TcSUH
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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Bi-Weekly Seminar
Interface Magic in Nanostructural Metamaterials
Date: Friday September 03, 2010
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Interface engineered metamaterial is not the materials engineering, it is the engineered materials with designed innovative properties. In the nanoscale materials, interface plays a key role in governing their physical properties and resulting in the discovery of various anomalous physical phenomena with excellent opportunity for new active device and engineered nanosystem developments. Recently, we have systematically focused on the studies of multilayered layered structures and nanoscale ordered structures such as multilayered ferroelectric BaTiO3/SrTiO3 heterostructures and ferromagnetic LaBaCo2O5.5 nanostructures and found many interesting physical phenomena, such as domain clamped ferroelectric phenomena, unusual magnetic properties, and ultrafast oxygen exchange chemical dynamic behavior. Also, a series of models were developed to understand these interface phenomena. Details will be presented in the talk.
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Bi-Weekly Seminar
Electrochemical Nanofabrication and Material Synthesis
Date: Friday July 23, 2010
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
In the last several decades, the electrochemical processes were proven to be enabling fabrication routes behind the train of high-tech enterprise. Today, the devices with sub-100 nm dimensions are produced routinely using the electrodeposi-tion and electropolishing as the standard processing operations in magnetic recording, microprocessors and MEMS/NEMS technologies. There are many other examples where the electrochemical processes are used as convenient if not "the only" approach to deliver desired structures, materials or catalytic surfaces. The traditional felds of electrochemical research like corrosion, anodization, or electrodeposition are rapidly being involved in different contemporary scientifc disciplines where the word "nano" is frequently used prefx. In this talk, several examples of the electrochemical nanofabrication and material synthesis are presented. In the first part, the optimum design and fabrication of Pt catalyst monolayer (ML) on Au(111) is demonstrated using surface limited red-ox reaction (SLRR). The morphology of the most active catalyst ML is identifed and compared with predictions of DFT calculations. The new synthesis route for catalyst ML fabrication using SLRR guided by molecular templates is discussed as well. Continuing the talk farther, the nanoporous Au electrodes produced by dealloying process are demonstrated as the generic ultrasound sensor devices. The sensitivity of these electrodes towards the incoming ultrasound excitation is compared against the state of the art piezoelectric ultrasound sensors. The transducer equation is presented highlighting the fundamental advantage of the electrochemical ultrasound sensors in terms of their design-determined sensitivity and reduced cost. The fnal part of the talk focuses on revealing the role of additives in electrodeposition process of magnetic flms. The functional relation between concentration of organic additives in the electrodeposition solution and their incorporation into magnetic alloys is described. The dependence of magnetic flms properties on additive concentration in the solution is quantifed shining a new light to the understanding and appreciation of the organic adsorption phenomenon at the electrode surface during electrodeposition of magnetic alloys.
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