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Biomedical subjects

P Ordejón

Publications and source records attributed to P Ordejón.

17 recordsLinked to original sources
PubMed2006

Electron transport via local polarons at interface atoms.

Electronic transport is profoundly modified in the presence of strong electron-vibration coupling. We show that in certain situations, the electron flow takes place only when vibrations are excited. By controlling the segregation of boron in semiconducting Si(111)-square root 3 x square root 3 R 30 degrees surfaces, we create a type of adatom with a dangling-bond state that is electronically decoupled from any other electronic state. However, probing this state with scanning tunnelling microscopy at 5 K yields high currents. These findings are rationalized by ab-initio calculations that show the formation of a local polaron in the transport process.

Journal Article↗
PubMed2004

Stability and mobility of mono- and di-interstitials in alpha-Fe.

We report a detailed ab initio study of the stability and migration of self-interstitial atoms (SIAs) and di-interstitials (di-SIAs) in alpha-Fe. The <110> dumbbell is confirmed to be the most stable SIA configuration, 0.7 eV below the <111> dumbbell. The lowest-energy migration path corresponds to a nearest-neighbor translation-rotation jump with a barrier of 0.34 eV. The most stable configuration for di-SIAs consists of <110> parallel dumbbells. Their migration mechanism is similar to that for SIAs, with an activation energy of 0.42 eV. These results are at variance with predictions from existing empirical potentials and allow one to reconcile theory with experiments.

Journal Article↗
PubMed2004

Phonon dispersion in graphite.

We measured the dispersion of the graphite optical phonons in the in-plane Brillouin zone by inelastic x-ray scattering. The longitudinal and transverse optical branches cross along the Gamma-K as well as the Gamma-M direction. The dispersion of the optical phonons was, in general, stronger than expected from the literature. At the K point the transverse optical mode has a minimum and is only approximately 70 cm(-1) higher in frequency than the longitudinal mode. We show that first-principles calculations describe very well the vibrational properties of graphene once the long-range character of the dynamical matrix is taken into account.

Journal Article↗
PubMed2001

Unexpected dynamics for self-interstitial clusters in silicon.

Ab initio molecular-dynamics simulations of self-interstitial clusters in Si show that I2 and the most stable of the I3 (" I(a)(3)") clusters diffuse extremely fast. In these clusters, the I's share a single bond-centered (BC) site. The metastable I(b)(3) cluster involves three adjacent BC sites. Simulations show that the three I's exchange sites with each other, but the center of the defect remains at the same place. Simulations with I1 and I4 show no diffusion on the same time scale.

Journal Article↗
PubMed2000

Absence of dc-conductivity in lambda-DNA.

The electrical conductivity of biomaterials on a molecular scale is of fundamental interest in the life sciences. We perform first principles electronic structure calculations, which clearly indicate that lambda-DNA chains should present large resistance values. We also present two direct procedures to measure electrical currents through DNA molecules adsorbed on mica. The lower limit for the resistivity is 10(6) Omega . cm, in agreement with our calculations. We also show that low energy electron bombardment induces a rapid contamination and dramatically affects the measured conductivity, thus providing an explanation to recent reports of high DNA conductivity.

Animals↗