  0.2eV A.C. Hewson, Cambridge University Press, Cambridge (1993) O. Ujsaghy et al., Phys. Rev. Lett.85, 2557 (2000) nd ~ overlap between adatom and substrate orbitals distance to nearest neighbor a extent of d-orbital coordination λd 1Å nNN=4 nNN=3

14. Model occupation nd Co/Ag(111) Co/Cu(100) TK (K) Co/Au(111) Co/Cu(111) Co/Ag(100) Test of the model: Position of the resonance eK (meV) occupation nd • excellent agreement with experimental data Phys. Rev. Lett.93, 176603 (2004)

15. The Kondo Effect of Molecules Ref. 1 Can we tune the spin by chemistry ? 1 http://www.webelements.com/webelements/elements/text/Co/key.html

16. Preparation (110) (110) U=-0.2V, I=2nA 2 nm • Preparation of Co(CO)n/Cu(100): • Deposition of Cobalt at ~150K (Θ~0.005ML) • Exposition to ~0.1L CO • Annealing to 260-320 K

17. DFT calculations Calculation STM image (110) (110) Image size: 1 nm2 U=-0.7V, I=2nA CO Co Cu Collaboration with A.P. Seitsonen, University of Zurich

18. STM induced chemical reaction Breaking ofCo-CO bonds 5Å U=-3V, I=0.6nA voltage sweep STS taken in open feedback mode with stabilization atU=-0.8V, I=0.6nA.

19. Chemical Identification 5 Å 2 nm Kondo feature2  Cobalt adatom vibrational features1  CO molecule 1 L.J. Lauhon and W. Ho, Phys. Rev. B60, R8525 (1999) 2 Phys. Rev. Lett.88, 096804 (2002) molecules are Co(CO)n

20. Partial Dissociation (110) (110) Co(CO)4 Co(CO)2 Co

21. Rotation of Dicarbonyl (110) (110) tip Co(CO)2

22. Cobaltcarbonyls on Cu(100) (110) (110) TK=88 K TK=165 K TK=170 K TK=283 K Co Co(CO)2 Co(CO)3 Co(CO)4

23. Irontetracarbonyl (110) (110) TK142K 5 nm 5 Å Preparation as for cobalt … Fe(CO)4 (Fe(CO)3)2 (Fe(CO)2)2

24. Copperdicarbonyl (110) (110) 5 Å Preparation as for cobalt … Cu(CO)2 no Kondo feature !

25. Spin tuning by ligands A.C. Hewson, Cambridge University Press, Cambridge (1993)

26. Spin Mapping Topography U=0.6V, I=2nA dI/dV (2mV)-dI/dV(-60mV) (110) (110) 5Å 5Å  Spatial mapping of the Kondo resonance

27. Spin Mapping (110) (110) 5Å 5Å TK138±21K TK176±13K (Co(CO)2)2 (Co(CO)3)2

28. Spin Mapping (110) (110) (Co(CO)2)2 (Co(CO)3)2

29. Interaction between Impurities I ­ J ­ ?

30. Preparation of cobalt nanostructures tip-induced dissociation 6.4Å 1 nm (Co(CO)3)2

31. Interaction between Impurities No Kondo 2.56Å TK180K 5.12Å 5.72Å TK100K TK=88 K

32. Interaction between Impurities TK368±37K T*78±13K 1D Kondo chain !

33. Arrays of Magnetic Impurities – 2D Fe TPA Coupling between Fe atoms ? Kondo Effect ?2D Kondo lattice ? M.A. Lingenfelder et al., Chem. Eur. J.10, 1913 (2004)

34. Inelastic Spin Flip Spectroscopy (Mn) (Al2O3) (NiAl(110)) • Spin is locked at kBT<gµBH • Spin flip can be excited for U>gµBH H Magnetic Adatom ­ insulating layer metal A. Heinrich et al., Science306, 466 (2004)

35. ESR-STM ESR-STM @ 210G BDPA/HOPG Ref. 2 10 nm Spin is fluctuating at kBT>gµBH H  Larmor precession L=gµBH ­ Detection of noise with L when the tip is placed on top of the atom. 1. Y. Manassen, R.J. Hamers, J.E. Demuth and A.J. Castellano Jr., Phys. Rev. Lett.62, 2531 (1989) 2. C. Durkan and M.E. Welland, Appl. Phys. Lett.80, 458 (2002)

36. Conclusions • The Kondo effect can be exploited to study the coupling of a single spin. • Chemical analysis by STM & Modification of magnetic properties by ligands • Spatial mapping of the Kondo resonance with submolecular resolution. • Interaction between Impurities.

37. Acknowledgments • MPI Stuttgart: • L. Diekhöner (now University of Aalborg) • G. Wittich • L. Vitali • M.A. Schneider • K. Kern • Theory: • A.P. Seitsonen (DFT) • O. Gunnarsson, J. Merino, H. Kroha (Kondo)