TcSUH
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Special Seminar
New Synthetic Approaches to Chalcogenide Nanoparticles - Molecular/Solid State LEGO - Chemical Concepts for Materials Design
by: Prof. Wolfgang Tremel
Date: Thursday December 10, 2009
Time: 4:00 pm – 5:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Not available.
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Special Seminar
The Sonic Crystal and its Novel Efects
by: Prof. Yan Chen
Date: Friday October 23, 2009
Time: 11:00 am – 12:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
The sonic crystal, analogous to the photonic crystal, has been studied for over two decades. Some of its important efects, such as band-gap and dispersion relation, have been well established. Very recently, abnormal phenomena, such as negative refraction, enhanced transmission of acoustic waves in sub-wavelength, negative bulk modulus, negative mass density, etc., were found. The much lower velocity of acoustic waves as compared to light makes it easier to directly measure the sonic crystal's amplitude and phase of propagation, providing evidence for establishing the basic efects shared by both photonic and sonic crystals. On the large scale, sonic crystals with complicated artifcial structures could be easily introduced not only to give rise to some novel efects for which the physics is unclear in photonic crystals, but also to revolutionize acoustic device applications.
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Special Seminar
Interfacial Behavior of Highly Epitaxial Ferroelectric (Pb,Sr)TiO3 Thin Films
by: Dr. Yuan Lin
Date: Friday August 14, 2009
Time: 11:00 am – 12:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
(Pb,Sr)TiO3 thin films have shown great potential in high frequency room temperature microwave elements due to their excellent properties such as large tunability and low dielectric loss. Interfacial properties have been considered to be related closely with the dielectric properties of these films. To fully understand the growth mechanism for achieving high quality epitaxial films with optimized physical properties, systematical study on the interfacial behavior is critical and important. We have investigated the strain states, dislocation density and strain distribution near the interface of epitaxial (Pb,Sr)TiO3 thin films on NdGaO3, LaAlO3, and MgO substrates, by using high-resolution X-ray diffraction. Dielectric properties of the films have been measured and demonstrate strong correlation with the interfacial properties such as strain and dislocation densities. Details about the interfacial behavior and strain relaxation will be discussed.
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Special Seminar
The Low-Energy Inhomogeneous States of the t- J Model
by: Dr. T. K. Lee
Date: Friday April 24, 2009
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
By using a variational appproach it is shown that periodic stripe states or randomly oriented stripe states are almost degenerate in energy with the uniform d-wave superconducting state in the t-J type models. It provides a simple explanation for the observation of the cluster-glass state in BSCO high temperature superconductors. There is no need to introduce other competing interactions to obtain these inhomogeneous states. However, to stabilize a long-range-order stripe state as seen in LaBaCuO with 1/8 doping, we propose a new model to include in the t-J model the short-ranged hopping modulations due to mass renormalization by electron-phonon coupling. In addition, our results show that the most stable stripe will have its charge modulation period scales with 1/2x where x is the hole density. Hence we are able to explain the so called Yamada plot observed in neutron scattering experiment.
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Special Seminar
What is Going on in the Iron Pnictides?
by: Dr. Qimiao Si
Date: Thursday April 09, 2009
Time: 4:00 pm – 5:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Since the surprising discovery of high Tc superconductivity in the iron pnictides in early 2008, a large body of physical properties have already been accumulated. In this talk, I will attempt to draw a coherent picture about the microscopic physics of these systems.
Based on the fact that they are "bad metals," I will make the case that these materials lie at the boundary between Mott localization and itinerancy [1,2,3]. The incoherent electronic excitations are modeled in terms of localized magnetic moments, with J1-J2 superexchange interactions on the iron square lattice. Such a magnetic frustration leads to a (pi,0) collinear antiferromagnetic ordering and a reduced ordered moment [1], and naturally yields a magnetism-induced structural phase transition; both are observed experimentally. The coupling of the local moments to the coherent electronic excitations tunes the strength of antiferromagnetic order, leading to a magnetic quantum critical point [2,3]; emerging evidence for quantum criticality will be summarized. The implications of these considerations for superconductivity will be discussed.
[1] Q. Si and E. Abrahams, PRL101, 076401 (2008)[2] J. Dai et al, PNAS 106, 4118 (2009)[3] Q. Si et al, NJP (2009) -- arXiv.org:0901.4112
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