Biomedical subjects

Eric M Dufresne

Publications and source records attributed to Eric M Dufresne.

6 recordsLinked to original sources

PubMed2006

Nanosecond domain wall dynamics in ferroelectric Pb(Zr, Ti)O(3) thin films.

Domain wall motion during polarization switching in ferroelectric thin films is fundamentally important and poses challenges for both experiments and modeling. We have visualized the switching of a Pb(Zr, Ti)O(3) capacitor using time-resolved x-ray microdiffraction. The structural signatures of switching include a reversal in the sign of the piezoelectric coefficient and a change in the intensity of x-ray reflections. The propagation of polarization domain walls is highly reproducible from cycle to cycle of the electric field. Domain wall velocities of 40 m s(-1) are consistent with the results of other methods, but are far less than saturation values expected at high electric fields.

Journal Article↗

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.

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PubMed2005

X-ray intensity fluctuation spectroscopy studies on phase-ordering systems.

The order-disorder phase transition in Cu3Au has been studied by x-ray intensity fluctuation spectroscopy. Following a quench from the high-temperature, disordered phase, the ordering kinetics is well described by a universal scaling form that can be measured by time-resolved (incoherent) x-ray scattering. By using coherent scattering, we have measured the fluctuations about this universal scaling form. In the late stages of the ordering process, these fluctuations give a two-time correlation function C(q,t1,t2) which has a scaling form with natural variables deltat=/t1-t2/ and t =(t1+t2) / 2. The scaling form crosses over from linear in t to t1/2. These present the first such results for a nonconserved system.

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PubMed2004

Structural visualization of polarization fatigue in epitaxial ferroelectric oxide devices.

Ferroelectric oxides, such as Pb(Zr,Ti)O(3), are useful for electronic and photonic devices because of their ability to retain two stable polarization states, which can form the basis for memory and logic circuitry. Requirements for long-term operation of practical devices such as non-volatile RAM (random access memory) include consistent polarization switching over many (more than 10(12)) cycles of the applied electric field, which represents a major challenge. As switching is largely controlled by the motion and pinning of domain walls, it is necessary to develop suitable tools that can directly probe the ferroelectric domain structures in operating devices-thin-film structures with electrical contacts. A recently developed synchrotron X-ray microdiffraction technique complements existing microscopic probes, and allows us to visualize directly the evolution of polarization domains in ferroelectric devices, through metal or oxide electrodes, and with submicrometre spatial resolution. The images reveal two regimes of fatigue, depending on the magnitude of the electric field pulses driving the device: a low-field regime in which fatigue can be reversed with higher electric field pulses, and a regime at very high electric fields in which there is a non-reversible crystallographic relaxation of the epitaxial ferroelectric film.

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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↗