TcSUH
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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Special Seminar
Magnetotransport in Heavy Fermion Metals CeMIn5 (M = Co, Ir): The Influence of Antiferromagnetic Fluctuations
by: Steffen Wirth
Date: Thursday November 06, 2008
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Heavy fermion metals are often characterized by a variety of relevant energy scales and competing interactions which may result in such fascinating phenomena as quantum criticality and unconventional superconductivity. Therefore, these materials have advanced to suitable model systems by means of which electronic interactions can be studied in detail. This will be discussed for two classes of heavy fermion metals: First, results of detailed magneto-transport investigations on YbRh2Si2 will be described. In the second part, we present an overview of magnetotransport measurements on CeCoIn5 and CeIrIn5. Pressure-dependent Hall measurements on CeCoIn5 exhibit a well developed feature that can unambiguously be related to spin fluctuations associated with the departure from Landau Fermi liquid behavior. Magnetotransport measurements on CeIrIn5 indicate a precursor state to superconductivity. The relation of this precursor state to the so-called pseudogap in high-TC cuprate superconductors will be discussed.
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Special Seminar
Brief Overview of Recent Developments on Iron-based Superconductors
by: Prof. Yan Chen
Date: Friday October 03, 2008
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Superconductivity has always attracted considerable attention from people in various fields due to its striking quantum phenomena. The recent discovery of high-temperature superconductivity in the iron-based pnictide materials has made a significant impact in the field of condensed matter physics. The superconducting transition temperature has been quickly raised to about 55K and this novel superconductor does not belong to the same categories as those of known superconductors such as copper oxide high-Tc superconductors and conventional BCS-type superconductors. In this talk, I will give a very brief introduction to the recent research progress as well as a perspective on these new iron-based superconductors, in particular, the theoretical and experimental efforts toward cracking the mechanism.
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