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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.

Special Seminar

Studies of Ferroelectric Thin Film Growth and Film-Based Device Processes via in situ Analytical Techniques

by: Dr. Orlando Auciello

Date: Friday January 22, 1999

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The science and technology of ferroelectric thin films has experienced explosive development during the last ten years. Low density memories are now being incorporated in numerous devices such as cellular phones and “smart cards.” However, substantial work remains to be done to develop materials integration strategies for high-density. non-volatile ferroelectric random access memories (NVFRAMs). We have demonstrated that the implementation of a variety of complimentary in situ or in situ, real-time characterization techniques is critical to understand film growth and device related processes. We are using time of flight ion scattering and recoil spectroscopy (TOF-ISARS) to perform in situ, real-time studies of film growth processes. TOF-ISARS involves three distinct but closely related experimental methods, namely ion scattering spectroscopy (ISS), direct recoil spectroscopy (DRS), and mass spectroscopy of recoiled ions (MSRI), which are capable of providing monolayer-specific information on film growth and surface segregation processes in the ambient conditions required for growing ferroelectric and other thin films. TOF-ISARS can see the surface of thin films, but not buried interfaces. Therefore, we have combined TOF-ISARS with Spectroscopic Ellipsometry, which permits us to investigate buried interfaces as they are being formed.

Recent work performed to understand growth processes and microstructure-property relationships of the layered perovskite SrBi2Ta2O9 (SBT) films will be discussed, because of their relevance for SBT capacitors now included in “ferroelectric smart cards.” The polarization state and polarization reversal in ferroelectric thin films are naturally linked to domains arrangements and their transformations. Therefore, direct imaging of domain structures and investigation of their behavior under the applied electric field can provide valuable information for a general understanding of switching phenomena and the role domains play in degradation effects such as polarization fatigue and polarization retention loss in ferroelectric films. We are using scanning force microscopy (SFM) to perform in situ nanoscale imaging of polarization domains in ferroelectric thin films to study polarization reversal and retention loss. Recent results from studies of domain structures at the nanoscale level in ferroelectric films will be discussed in terms of the basic scientific and technological implications. We are also investigating domain dynamics in collaboration with Prof. Dravid (NWU) using in situ TEM analysis.

Another powerful technique suitable for in situ, real-time characterization of thin film growth processes and some film-based device operation is based on the use of synchrotron X-ray analysis at the APS. We have started a program to study film growth processes using an MOCVD system attached to the Advanced Photon Source at Argonne National Laboratory, and to perform in situ characterization of switching phenomena in ferroelectric thin films. Initial results will be presented.

Special Seminar

Braided Rivers and Superconducting Vortex Avalanches

Prof. Kevin E. Bassler

by: Prof. Kevin E. Bassler

Date: Thursday January 21, 1999

Time: 3:00 pm – 4:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Magnetic vortices have experimentally been found to intermittently flow through preferred channels when they are forced in or out of a superconductor. This behavior is studied using a simple cellular model. The vortex flow is found to make braided rivers strikingly similar to aerial photographs of braided fluvial rivers, such as the Brahmaputra. The scaling properties of the braided vortex rivers in the model are quantitatively compared with those of naturally occurring braided fluvial rivers, giving general agreement. This suggests that a common dynamical mechanism exists for braiding. This mechanism is avalanches of stick-slip events, either of sediment or vortices, which organize the system into a critical braided state.

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