Jungiusstr. 11, Hörsaal AP
SFB 668 - Kolloquium
Prof. Dr. P. Gambardella (University of Barcelona):
Charge transfer, electron correlation, and spin coupling in molecule-metal contacts
Metal-organic complexes provide a variety of charge and spin states that can be used to tailor the conductance of single-molecule electronic devices. Key to achieving this control is understanding how the molecular spin couples to electrons inside and outside a molecule, providing different pathways for spin excitation and relaxation. Here, starting from a description of the valence state and magnetic moments of 3d impurities at surfaces1-3, we will discuss the competition between local correlation and hybridization effects for metal-organic complexes adsorbed on metals4. By combining scanning tunnelling microscopy and x-ray magnetic circular dichroism measurements, we argue that metal-organic adsorbates should be generally considered as strongly-correlated electron systems with either pure valence5,6 or mixed-valence7 behavior. Examples of molecules with strong, weak, and zero mixed-valence character will be presented, leading to a consistent description of the magnetic moment and Kondo behavior of metal-organic adsorbates. Further, we will show that charge transfer from a metal electrode to ligand orbitals can increase the spin degeneracy of both paramagnetic and diamagnetic molecules, opening multiple spin relaxation channels with distinctive spatial localization and decay rates8. Our data illustrate how the interplay of charge, spin, and vibrational degrees of freedom determine the site-specific magnetic moment and electron conductance of molecule-metal contacts.
1 Localized magnetic states of Fe, Co, and Ni impurities on alkali metal films, P. Gambardella et al., Phys. Rev. Lett. 88, 047202 (2002).
2 Correlated electrons step by step: itinerant-to-localized transition of Fe impurities in free-electron metal hosts, C. Carbone et al., Phys. Rev. Lett. 104, 117601 (2010).
3 Magnetism of individual atoms adsorbed on surfaces, H. Brune and P. Gambardella, Surf. Sci. 603, 1830 (2009).
4 Supramolecular control of the magnetic anisotropy in two-dimensional high-spin Fe arrays at a metal interface, P. Gambardella et al., Nature Mater. 8, 189 (2009).
5 Giant spin and orbital moment anisotropies of a Cu-Phthalocyanine monolayer, S. Stepanow et al., Phys. Rev. B 82, 014405 (2010).
6 Spin and Orbital Magnetic Moment Anisotropies of Monodispersed Bis(Phthalocyaninato)Terbium on a Copper Surface, S. Stepanow et al., J. Am. Chem. Soc., 132, 11900 (2010).
7 Mixed-valence behavior and strong correlation effects of metal phthalocyanines adsorbed on metals, S. Stepanow et al., Phys. Rev. B 83, 220401(R) (2011).
8 Spin coupling and relaxation inside molecule-metal contacts A. Mugarza, C. Krull, R. Robles, S. Stepanow, G. Ceballos, and P. Gambardella, submitted.