Scanning Tunneling Spectrosopy of single magnetic adatoms and complexes at surfaces Peter Wahl Max-Planck-Institute for Solid State Research Stuttgart 1. STS of adsorbates 2. Spin detection via the Kondo effect 3. Scaling behavior of single Kondo impurities 4.Chemical analysis by STM 5. The Kondo effect of molecules

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

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

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

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

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

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

The Renaissance of the Kondo Effect V. Madhavan et al., Science 280, 567 (1998) J. Li et al., Phys. Rev. Lett. 80, 2893 (1998) STS on Co/Au(111) 5 nm dI/dV (a. u.) Bias (mV) V ds (mV) VgVg D. Goldhaber-Gordon et al., Nature 391, 156 (1998) S. M. Cronenwett et al., Science 281, 540 (1998) 1998: Single spin in a quantum dot

STS on Cobalt Adatoms Phys. Rev. B 65, (2002) Friedel oscillations in surface state LDOS width = 2 k B T K 10 nm Co/Ag(111)

Lineshape M. Plihal and J.W. Gadzuk, Phys. Rev. B 63, (2001) indirect direct q=100 q=1 q=0  Fano lineshape

STS on Cobalt Adatoms What determines T K ? SubstrateT K (K)ε K (meV) Cu(111) Cu(100) Ag(111) Ag(100) Au(111) Phys. Rev. Lett. 88, (2002); 2 H.C. Manoharan, C.P. Lutz, and D.M. Eigler, Nature 403, 512 (2000); 3 Phys. Rev. B 65, (2002); 4 Phys. Rev. Lett. 93, (2004); 5 N. Knorr, PhD Thesis, Lausanne (2002); 6 V. Madhavan et al., Science 280, 567 (1998)

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

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

Model excellent agreement with experimental data Test of the model: Position of the resonance occupation n d  K (meV) Co/Ag(111) Co/Cu(111)Co/Ag(100) Co/Cu(100) Co/Au(111) occupation n d T K (K) Phys. Rev. Lett. 93, (2004)

The Kondo Effect of Molecules 1 Ref. 1 Can we tune the spin by chemistry ?

Preparation 2 nm U=-0.2V, I=2nA (110) Preparation of Co(CO) n /Cu(100): 1.Deposition of Cobalt at ~150K (Θ~0.005ML) 2.Exposition to ~0.1L CO 3.Annealing to K

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

Breaking of Co-CO bonds voltage sweep STS taken in open feedback mode with stabilization at U=-0.8V, I=0.6nA. 5Å5Å U=-3V, I=0.6nA STM induced chemical reaction

 molecules are Co(CO) n 2 nm 5 Å Kondo feature 2  Cobalt adatom 2 Phys. Rev. Lett. 88, (2002) Chemical Identification vibrational features 1  CO molecule 1 L.J. Lauhon and W. Ho, Phys. Rev. B 60, R8525 (1999)

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

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

Cobaltcarbonyls on Cu(100) (110) Co(CO) 3 Co(CO) 4 CoCo(CO) 2 T K =88 KT K =170 KT K =283 KT K =165 K

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

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

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

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

Spin Mapping 5Å (Co(CO) 2 ) 2 (Co(CO) 3 ) 2 T K  176±13KT K  138±21K (110)

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

Interaction between Impurities   J I ?

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

Interaction between Impurities 5.12Å T K  180K 5.72Å T K  100K 2.56Å No Kondo T K =88 K

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

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

Inelastic Spin Flip Spectroscopy A. Heinrich et al., Science 306, 466 (2004)  Spin is locked at k B T<gµ B H Spin flip can be excited for U>gµ B H H insulating layer metal Magnetic Adatom(Mn) (NiAl(110)) (Al 2 O 3 )

ESR-STM 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)  Spin is fluctuating at k B T>gµ B H H  Larmor precession  L =gµ B H Detection of noise with  L when the tip is placed on top of the atom. BDPA/HOPG 10 nm 210G Ref. 2

Conclusions 1.The Kondo effect can be exploited to study the coupling of a single spin. 2.Chemical analysis by STM & Modification of magnetic properties by ligands 3.Spatial mapping of the Kondo resonance with submolecular resolution. 4.Interaction between Impurities.

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)