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Biomedical subjects

H van Kempen

Publications and source records attributed to H van Kempen.

7 recordsLinked to original sources

Atomic-size oscillations in conductance histograms for gold nanowires and the influence of work hardening.

Nanowires of different natures have been shown to self-assemble as a function of stress at the contact between two macroscopic metallic leads. Here we demonstrate for Au wires that the balance between various metastable nanowire configurations is influenced by the microstructure of the starting materials, and we discover a new set of periodic structures, which we interpret as due to the atomic discreteness of the contact size for the three principal crystal orientations.

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Amblyopia & Strabismus Questionnaire: design and initial validation.

Literature on the benefits of screening and treatment of amblyopia emphasizes the costs of insufficiently treated amblyopic patients who lose their better eye. However, patients with insufficiently treated amblyopia and strabismus who do not lose their better eye only experience a slight decrease in their quality of life, and such patients occur more frequently. We designed and validated a questionnaire for amblyopia and strabismus in order to assess the decrease in quality of life of such patients. Complaints were collected from outpatients and reduced to five domains, pertaining to distance estimation, visual disorientation, problems with social contacts and cosmetic problems, diplopia, and fear of losing the better eye. For these domains 26 questions were formulated that constitute the Amblyopia & Strabismus Questionnaire (A&SQ). The A&SQ, the National Eye Institute Visual Function Questionnaire-25 (VFQ-25) and the Short Form-12 Health Survey (SF-12) were presented to 53 healthy controls, 68 outpatients and a cohort of 174 outpatients born between 1962 and 1972 and treated between 1968 and 1974 for amblyopia and strabismus. The quality of life was best in healthy controls and worst in current outpatients, not only on the SF-12 and the VFQ-25 but also on the A&SQ, demonstrating an acceptable construct validity of the A&SQ. The decrease in the quality of life as measured by the A&SQ was most outspoken in our outpatient group of amblyopia and strabismus patients, less in the cohort that had been treated 30 years previously and least in the healthy controls, demonstrating an acceptable discriminatory validity of the A&SQ.

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Fractional conductance in hydrogen-embedded gold nanowires.

Interaction of the physically adsorbed molecular hydrogen with a breaking gold nanowire results in additional stable atomic configurations in few atom contacts and appearance of fractional peaks in the conductance histogram. This effect is explained by peculiar dynamic evolution of the hydrogen-embedded nanoconstriction due to competition between tensile and capillary forces. Dimerization within the atomic wire and hydrogen-assisted stabilization of gold dimers results in preferable atomic arrangements with conductivity close to 0.5 and 1.5 of quantum conductance unit G(0)=2e(2)/h.

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Real-space imaging of an orbital Kondo resonance on the Cr(001) surface.

The Kondo effect is usually connected with the interaction between a localized spin moment and itinerant electrons. This interaction leads to the formation of a narrow resonance at the Fermi level, which is called the Abrikosov-Suhl or Kondo resonance. Scanning tunnelling microscopy is an ideal technique for real-space investigations of complicated electronic structures and many-body phenomena, such as the formation of the Kondo resonance or d-wave pairing in high-T(c) superconductors. Theory has predicted that similar, Kondo-like many-electron resonances are possible for scattering centres with orbital instead of spin degrees of freedom--the quadruple momenta in uranium-based compounds or two-level systems in metallic glasses are examples of such 'pseudo-Kondo' scattering centres. Here we present evidence for the orbital Kondo resonance on a transition-metal surface. Investigations of an atomically clean Cr(001) surface at low temperature using scanning tunnelling microscopy reveal a very narrow resonance at 26 meV above the Fermi level, and enable us to visualize the orbital character of the corresponding state. The experimental data, together with many-body calculations, demonstrate that the observed resonance is an orbital Kondo resonance formed by two degenerate d(xz), d(yz) surface states.

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Direct observation of surface alloying and interface roughening: growth of Au on Fe(001).

Scanning tunneling microscopy studies on the growth of Au on Fe(001) are reported. A surface alloy is observed for submonolayer deposition ( <0.5 monolayer) at temperatures higher than 370 K. This surface-confined alloy demixes when it is covered with Au and in combination with imperfect layer-by-layer growth a rough interface consisting of Au islands in and Fe islands on the original Fe(001) substrate is created. A real-space high resolution study of this buried interface is possible because of the large difference in interlayer spacing between bcc Fe(001) and fcc Au(001).

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Influence of impurities on localized transition metal surface states: scanning tunneling spectroscopy on V(001).

The first scanning tunneling spectroscopy measurements on V(001) are reported. A strong surface state is detected which is very sensitive to the presence of segregated carbon impurities. The surface state energy shifts from 0.03 eV below the Fermi level at clean areas towards higher energies (up to approximately 0.2 eV) at contaminated areas. Because of the negative dispersion of this state, the upward shift cannot be described in a simple confinement picture. Rather, the surface state energy is governed by vanadium surface s- d interactions which are altered by carbon coverage.

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Optical detection of ballistic electrons injected by a scanning-tunneling microscope.

We demonstrate a spectroscopic technique which is based on ballistic injection of minority carriers from the tip of a scanning-tunneling microscope into a semiconductor heterostructure. By analyzing the resulting electroluminescence spectrum as a function of tip-sample bias, both the injection barrier height and the carrier scattering rate in the semiconductor can be determined. This technique is complementary to ballistic electron emission spectroscopy since minority instead of majority carriers are injected, which give the opportunity to study the carrier trajectory after injection.

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