TcSUH
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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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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.
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Special Seminar
Braided Rivers and Superconducting Vortex Avalanches
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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Special Seminar
Scattering Studies of Staging and Phase Transitions in Layered Graphite Intercalation Compounds and Superconductors (La2CuO4+δ)
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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Special Seminar
A Study of the Electrochemically Oxidized La2CuO4+δ
by: Prof. Pei Hor
Date: Friday October 30, 1998
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
It is recognized that the single most important and universal parameter in high [Tc] is the “nominal” hole concentration p. We argue that the La2CuO4+δ system exhibits real intrinsic behaviors of doped holes around room temperature. Using the La2CuO4+δ as a model system and taking advantage of precise control of p by electrochemical intercalation technique, we will address some critical issues concerning the universal electronic phase diagram of high [Tc]. We will first focus on the metal-insulator transition and the onset of superconductivity at the critical hole concentration pc = 0.0625. It is found that the metallic state of high [Tc] is composed of two types of holes: Free holes on top of an underlying more localized condense. The existence and physical significance of some special hole concentrations will also be discussed.
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