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Special Seminar

The Low-Energy Inhomogeneous States of the t- J Model

Dr. T. K. Lee

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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Bi-Weekly Seminar

Direct Probe of the Key Building Block of the Fe-based Superconductors with Scanning Tunneling Microscopy/Spectroscopy (STM/S)

Dr. Shuheng H. Pan

by: Dr. Shuheng H. Pan

Date: Thursday April 23, 2009

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The recently discovered superconductivity in iron (Fe)-based compounds is another exciting advancement in condensed matter physics since the discovery of high-Tc superconductivity in cuprates. Using a UHV Low Temperature Scanning Tunneling Microscope, we have been studying the structural and electronic properties of the parent and Co-doped BaFe2As2 compound. We find that, by low temperature in situ cleaving, we are able to expose the key building block - the Fe-As layer of this compound, where superconductivity is believed to occur. With STM/S, we directly probe this key building block with spatial resolution down to atomic scale. STM is a surface sensitive technique. Keeping this in mind, I will demonstrate how we use this high real-space resolution and surface sensitive technique to learn the structural and electronic properties within the bulk. I will also discuss our results on the density-of-states (DOS) evolution with doping, the scaling of the superconducting energy gap, and some electronic local effects that may be used to help determine the pairing symmetry.

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Bi-Weekly Seminar

Interface Engineered Nanostructural Metamaterials with Anomalous Physical Phenomena

Prof. Chonglin  Chen

by: Prof. Chonglin Chen

Date: Tuesday April 14, 2009

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Interface engineered material has attracted more and more attention in the multifunctional materials research and active device fabrication. It plays a key role to control the physical properties of advanced nanomaterials and results in the discovery of various new physical phenomena with excellent opportunity for developing new metamaterials for active devices and engineered nanosystems. We have focused on the systematic studies on the formations and the characterizations of various highly epitaxial oxide thin films and multilayered layered structures to understand the nature of interface induced anomalous physical phenomena. Recently, by optimizing the epitaxial conditions we have successfully controlled and systematically investigated the highly epitaxial ferroelectric thin films and highly ionic conductive oxide thin films and the multilayered nanostructures. We have observed strong anisotropic phenomena in highly epitaxial (Pb,Sr)TiO3 thin films, and observed various anomalous physical phenomena such as locked ferroelectric domain formation from the multilayered BaTiO3/SrTiO3 superlattices for memory capacitance device and active actuator applications, extremely high ionic conductivity in the multilayered YSZ/GCO structures solid state fuel cells, and many others. Also, a series of models were developed to understand these interface phenomena. Details will be presented in the talk.

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Special Seminar

What is Going on in the Iron Pnictides?

Dr. Qimiao  Si

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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Bi-Weekly Seminar

Fe-As Based High-Temperature Superconductors: The Breakthrough of the Year (2008)

 Bernd  Lorenz

by: Bernd Lorenz

Date: Friday April 03, 2009

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The discovery of superconductivity in rare earth (R) oxypnictides, ROFeAs, by Hosono et al. has revived the field of high-temperature superconductivity. With transition temperatures of up to 55 K the new class of superconducting compounds has given hope to reach even higher Tc's exceeding those of the copper oxide superconductors. At the same time, questions have been raised concerning possible similarities and differences between the two high-Tc systems with the perspective that studying the FeAs superconductors might also help to better understand the cuprates. I will present a brief overview of some recent results and discuss examples of FeAs-based superconductors crystallizing in different basic structure types: (i) The PbFCl-type structure (LiFeAs) and (ii) the ThCr2Si2-type structure (AFe2As2, A=K, Rb, Cs, and the solid solution (K/Sr)Fe2As2). The ternary compounds are all self-doped superconductors. The (K/Sr)Fe2As2 - system reveals an interesting phase diagram that seems to be generic to most FeAs-systems, with a maximum Tc at an optimal composition and a spin density wave (SDW) state at the Sr-rich side. The extrapolation of the SDW phase boundary suggests the possible existence of a quantum critical point. Evidence for quantum critical scaling is found in resistivity and thermoelectric measurements.

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