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.

We have used scanning tunneling microscopy and spectroscopy to study the electronic structure of individual C 60 molecules adsorbed onto the Au(111) and Ag(100) surfaces. C 60 molecules on Au (111) show an increase in the HOMO-LUMO gap of 0.6 eV compared to C 60 on Ag(100). Splitting of the C 60 LUMO manifold is suppressed for C 60 on Au(111), in contrast to the strong splitting observed for C 60 on Ag(100). Our data implies a 0.6 eV increase in intramolecular Coulomb energy for C 60 on Au(111) as compared to C 60 on Ag(100). Topographs and energy-resolved spectral maps, however, show nearly identical features and indicate a similar influence of the two substrates on molecular-orbital geometry. C 60 -substrate bonding and charge transfer is further investigated by calculating C 60 charge redistribution using ab initio pseudopotential densityfunctional theory methods. These calculations indicate that a negligible amount of charge is transferred from Au(111) to adsorbed C 60 , while about 0.2 electron is transferred to C 60 resting on Ag (100), although the precise amount depends on the definition used. This charge transfer likely changes the electronic screening properties of C 60 , providing an explanation for observed spectroscopic differences on these two substrates.

Scaling laws and universality play an important role in our understanding of critical phenomena and the Kondo effect. We present measurements of nonequilibrium transport through a single-channel Kondo quantum dot at low temperature and bias. We find that the low-energy Kondo conductance is consistent with universality between temperature and bias and is characterized by a quadratic scaling exponent, as expected for the spin-1/2 Kondo effect. We show that the nonequilibrium Kondo transport measurements are well described by a universal scaling function with two scaling parameters.

We have used scanning tunneling spectroscopy to spatially map the energy-resolved local density of states of individual C60 molecules on the Ag(100) surface. Spectral maps were obtained for molecular states derived from the C60 HOMO, LUMO, and LUMO+1 orbitals, revealing new details of the spatially inhomogeneous C60 local electronic structure. Spatial inhomogeneities are explained using ab initio pseudopotential density functional calculations. These calculations emphasize the need for explicitly including the C60-Ag interaction and STM tip trajectory to understand the observed C60 local electronic structure.