Fall 2010 Schedule
BYU Condensed Matter Physics Seminar, Fall Semester 2010
Assumed to be in N209 ESC on Tuesdays at 2:00 PM unless otherwise specified.
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Branton J. Campbell, BYU Physics & Astronomy
Symmetry detection via symmetry-mode analysis
Consider a subtle low-symmetry distortion of an otherwise high-symmetry parent structure that gives rise to a large supercell and a low point symmetry. If the peak splittings are small and the superlattice intensities weak, structural complexity can increase dramatically without a comparable increase in the information content of an experimental powder-diffraction pattern. Knowing the symmetry of the low-symmetry phase reduces the number of variable structural parameters. And we have previously demonstrated that symmetry-mode analysis provides additional simplification. But what can one do when the symmetry is indeterminable using traditional methods? I will show that symmetry-mode analysis, combined with statistical-search tools, can be used to detect the symmetry of a distorted structure.
Karine Chesnel, BYU Physics & Astronomy
A speckle view of nanomagnetism
Coherent X-ray Resonant Magnetic Scattering (C-XRMS) is a unique tool for the investigation of nanoscale structural and magnetic domain topologies. The use of a spatially-filtered coherent X-ray beam implies interference effects between the scattering paths and therefore gives information about specific local morphologies1. Furthermore, the resonance effect provides a specific sensitivity to magnetic structure. The resulting coherent scattering pattern exhibits speckle features, which give an indirect image of the specific magnetic topologies2. I will present here results obtained on perpendicular exchange bias thin Co/Pt/IrMn films3. These ferromagnetic layers exhibit a perpendicular magnetization leading to striped domain structures in the plane of the film. Our C-XRMS study shows that when the film is cooled in ZFC state, it acquires a strong magnetic domain memory through exchange coupling effects with the antiferromagnetic layer4. This phenomenon is interpreted by the formation of a reference magnetic template in the antiferromagnetic underlayer during the cooling process. I will also present an update of the magnetometry instruments we have developed here at BYU – including the VSM, the EHE and SMOKE setup, and that we have used to characterize the magnetic behavior of the nanomaterials.
Wednesday September 29th at 2:00 PM in C247 ESC
Michael Bartl, University of Utah, Departments of Chemistry and Physics
Bioinspired Three-Dimensional Photonic Band Gap Crystals
The strikingly colorful world of insects is in large part the result of optical interference produced by the interaction of light with precisely ordered, periodic bio-polymeric structures, incorporated into their exoskeletons. Such structural colors have recently gained tremendous interest for the use as photonic crystals with promising potential for energy and information technology applications. While our current photonic engineering capabilities at visible wavelengths are rather limited, biological systems have evolved to create the most complex photonic architectures – structures that are still far out of our synthetic reach. For example, we discovered recently that the brilliant coloration of several beetles is the result of photonic structures with a diamond-based lattice – one of the most efficient photonic architectures. I will also present bioinspired fabrication routes that take advantage of the synergistic combination of photonic engineering in biology with sol-gel chemistry-based materials synthesis. Using this approach, we create high-dielectric three-dimensional photonic crystals with a variety of lattice geometries and band gaps at visible frequencies. In order to evaluate the properties of these novel photonic architectures, we apply a range of structural and optical characterization tools, including multi-directional optical reflectance micro-spectroscopy, optical and electron microscopy as well as photonic band structure calculations.
October 5th: APS Four-Corners Presentations
Ryan Smith, BYU Physics & Astronomy
The Ising model as a pedagogical tool
Derek Carr, BYU Physics & Astronomy
Cluster Expansion for Pt/Pd-Al binary alloys
October 12th: APS Four-Corners Presentations
Lauren Richey, BYU Physics & Astronomy
The experimental search for new predicted binary-alloy structures
Ken Clark, BYU Physics & Astronomy
Photoluminescent Properties of InGaAs Quantum Dot Structures
Gus W. Hart, BYU Physics & Astronomy
Something red, something blue, something ordered, something new.
Daniel Austin, BYU Department of Chemistry and Biochemistry
Microfabricated Ion Trap Mass Spectrometry
Abstract: I will discuss efforts to develop miniaturized radiofrequency ion trap mass analyzers using assemblies of lithographically patterned plates. While these developments are aimed at portable analytical instrumentation, we have also made progress in novel trapping geometries, electric field configurations, and applications to other ion optics devices.
Haeyeon Yang, Utah State University, Department of Physics
Guided assembly of nanodots through selective heating
Difficulties in control of the size and position of self-assembled nanostructures have been regarded as major obstacles to bringing their full potentials into commercialization, such as high efficiency solar cells. Many approaches were tried to control placements of nano dots, which includes nano patterning to take advantage of different diffusion kinetics depending on facet orientations, nano patterning then growth of stressor layers, and use of high index surfaces.
Interferential irradiation is a relatively simple process to control the placement and can be applied to any substrates and coupled with various growth techniques. Interferential irradiation of high power laser pulses has been used to create and align self-assembled metallic nano dots on glass substrates. Self-assembled nano dots by applying interferential irradiations of high power laser pulses on semiconductor surfaces have not been reported yet.
In this talk, guided assembly of nanodots on InGaAs epilayers on GaAs(001) substrate will be discussed. Spatial thermal modulations in nanoscale were created in-situ on the epitaxial growth fronts in the Molecular Beam Epitaxy (MBE) growth chamber by employing interferential irradiations of high power laser pulses. As-irradiated surfaces were examined using the attached ultra-high vacuum Scanning Tunneling Microscope (STM). STM images indicate that self-assembled dots are formed due to the irradiation. Furthermore, the dot density modulates sinusoidally with a periodicity similar to that of the interference. The morphological analysis of dots and the implications on the growth mechanism will be discussed. This work is supported by the National Science Foundation, grant number CBET-0854314.
John Kitchin, Carnegie Mellon, Department of Chemical Engineering
Modeling the interactions of adsorbates with each other and with metal surfaces
The interactions of molecules with metallic surfaces are fundamental to the ability of metals to catalyze reactions. One often thinks of a metal like platinum as the catalyst, but under reaction conditions the reactivity of the metal surfaces is modified by the molecules that adsorb on them. We have used quantum chemical calculations in conjunction with cluster expansions to probe the adsorption behavior of atomic adsorbates such as C, N, O, and S on late transition metal surfaces such as Rh, Ir, Pd, Pt, Cu, Ag, and Au(111). There are remarkable similarities in the adsorption behavior of these adsorbates that can be interpreted in terms of a simple adsorbate-induced surface electronic structure modification mechanism that is common to all the adsorbates and surfaces. The variations between the adsorbates and metals are readily explained in terms of the size of the metal and adsorbate orbitals and the geometry dependent overlap of these orbitals. We have constructed a new Solid State Table of these orbital radii from the quantum chemical calculations that can be used in conjunction with a simple model to rapidly estimate the electronic structure of metal and alloy surfaces with adsorbates on them.
Robert C. Davis, BYU Physics & Astronomy
Making Nano go Micro
November 23th: Virtual Friday - no meeting
November 30th: No seminar
John Colton, BYU Physics & Astronomy
Lightly-doped GaAs: A decade of spin lifetimes and spin resonance (Part I)
Send email to Branton Campbell if you have questions about the schedule.