Kondo effect of a Co atom on Cu(111) in contact with an Fe tip

Single Co atoms, which exhibit a Kondo effect on Cu(111), are contacted with Cu and Fe tips in a low-temperature scanning tunneling microscope. With Fe tips, the Kondo effect persists with the Abrikosov-Suhl resonance significantly broadened. In contrast, for Cu-covered W tips, the resonance width remains almost constant throughout the tunneling and contact ranges. The distinct changes of the line width are interpreted in terms of modifications of the Co d state occupation owing to hybridization with the tip apex atoms.Comment: 4 pages, 3 figure

Quantized Conductance of a Single Magnetic Atom

A single Co atom adsorbed on Cu(111) or on ferromagnetic Co islands is contacted with non-magnetic W or ferromagnetic Ni tips in a scanning tunneling microscope. When the Co atom bridges two non-magnetic electrodes conductances of 2e^2/h are found. With two ferromagnetic electrodes a conductance of e^2/h is observed which may indicate fully spin-polarized transport.Comment: 3 pages, 2 figure

Isotropization in the approach to big rip singularities for Cardassian models

Cardassian models are an alternative to general relativity which have been proposed as an approach to explaining accelerated cosmic expansion while avoiding directly introducing dark energy. They are generally formulated only in the homogeneous and isotropic case. In this paper an extension of the usual formulation to general spatially homogeneous geometries is given. A characteristic feature of many classes of Cardassian models is the occurrence of big rip singularities where the scale factor tends to infinity after a finite time. It is shown that big rip singularities are also widespread in more general homogeneous cases. It is also shown that there is isotropization in the approach to a big rip singularity which bears a strong resemblance to the late-time isotropization observed in cosmological spacetimes which accelerate forever in the future

Controlled Contact to a C60 Molecule

The conductance of C60 on Cu(100) is investigated with a low-temperature scanning tunneling microscope. At the transition from tunneling to the contact regime the conductance of C60 adsorbed with a pentagon-hexagon bond rises rapidly to 0.25 conductance quanta G0. An abrupt conductance jump to G0 is observed upon further decreasing the distance between the instrument's tip and the surface. Ab-initio calculations within density functional theory and non-equilibrium Green's function techniques explain the experimental data in terms of the conductance of an essentially undeformed C60. From a detailed analysis of the crossover from tunneling to contact we conclude that the conductance in this region is strongly affected by structural fluctuations which modulate the tip-molecule distance.Comment: 4 pages, 3 figure

Controlling the Kondo Effect in CoCu_n Clusters Atom by Atom

Clusters containing a single magnetic impurity were investigated by scanning tunneling microscopy, spectroscopy, and ab initio electronic structure calculations. The Kondo temperature of a Co atom embedded in Cu clusters on Cu(111) exhibits a non-monotonic variation with the cluster size. Calculations model the experimental observations and demonstrate the importance of the local and anisotropic electronic structure for correlation effects in small clusters.Comment: 4 pages, 4 figure

Hot electron cascades in the scanning tunneling microscope

The nonequilibrium distribution of electrons at the junction of a scanning tunneling microscope is investigated by detecting photons with energies hv > eV, where V is the bias voltage. Electrons are found at energies exceeding the Fermi level by almost eV. While their distribution deviates from a Fermi-Dirac function it is consistent with a model of hot electrons and holes that diffuse in energy and real space