Search Results (7 total)

We have used atomic manipulation and scanning tunneling spectroscopy to study the evolution in electronic properties that occurs as two Ni atoms are merged into a single magnetic molecule on Au(111). We observe energetic shifting of molecular d-orbitals and a strong decrease in the molecular Kondo temperature as Ni-Ni separation is reduced to 3.4±0.3 Å. These results are qualitatively explained by a combination of a spin-1/2 s-d model and density-functional calculations.

Recent advances in scanning tunneling microscopy have allowed the observation of the Kondo effect for individual magnetic atoms. One hallmark of the Kondo effect is a strong temperature-induced broadening of the Kondo resonance. In order to test this prediction for individual impurities, we have investigated the temperature dependent electronic structure of isolated Ti atoms on Ag(100). We find that the Kondo resonance is strongly broadened in the temperature range T  =  6.8 K to T  =  49.0 K. These results are in good agreement with theoretical predictions for Kondo impurities in the Fermi liquid regime, and confirm the role of electron-electron scattering as the main thermal broadening mechanism.

The triangular Cr trimer (Cr₃) is a fundamental component in a number of frustrated, antiferromagnetic systems. We have used atomic manipulation and scanning tunneling spectroscopy to probe the local behavior of this basic magnetic substructure by fabricating and analyzing individual Cr trimers at the surface of gold. We find that Cr trimers can be reversibly switched between two distinct electronic states. This phenomenon can be explained as the Kondo response of a spin-switching, magnetically frustrated nanocluster. Such behavior is consistent with noncollinear magnetic states predicted for Cr trimers whose structures differ by the position of a single atom.

We present a detailed study of the local electronic properties of the Kondo system formed from cobalt adatoms deposited onto Au(111) at a temperature of 6.6 K. Cryogenic scanning-tunneling spectroscopy was used to observe impurity-induced resonances at the Fermi energy and at the Au(111) surface-state band edge. The line shape of the Fermi-energy resonance, identified as a Kondo resonance, is observed to vary with lateral position from the impurity center and with impurity binding position on the reconstructed Au(111) surface. Little vertical dependence is seen in the resonance line shape for positions above the center of the impurity. Interaction effects between Kondo impurities are observed to remain small as cobalt coverage is increased up to 1 ML on the gold surface. The Kondo resonance is shown theoretically to be a member of a general class of Fano resonances arising from the interaction of a discrete impurity state with a conduction-electron continuum. The asymmetric line shape of the resonance thus reflects quantum interference between the d orbital and continuum conduction electron channels, as well as their coupling to the STM tip.

The magnetic properties of an impurity atom vary greatly depending on the nature of the impurity d level. We have used scanning tunneling spectroscopy to systematically probe the local electronic structure of individual transition-metal impurities having different d-level configurations. Atoms from the 3d row of the periodic table were adsorbed onto a Au(111) substrate and spectroscopically probed with an ultrahigh vacuum scanning tunneling microscope at a temperature of 6 K. Elements near the center of the 3d row (V, Cr, Mn, and Fe) displayed featureless electronic structure over the energy range studied, while elements near the ends of the row (Ti, Co, and Ni) showed narrow resonances near the Fermi energy. These spectroscopic features are interpreted as a combination of the Kondo resonance and bare d resonance, and are consistent with trends observed in the Kondo temperature of bulk impurities.

We have used scanning tunneling spectroscopy and atomic manipulation to study the interaction between a single pair of magnetic atoms at the surface of a nonmagnetic metal. The local electronic structure of cobalt adatoms on Au(111) was measured for different cobalt-cobalt interatomic spacings at T=6 K. Artificially fabricated cobalt dimers are found to show an abrupt disappearance of the Kondo resonance for cobalt-cobalt separations less than 6 Å. This behavior is explained as the result of reduced exchange coupling between gold conduction electrons and ferromagnetic cobalt dimers.

We have used scanning tunneling spectroscopy to spatially resolve the electronic structure of clean Au(111) at low temperature. We find that the long-range herringbone reconstruction on Au(111) acts as a superlattice for surface-state electrons, creating a new band structure and modulated electronic density. Low energy electrons respond to the superlattice by localizing in the hexagonal-close-packed (hcp) region of the reconstruction, while higher energy electrons reverse this trend, shifting density back to the adjacent face-centered-cubic (fcc) region. These observations are quantitatively explained by an extended Kronig-Penney model, from which we estimate the well-depth of the reconstruction-induced surface superlattice.