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Special Seminar

A Backdoor Approach to Cuprate Superconductivity

by: Prof. Anthony J. Leggett

Date: Wednesday April 07, 1999

Time: 11:00 am – 12:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

A question which has been asked with insufficient persistence concerning cuprate superconductivty is: If we assume that the driving mechanism for the transition is primarily electronic rather than phononic and is associated with a saving of repulsive Coulomb energy, then exactly where in the space of wave vector and frequency does the saving occur? I first point out that, quite irrespective of any theoretical preconceptions, this question can in principle be answered by differential EELS experiments. I then conjecture the answer: In the region of small q and midinfrared omega! This conjecture, if correct, explains in a natural way the systematics of [Tc] within the various homologous series, and moreover predicts changes, on the transition, in both the EELS and the optical spectra in the midinfrared region which are one or two orders of magnitude larger than would follow from a simple BCS picture and hence of an observable order. Implications for our general theoretical understanding of cuprate superconductivity are discussed.

Special Seminar

X-Ray Emission Spectra as a Tool for Study of Local and Electronic Structure and Characterization of Materials

by: Prof. Ernst Z. Kourmaev

Date: Tuesday April 06, 1999

Time: 11:00 am – 12:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The results of study of the local and electronic structure of advanced materials by means of X-ray emission spectroscopy (XES) are presented. The small-spot XES with variable electron excitation is used for the characterization of phase distribution in depth in buried solid-solid interfaces after heat treatment. The site selective X-ray fluorescence (excited with tuneable synchrotron radiation) is used for the band mapping of in-plane and apical oxygen 2p-states in the valence band of Sr2CuO4 superconductor. The XES of impurity atoms are measured in HTSC: B, C, F, P and Ni3Al:B and their local structure is determined. The chemical reactions in polymer films induced by ion irradiation are studied with help of X-ray fluorescence measurements. The results of XES measurements of organic superconductors: (SN)x, k-(ET)2Cu[N(CN)2]Br and k-(ET)2Cu(NCS)2 are compared with UPS (XPS) spectra and band structure calculations and orbital composition of energy bands is determined.

Special Seminar

Magnetostriction and Internal Stress due to Flux Pinning in Superconductors

by: Prof. Tom H. Johansen

Date: Tuesday March 09, 1999

Time: 11:00 am – 12:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The giant magnetostriction phenomenon was discovered a few years ago by Ikuta et al., who observed wide hysteresis loops in dilatation versus magnetic field in Bi-2212 crystals. The effect was immediately recognized as a general manifestation of the flux-pinning forces. Of particular current interest is the magneto-elastic behavior of the large-size single-domain RE-123 bulks. As first analysed by Ren et al., the pinning-induced stresses can easily cause material fracture [en] a disaster for applications like HTS trapped-field magnets. In the talk I will focus on the recent progress made in the analysis of such magneto-elastic problems. Exact solutions, valid for any critical-state type of behavior, will be presented for several realistic sample shapes. Also some new experimental strain results are reported.

Special Seminar

Processing and Characterization of Single-Domain YBCO: Fundamental Electromagnetic Properties and Engineering Applications

by: Dr. Donglu Shi

Date: Wednesday March 03, 1999

Time: 10:00 am – 11:00 am

Location: Houston Science Center – Building 593 — Room 102

Overview

The general and modified seeded melt growth (SMG) procedures of large-domain YBCO will be reviewed. Growth anisotropy will be discuused based on the results of a series quenching experiments below the peritectic temperature. A study on the electromagnetic properties of single-domain YBa2Cu3Ox has also been carried out including angle dependence of magnetization, RF behavior, and levitation force. Crystal angle dependence of magnetization has been measured in a spherically-shaped, single domain YBa2Cu3Ox (YBCO). In contrast to the previously reported results, we have found that the angle dependence exhibits a saw-tooth wave between 0[deg] and 360[deg] in a wide range of temperatures and fields. A physical model has been developed to explain the angle dependence of magnetization observed in this experiment. Based on a new design, a cavity resonator has been constructed using the single-domain YBa2Cu3Ox processed by SMG. All cavity parts are made of single-domain YBa2Cu3Ox without any dielectric materials. The measured Q has reached a high value of 10,200 at 18.4 GHz. Our experimental data show great promise in the development of RF components using single domain high temperature superconductors.

Special Seminar

Unusual Experiments on the Nature of High Tc Superconductivity

by: Dr. T. Venkatesan

Date: Friday February 26, 1999

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Three different experiments which explore the roles of lattice, electron, and spin dynamics in the cuprate superconductors will be described.

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