Biomedical subjects

Roy Clarke

Publications and source records attributed to Roy Clarke.

3 recordsLinked to original sources

PubMed2005

X-ray synchrotron studies of ultrafast crystalline dynamics.

Ultrafast X-ray experiments at synchrotron sources hold tremendous promise for measuring the atomistic dynamics of materials under a wide variety of transient conditions. In particular, the marriage of synchrotron radiation and ultrafast laser technology is opening up a new frontier of materials research. Structural changes initiated by femtosecond laser pulses can be tracked in real time using time-resolved X-ray diffraction on picosecond time scales or shorter. Here, research at the Advanced Photon Source is described, illustrating the opportunities for ultrafast diffraction with some recent work on the generation of impulsive strain, coherent phonon generation and supersonic diffusion of electron-hole plasmas. The flexibility of time-resolved Bragg and Laue diffraction geometries are both utilized to illuminate the strain generation and evolution process. Time-resolved X-ray science will become increasingly important with the construction of linac-based ultrafast X-ray sources.

Journal Article↗

PubMed2002

Concentration fluctuations in the binary mixture hexane-nitrobenzene with static and dynamic x-ray scattering.

We report on a small-angle x-ray scattering study of the hexane-nitrobenzene binary fluid mixture near its critical point. The use of an ultrabright x-ray undulator synchrotron source enabled us to measure the temperature dependence of the static structure factor with unprecedented contrast, and the large coherent flux of this source provided a probe for the fluctuation dynamics via the x-ray photon correlation spectroscopy technique. We find that the intensity and correlation lengths diverge with the expected three-dimensional Ising critical exponents, and the dynamical correlation function decays exponentially with correlation times as small as 250 micros. In the range of wave vector studied here (1.2-2.6 x 10(-3) A(-1)) the concentration fluctuations relaxed diffusively with a diffusion constant consistent with that determined from visible light scattering measurements.

Journal Article↗

PubMed2002

Direct determination of epitaxial interface structure in Gd2O3 passivation of GaAs.

Obtaining accurate structural information on epitaxial films and interfaces is nowhere more critical than in semiconductor passivation layers, where details of the atomic structure and bonding determine the nature of the interface electronic states. Various non-destructive methods have been used to investigate the structure of films and interfaces, but their interpretation is model-dependent, leading occasionally to wrong conclusions. We have developed a new X-ray method for the direct determination of epitaxial structures, coherent Bragg rod analysis (COBRA). The usefulness of our technique is demonstrated by mapping, with atomic precision, the structure of the interfacial region of a Gd2O3 film grown epitaxially on a (100) GaAs substrate. Our findings reveal interesting behaviour not previously suggested by existing structural methods, in particular a lock-in of the in-plane Gd atomic positions to those of the Ga/As atoms of the substrate. Moreover, we find that the bulk stacking of the Gd2O3 atomic layers is abandoned in favour of a new structure that is directly correlated with the stacking sequence of the substrate. These results have important implications for Gd2O3 as an effective passivation layer for GaAs (ref. 7). Our work shows that the COBRA technique, taking advantage of the brilliance of insertion device synchrotron X-ray sources, is widely applicable to epitaxial films and interfaces.

Arsenicals↗