68 research outputs found
Surface-State Localization at Adatoms
Low-temperature scanning tunneling spectroscopy of magnetic and non-magnetic
metal atoms on Ag(111) and on Cu(111) surfaces reveals the existence of a
common electronic resonance at an energy below the binding energies of the
surface states. Using an extended Newns-Anderson model, we assign this
resonance to an adsorbate-induced bound state, split off from the bottom of the
surface-state band, and broadened by the interaction with bulk states. A
lineshape analysis of the bound state indicates that native adatoms decrease
the surface-state lifetime, while a cobalt adatom causes no significant change.Comment: 4 pages, 4 figure
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
Engineering Negative Differential Conductance with the Cu(111) Surface State
Low-temperature scanning tunneling microscopy and spectroscopy are employed
to investigate electron tunneling from a C60-terminated tip into a Cu(111)
surface. Tunneling between a C60 orbital and the Shockley surface states of
copper is shown to produce negative differential conductance (NDC) contrary to
conventional expectations. NDC can be tuned through barrier thickness or C60
orientation up to complete extinction. The orientation dependence of NDC is a
result of a symmetry matching between the molecular tip and the surface states.Comment: 5 pages, 4 figures, 1 tabl
Kondo resonance of a Co atom exchange coupled to a ferromagnetic tip
The Kondo effect of a Co atom on Cu(100) was investigated with a
low-temperature scanning tunneling microscope using a monoatomically sharp
nickel tip. Upon a tip-Co contact, the differential conductance spectra exhibit
a spin-split asymmetric Kondo resonance. The computed ab initio value of the
exchange coupling is too small to suppress the Kondo effect, but sufficiently
large to produce the splitting observed. A quantitative analysis of the line
shape using the numerical renormalization group technique indicates that the
junction spin polarization is weak.Comment: 5 pages, 4 figure
Atom Transfer and Single-Adatom Contacts
The point contact of a tunnel tip approaching towards Ag(111) and Cu(111)
surfaces is investigated with a low temperature scanning tunneling microscope.
A sharp jump-to-contact, random in nature, is observed in the conductance.
After point contact, the tip-apex atom is transferred to the surface,
indicating that a one-atom contact is formed during the approach. In sharp
contrast, the conductance over single silver and copper adatoms exhibits a
smooth and reproducible transition from tunneling to contact regime. Numerical
simulations show that this is a consequence of the additional dipolar bonding
between the homoepitaxial adatom and the surface atoms.Comment: 4 pages, 4 figure
- …