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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.

Special Seminar

Scattering Studies of Staging and Phase Transitions in Layered Graphite Intercalation Compounds and Superconductors (La2CuO4+δ)

Prof. Simon C. Moss

by: Prof. Simon C. Moss

Date: Friday November 20, 1998

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

In intercalation systems a second species of atom or molecule is inserted within the layers of the host. The most highly studied materials are the graphite intercalation compounds (GIC's) which incorporate both alkali donors and molecular acceptors in between the graphite sheets. Via a repulsive interaction (both Coulomb and elastic) these intercalated species tend to stage, i.e. to form a periodic stacking . In La2CuO4+δ, the extra oxygen also goes into random interstitial sites in layers of the Fmmm phase and with decreasing temperature, the oxygen phase separates into δ ~ 0 and δ ~ 0.05 phases. The O-rich phase then undergoes a periodic layering (staging) which depends on both temperature and composition. The phase diagram will be discussed and the (neutron) scattering evidence for staging via periodic tilts of the CuO6 octahedra will be shown. The mechanism for the continuous change in staging period [en] the Daumas/Herold model [en] will be discussed as will the gradual evolution of a striped phase in La2CuO4+δ at low temperature.

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