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Hund’s exchange controls the physics of 3d adatoms on surfaces

Understanding the interaction of isolated transition metal atoms with metal surfaces is crucial in order to rationalize the properties of nanoscopic magnetic devices. Magnetic nanostructures consisting of a few atoms on nonmagnetic substrates are explored as model systems for miniaturized data storage, spintronic devices and for the implementation of quantum computing.
The experimental and theoretical understanding of the physical mechanisms determining the electronic and magnetic properties of isolated atoms on surfaces is still at a rather basic qualitative level. The ground state of a free standing transition metal atom possesses localized large spin and orbital magnetic moments given by Hund’s rules: the tendency of electrons to distribute in different orbitals and in high-spin states. When such atoms are in contact with metallic surfaces, the electronic structure of the impurity and the Hund's rule coupling is less understood. Often experiments on transition metal impurity systems are interpreted in terms of Kondo-type models where a generalized spin is coupled to a bath of conduction electrons or in terms of single orbital Anderson models. Connections between models and realistic systems are usually questionable and can typically only be made a posteriori. It is often largely unclear which microscopic degrees of freedom are effective at a given energy scale and how their contribution in excitation spectra measured with different spectroscopy techniques can be unravel.
In our work we investigated the series of isolated Mn, Fe, Co, and Ni adatoms on the Ag(100) surface and explain their excitation spectra. In order to successfully access valence electron photoemission spectra of diluted magnetic atoms on surfaces (coverage in the range of 2 x 10-2 to 10-3 monolayers) we require a very high photon flux, a photoelectron spectroscopy apparatus optimized for the fast acquisition, a cryogenic manipulator (temperature of the order of 20 K) and in-situ sample preparation. All these requirements were available at the SuperESCA beamline where we performed the experiment.The measured valence band spectra revealed the complex evolution of the electronic spectra along the series.Figure 1(a)shows the experimental photoelectron energy distribution curves of the 3d states, in the valence band of isolated Mn, Fe, Co, and Ni atoms on the Ag(100) surface. Mn possess one feature (1) at binding energy (BE) of 3.25 eV; Fe has two features, one (1) at 2.32 eV BE and the other one (2) near the Fermi Energy (EF); Co has one (1) broad feature at 2.57 eV BE, one feature (2) at 0.8 eV BE, and one feature (3) close to the EF; and Ni has one broad feature (1) at 0.35 eV BE. We observed a monotonic decrease in the splitting of higher energy structures and a nonmonotonic variation of low energy spectral weight.
To explain the experimental spectra we performed first principles calculations and we obtained the adatom spectra including Kondo screening as well as the atomic multiplet structures (Fig. 2). We, thus, use the theoretical results to understand the physical mechanisms behind the evolution of the spectra in theseries of 3d adatoms. First, the splitting between final state multiplets with different spin decreases monotonically due to a monotonic reduction of effective exchange splittings with increasing filling of the 3d shell (n). Second, the effective charging energies vary due to Hund’s exchange in a strongly nonmonotonic way from Mn to Ni. Therefore, the amount of charge fluctuations and the weight of quasiparticle peaks at the Fermi level evolves nonmonotonically through this 3d series. We find sizable charge fluctuations and mixed valence behavior for Fe and Ni. In contrast, Mn and Co come closer to a multiorbital Kondo limit with a generalized impurity spin being coupled to a bath of conduction electrons and less charge fluctuations.
Our joint experimental and theoretical study shows that Hund’s exchange controls the physics of 3d adatoms on the surfaces of Ag(100).

Figure 1. Valence band spectra of Mn, Fe, Co, and Ni adatoms (from top to bottom) on Ag (100). (a) Experimental photoemission spectra. The curves are difference spectra between the clean Ag surface and the surface covered with a few adatoms and, thus, correspond to the contribution from 3d impurity electronic states (b) Theoretical spectra obtained first principles calculationscalculations. The 3d shell occupancies used in the simulations are n=5:0 for Mn,n=6:0 for Fe, n=7:8 for Co, and n=8:4 for Ni.

Figure 2:  Orbitally resolved spectral functions of Fe and Co impurities on Ag(100) as obtained from first principle calculation. n is the 3d shell occupancy used in the simulation.

This research was conducted by the following research team:

  • Sandra Gardonio, University of Nova Gorica, Nova Gorica, Slovenia
  • Tim Oliver Wehling, Universität Bremen, Bremen, Germany
  • Michael Karolakand Alexander Lichtenstein, Universität Hamburg, Hamburg, Germany
  • Luca Petaccia, Silvano Lizzit and Andrea Goldoni, Elettra - Sincrotrone Trieste, Trieste, Italy
  • Carlo Carbone, Istituto di Struttura della Materia-CNR, Trieste, Italy

Contact persons:
Sandra Gardonio: sandra.gardonio@ung.si


S. Gardonio, M. Karolak,T. O. Wehling,L. Petaccia, S. Lizzit, A. Goldoni, A. Lichtenstein and C. Carbone,”Excitation spectra of transition metal atoms on the Ag(100) surface controlled by Hund’s exchange”, Phys. Rev. Lett. 110, 186404 (2013). DOI: 10.1103/PhysRevLett.110.186404  
Last Updated on Friday, 22 November 2013 13:47