80 research outputs found
Theory of real space imaging of Fermi surfaces
A scanning tunneling microscope can be used to visualize in real space Fermi
surfaces with buried impurities far below substrates acting as local probes. A
theory describing this feature is developed based on the stationary phase
approximation. It is demonstrated how a Fermi surface of a material acts as a
mirror focusing electrons that scatter at hidden impurities.Comment: 10 pages, 4 figure
Resonance-enhanced vibrational spectroscopy of molecules on a superconductor
Molecular vibrational spectroscopy with the scanning tunneling microscope is
feasible but usually detects few vibrational modes. We harness sharp
Yu-Shiba-Rusinov (YSR) states observed from molecules on a superconductor to
significantly enhance the vibrational signal. From a lead phthalocyanin
molecule 46 vibrational peaks are resolved enabling a comparison with
calculated modes. The energy resolution is improved beyond the thermal
broadening limit and shifts induced by neighbor molecules or the position of
the microscope tip are determined. Vice versa, spectra of vibrational modes are
used to measure the effect of an electrical field on the energy of YSR states.
The method may help to further probe the interaction of molecules with their
environment and to better understand selection rules for vibrational
excitations
Long-range Kondo signature of a single magnetic impurity
The Kondo effect, one of the oldest correlation phenomena known in condensed
matter physics, has regained attention due to scanning tunneling spectroscopy
(STS) experiments performed on single magnetic impurities. Despite the
sub-nanometer resolution capability of local probe techniques one of the
fundamental aspects of Kondo physics, its spatial extension, is still subject
to discussion. Up to now all STS studies on single adsorbed atoms have shown
that observable Kondo features rapidly vanish with increasing distance from the
impurity. Here we report on a hitherto unobserved long range Kondo signature
for single magnetic atoms of Fe and Co buried under a Cu(100) surface. We
present a theoretical interpretation of the measured signatures using a
combined approach of band structure and many-body numerical renormalization
group (NRG) calculations. These are in excellent agreement with the rich
spatially and spectroscopically resolved experimental data.Comment: 7 pages, 3 figures + 8 pages supplementary material; Nature Physics
(Jan 2011 - advanced online publication
Spin Manipulation by Creation of Single-Molecule Radical Cations
All-trans-retinoic acid (ReA), a closed-shell organic molecule comprising
only C, H, and O atoms, is investigated on a Au(111) substrate using scanning
tunneling microscopy and spectroscopy. In dense arrays single ReA molecules are
switched to a number of states, three of which carry a localized spin as
evidenced by conductance spectroscopy in high magnetic fields. The spin of a
single molecule may be reversibly switched on and off without affecting its
neighbors. We suggest that ReA on Au is readily converted to a radical by the
abstraction of an electron.Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev. Let
Exchange coupling in transition-metal nano-clusters on Cu(001) and Cu(111) surfaces
We present results of density-functional calculations on the magnetic
properties of Cr, Mn, Fe and Co nano-clusters (1 to 9 atoms large) supported on
Cu(001) and Cu(111). The inter-atomic exchange coupling is found to depend on
competing mechanisms, namely ferromagnetic double exchange and
antiferromagnetic kinetic exchange. Hybridization-induced broadening of the
resonances is shown to be important for the coupling strength. The cluster
shape is found to weaken the coupling via a mechanism that comprises the
different orientation of the atomic d-orbitals and the strength of
nearest-neighbour hopping. Especially in Fe clusters, a correlation of binding
energy and exchange coupling is also revealed
Learning From Early Attempts to Generalize Darwinian Principles to Social Evolution
Copyright University of Hertfordshire & author.Evolutionary psychology places the human psyche in the context of evolution, and addresses the Darwinian processes involved, particularly at the level of genetic evolution. A logically separate and potentially complementary argument is to consider the application of Darwinian principles not only to genes but also to social entities and processes. This idea of extending Darwinian principles was suggested by Darwin himself. Attempts to do this appeared as early as the 1870s and proliferated until the early twentieth century. But such ideas remained dormant in the social sciences from the 1920s until after the Second World War. Some lessons can be learned from this earlier period, particularly concerning the problem of specifying the social units of selection or replication
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