TcSUH
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Bi-Weekly Seminar
Andreev reflection spectroscopy in uniquely designed nano-scale normal metal/superconductor devices featuring Fe1+yTe1-xSex
Date: Friday November 18, 2011
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Andreev reflection spectroscopy in uniquely designed nano-scale normal metal/superconductor devices featuring Fe1+yTe1-xSex
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Bi-Weekly Seminar
The Nature of Highly Epitaxial Double Perovskite LaBaCo2O5.5+δ Thin Films
Date: Friday September 09, 2011
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
The Nature of Highly Epitaxial Double Perovskite LaBaCo2O5.5+δ Thin Films
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Bi-Weekly Seminar
Shedding Light on Iron-Chalcogenide/Pnictide Superconductors by Inelastic Scattering
Date: Friday July 22, 2011
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Iron-based superconductors are materials with many intriguing properties as competing magnetic and superconducting orders, non-phonon mediated pairing mechanism, direct substitutional doping into the active pairing layer, and metallic multiband nature of the parent compounds. Although there is no a “silver bullet” approach to such a complexity, the light scattering techniques have proven to be a useful tool for studying these superconductors. Inelastic scattering of light from solids is commonly known as Raman scattering by phonons. The scattering process, however, always proceeds through electrons and can also involve a broad range of elementary excitations as intra-ionic electronic transitions (crystal-field excitations), intra and inter-band electronic transitions, magnons through spin-orbit coupling, and electronic pair-breaking excitations in superconductors. I will address the achievements, challenges, and perspectives in studying iron chalcogenides and pnictides by a combined experimental Raman scattering and first-principles simulation approach.
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Bi-Weekly Seminar
Image Noise in Helium Lithography
Date: Friday June 24, 2011
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
A distinctive feature of resist exposure by energetic helium ions or neutral atoms is that critical exposure densities are very low, about 100 times smaller than for electrons. Thus, particle distributions are sparse, leading to significant statistical fluctuations in the deposited energy density even in Polymethylmethacrylate (PMMA), a relatively insensitive, non-chemically amplified resist. The impact is first seen as roughness in the region of partial exposure on a feature’s edge where the bunching of just a few particles may cause the energy density to shift above or below the resist development threshold. As feature size is reduced, however, fluctuations in the total number of particles (shot noise) become larger as a fraction of average dose, potentially causing over and under exposure of the entire feature. This talk presents an integrated study of image noise in helium lithography that compares shape variations in neutral particle mask images with the predictions of a Monte-Carlo model. The model acco unts for the following: 1) Poisson statistics of the particle emission process, 2) the variable spatial distribution of the particles within the aerial image, 3) the effect of scattering on the particle distribution at various depths in the resist, and 4) smoothing of the deposited energy distribution by exposure and development processes. Proximity lithography experiments were carried out using 10 keV neutral helium atoms in 20 nm thick PMMA resist under conditions of 12.7 nm [FWHM] penumbral image blur. The energy smoothing function is assumed, based on previous experiments, to be Gaussian and its standard deviation s treated as a free parameter. Model predictions of the power spectral density of line edge roughness agree with experiment for σ =5.0 ± 0.5 nm. The model predicts that using a resist with a critical dose 20 times higher than PMMA and reducing penumbra to 0.5 nm [FWHM], for example by reducing the proximity gap, would reduce shape fluctuations to less than 0.5 nm [FWHM] for dense 10 nm dot arrays.
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Bi-Weekly Seminar
Understanding of the Superconductivity and Spin Density Wave phase in Iron-Pnictides
Date: Thursday May 12, 2011
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Based on a simple band model of two orbitals proposed by our group (D. Zhang, PRL (2009), where the asymmetry of As atoms above and below the Fe-plane is considered, we studied theoretically the coexistence of the spin-density-wave (SDW) and superconductivity in electron-doped iron-pnictide superconductors [such as BaFe_(2−x)Co_xAs_2] using Bogoliubov-deGennes equations. The phase diagram is mapped out and the evolution of the Fermi surface as the doping varies is demonstrated. For the parent compound with only SDW order, the low-energy part of the density of states and the magnetic band structure have been investigated. These results are consistent with ARPES experiments. The local density of states (LDOS) has also been calculated from low to high doping, and in the presence of a magnetic field. We show that the asymmetry in the coherent peaks, the domain wall formation and the vortex core states observed by STM experiments can all be explained by our theoretical study. It needs to point out that the band model
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