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Biomedical subjects

S D Kevan

Publications and source records attributed to S D Kevan.

9 recordsLinked to original sources
PubMed2005

Disorder-induced microscopic magnetic memory.

Using coherent x-ray speckle metrology, we have measured the influence of disorder on major loop return point memory (RPM) and complementary point memory (CPM) for a series of perpendicular anisotropy Co/Pt multilayer films. In the low disorder limit, the domain structures show no memory with field cycling--no RPM and no CPM. With increasing disorder, we observe the onset and the saturation of both the RPM and the CPM. These results provide the first direct ensemble-sensitive experimental study of the effects of varying disorder on microscopic magnetic memory and are compared against the predictions of existing theories.

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PubMed2004

Mechanism of gap opening in a triple-band Peierls system: in atomic wires on Si.

One dimensional (1D) metals are unstable at low temperature undergoing a metal-insulator transition coupled with a periodic lattice distortion, a Peierls transition. Angle-resolved photoemission study for the 1D metallic chains of In on Si(111), featuring a metal-insulator transition and triple metallic bands, clarifies in detail how the multiple band gaps are formed at low temperature. In addition to the gap opening for a half-filled ideal 1D band with a proper Fermi surface nesting, two other quasi-1D metallic bands are found to merge into a single band, opening a unique but k-dependent energy gap through an interband charge transfer. This result introduces a novel gap-opening mechanism for a multiband Peierls system where the interband interaction is important.

Journal Article↗
PubMed2003

Indium square root 7 x square root 3 on Si(111): a nearly free electron metal in two dimensions.

We present measurements of the Fermi surface and underlying band structure of a single layer of indium on Si(111) with square root 7 x square root 3 periodicity. Electrons from both indium valence electrons and silicon dangling bonds contribute to a nearly free, two-dimensional metal on a pseudo-4-fold lattice, which is almost completely decoupled at the Fermi level from the underlying hexagonal silicon lattice. The mean free path inferred from our data is quite long, suggesting the system might be a suitable model for studying the ground state of two-dimensional metals.

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PubMed2003

Unusual spectral behavior of charge-density waves with imperfect nesting in a quasi-one-dimensional metal.

Low-temperature electronic properties of the charge-density-wave system NbSe3 are reported from angle-resolved photoemission at 15 K. The effect of two instabilities q(1) and q(2) on the k-resolved spectral function is observed for the first time. With a pseudogap background, the gap spectra exhibit maxima at Delta*(1) approximately 110 meV and Delta*(2) approximately 45 meV. Imperfectly nested sections of the Fermi surface lack a Fermi-Dirac edge, and show the signature of a dispersion that is modified by self-energy effects. The energy scale is of the order of the effective gap 2 Delta*(2). The effect disappears above T2, suggesting a correlation with the charge-density-wave state.

Journal Article↗
PubMed2002

Spin-resolved photoemission of surface states of W(110)-(1 x 1)H.

The surface electronic states of W(110)-(1 x 1)H have been measured using spin- and angle-resolved photoemission. We directly demonstrate that the surface bands are both split and spin-polarized by the spin-orbit interaction in association with the loss of inversion symmetry near a surface. We observe 100% spin polarization of the surface states, with the spins aligned in the plane of the surface and oriented in a circular fashion relative to the Smacr; symmetry point. In contrast, no measurable polarization of nearby bulk states is observed.

Journal Article↗
PubMed2002

Differential photoelectron holography: a new approach for three-dimensional atomic imaging.

We propose differential holography as a method to overcome the long-standing forward-scattering problem in photoelectron holography and related techniques for the three-dimensional imaging of atoms. Atomic images reconstructed from experimental and theoretical Cu 3p holograms from Cu(001) demonstrate that this method suppresses strong forward-scattering effects so as to yield more accurate three-dimensional images of side- and backscattering atoms.

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PubMed2001

High-temperature symmetry breaking in the electronic band structure of the quasi-one-dimensional solid NbSe3.

The electronic band structure of the Peierls compound NbSe3 has been explored for its symmetries with microspot synchrotron photoemission. The Fermi level crossings and deviations from one-dimensional behavior are identified. Density-functional calculations of the Fermi surfaces confirm the nesting conditions relevant for the two phase transitions. The instability along the chains with superstructure periodicity q = 0.44 A(-1) induces a backfolding of the electronic bands, and the Fermi level crossings appear suppressed. This broken symmetry is observed in the fluctuation regime at more than twice the critical temperature, where the correlation length is strongly reduced.

Journal Article↗
PubMed1996

Effects of age and coion (K+ and Na+) on the toxicity of thiocyanate to rainbow trout (Oncorhynchus mykiss) during pulse or continuous exposure.

Thiocyanate (SCN-) is released to the aquatic environment as a result of the treatment of cyanide-bearing wastes from precious-metal mining activity. During continuous exposure alevin rainbow trout (1- and 10-day-old) were approximately 90% more tolerant of SCN- than juveniles (2-month-old). Pulse exposure of alevins for periods > or = 48 hr were required before postexposure stress reduced tolerance (by approximately 30%) in the 24-hr period following exposure. In contrast, stressing of juveniles after exposures of > or= 9 hr consistently reduced tolerance by approximately 60%. While coion (K+ or Na+ for SCN- derived from either KSCN or NaSCN) had no influence on SCN- toxicity for alevins, juveniles exhibited reduced tolerance in the presence on Na+. Although sudden death syndrome was apparent in juveniles, none occurred with alevins. Overall, juveniles were less tolerant of SCN- than alevins, possibly because of the enhanced efficiency of SCN- uptake by gill respiration versus the predominantly vitelline membrane respiration of alevins.

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