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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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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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Bi-Weekly Seminar

Electropolymerizable Dendrimer and Hybrid Nanomaterials

Prof. Rigoberto  Advincula

by: Prof. Rigoberto Advincula

Date: Thursday March 26, 2009

Time: 4:00 pm – 5:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

In this talk, we describe the investigation of dendrimeric precursor and hybrid nanomaterials towards the formation of conjugated polymer nanoparticles, networks, nanopatterning, and nanobjects. Electropolymerizable precursor polymer materials have been widely reported by our group and have been used to modify electrode surfaces with the formation of conjugated polymer network films. Most of these materials are based on linear polymers and block/graft copolymers. Very few reports have been given on dendritic precursor materials utilized for their electrochemical activity. In this talk, we will describe several strategies in which we have synthesized dendritic precursor polymers. These materials have applications for conducting polymer-based energy transfer materials, nanoelectronics, and sensing.

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Bi-Weekly Seminar

Pseudogap State of High-Temperature Superconductors

by: Dr. Kim Wonkee

Date: Friday February 06, 2009

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

One of the most defining features of high-temperature superconductors is the pseudogap state. It is important not only because high-temperature superconductors are significantly different from conventional superconductors above the superconducting transition temperature (Tc) but also because it is related with the natures of the superconducting transition of cuprate superconductors. In this talk, I briefly review experimental observations of the pseudogap state of high-Tc superconductors. Various experiments seem to indicate that the pseudogap state exists in all high-Tc superconductors regardless of the doping level. This is an important fact for the phase diagram of the cuprates. There are two commonly believed phase diagrams. These phase diagrams lead to quite different theoretical models for the pseudogap state of high-Tc superconductors. In general, the theoretical models can be categorized into two pictures. One is the preformed-pair scenario and the other the competing gap model. I compare the two scenarios based on the basic ideas of the models. Nonetheless, the emphasis in the talk goes to the preformed-pair model because growing experimental evidence appears to favor this picture. The preformed pair model is partially motivated by a two-dimensional nature of copper-oxide planes. Unlike the conventional BCS theory, this model does not assume that the Cooper pair formation and the phase coherence take place at the same time. The pairs form above Tc while the phase is locked in via the Kosterliz-Thouless (KT) transition at the KT transition temperature. Consequently, this temperature is identified as Tc. Since the KT phenomenon can be described within the classical XY model, I explain it pictorially. Recent experiments on spatial variations of the order parameters visualized in topographic images revel local structures of the order parameters in the pseudogap state of cuprate superconductors. Within the preformed-pair scenario (also known as phase fluctuation model), we incorporate the phase fluctuations generated by the classical XY model with the Bogoliubov-de Gennes formalism utilizing a field-theoretical method. This picture delineates the inhomogeneous characteristics of local order parameters observed in high-Tc superconductors above Tc. I also present the local density of states near a non-magnetic impurity with a strong scattering potential computed based on the model. The resonance peak smoothly evolves as temperature increases through Tc as observed in a recent experiment. Possible application of the theoretical framework would be discussed.

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