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Special Seminar

The Effect of Stress on the Microwave Dielectric Properties of Ba0.5Sr0.5TiO3 Thin Films

by: Dr. James S. Horwitz

Date: Friday October 15, 1999

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Epitaxial Ba0.5Sr0.5TiO3 (BST) films have been deposited onto (100) MgO and (100) LaAlO3 (LAO) substrates by pulsed laser deposition for the development of tunable microwave devices. The dielectric constant and loss tangent of the films have been measured at room temperature from 1-20 GHz as a function of electric field ([le] 200 kV/cm) using Ag interdigitated electrodes deposited on top of the ferroelectric film. The dielectric properties of the film are strongly affected by the growth conditions, substrate type and post-deposition annealing temperature. For BST films deposited onto MgO, it is observed that, after a post-deposition anneal in O2 (T [le] 1000 [deg]C) both the dielectric constant and the dielectric loss decrease. The opposite behavior is observed when annealing BST films on LAO. In general, the dielectric constant at microwave frequencies for BST films on LAO is higher (~1500) compared to BST films on MgO (~1000). However, films on MgO have lower dielectric loss (0.004 < tan[delta] < 0.022). On both substrates, the change in the dielectric constant with an applied DC field is directly proportional to the dielectric constant and films deposited onto LAO exhibit more tuning with an applied DC field. An analysis of X-ray diffraction measurements indicates that the BST films are tetragonally distorted, with the unit cell enlarged in the direction normal to the substrate surface. The magnitude of the distortion is affected by the substrate type, oxygen deposition pressure and post-deposition annealing temperature. The magnitude of the tetragonal distortion is directly proportional to the dielectric constant of the deposited film. The variations in the dielectric properties of epitaxial BST films deposited on different substrates can be attributed to the differences in film stress caused by the mismatch between the lattice constants and thermal expansion coefficients of the film and substrate. A thin amorphous buffer layer of BST (~50 [Aring]) has been used to relieve film stress. Stress relieved films show improved dielectric behavior for tunable microwave applications

Special Seminar

Phase-Coherent Quantum Phenomena in Charge and Spin Density Waves

Prof. John H. Miller

by: Prof. John H. Miller

Date: Friday September 24, 1999

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

This talk addresses a number of developments that strongly suggest the occurrence of high-temperature, phase-coherent quantum phenomena in charge and spin density waves. First, several experiments demonstrate that the charge density wave (CDW) is not polarized significantly below the threshold electric field for nonlinear transport. This shows that the threshold field is NOT the classical depinning field as commonly believed. An alternative model, based on Coulomb blockade and time-correlated tunneling of density wave solitons is presented. Additional experiments in our lab show the occurrence of a zero-bias resistance anomaly near the boundary between ion implanted and unimplanted regions of a CDW. This zero-bias resistance is found to drop abruptly below a new phase transition temperature equal to about 3/4 of Peierls transition temperature. One interpretation might be a Bose-like condensation of charge solitons. Finally, the talk will discuss the recent observation of Aharonov-Bohm oscillations, of period h/2e, in the magneto-conductance of a CDW containing columnar defects.

Special Seminar

Berry Phase Theory of the Anomalous Hall Effect in Colossal Magneto-Resistance (CMR) Manganites

by: Dr. Jin Wu Ye

Date: Wednesday June 16, 1999

Time: 11:00 am – 12:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

We show that the Anomalous Hall Effect (AHE) observed in Colossal Magnetoresistance Manganites is a manifestation of Berry phase effects caused by carrier hopping in a non-trivial spin background. We determine the magnitude and temperature dependence of AHE, finding that it increases rapidly in magnitude as the temperature is raised from zero through the magnetic transition temperature [Tc], peaks at a temperature Tmax > [Tc] and decays as a power of T, in agreement with experimental data.

Special Seminar

Investigations of Static and Dynamic Structural Distortions and Their Correlation with the Complex Magnetic Behavior in CMR Manganites

by: Dr. Rajeshwar P. Sharma

Date: Friday June 04, 1999

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The role of lattice distortions in the transport property of doped perovskite-type manganites, exhibitting colossal magnetoresistance (CMR), is becoming one of the cutting edge problems in condensed matter research today. Phenomena such as Jahn-Teller distortions, charge and orbital ordering, which strongly influence the phase diagram of these materials, tend to be accompanied by either or both of static and dynamic lattice distortions. Ion channeling is a unique method which provides a direct real space probe of small (< picometer) uncorrelated displacements (static and dynamic) of atoms in single crystalline materials. Using this technique a direct correlation between the dynamic structural distortions and the ferromagnetic transition in CMR doped manganite films as well as in layered double sheet (n=2) perovskite La1.2Sr1.8Mn2O7 (hole concentration x=0.4 and 0.3) systems has been found. The transport properties in these materials depend strongly on the hole concentration and R-site ionic radius. The Nd1.2Sr1.8Mn2O7 (n=2, x=0.4) system has not shown any significant structural anomaly, as in this case the ferromagnetic transition is suppressed by possible charge ordering around 90 K, rendering the material insulating with an antiferromagnetic ordering. It appears that the incoherent atomic displacements have important participation in the transport process and at the onset of the ferromagnetic phase transitions. The importance of dynamic lattice effects in the phenomena of colossal magnetoresistance is discussed.

Special Seminar

Exploring Complex Material Systems Using Continuous Phase-Diagrams and Materials Chips

by: Dr. Xiao-Dong Xiang

Date: Thursday May 27, 1999

Time: 11:00 am – 12:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Conventional approach to mapping phase diagrams or exploring new materials is to make and characterize samples of discrete composition one at a time. In an effort to speed up this process, “integrated materials chips” (IMCs) and “continuous phase diagrams” (CPDs) are fabricated by thin film deposition of elemental precursors through “combinatorial masks” or linear shutters. Followed by proper annealing processes, thousands of distinct compounds or continuous ternary phase-diagrams are formed, in either polycrystalline or more often epitaxial thin film format, on a small (e.g. inch2) substrate. Various physical properties, including electrical impedance, optical and magnetic properties, of these compounds are then mapped using various imaging instruments. We are routinely applying this approach to explore and optimize existing function materials and to study materials phase diagrams. Application areas include exploring superconductors, ferroelectrics/dielectrics, electro-optical, luminescent, piezoelectric and magnetic materials. I will discuss a recent study of spin & charge stripe phases in doped Mott insulators with CPDs. I will show evidence of smetic phase and commensurate static charge orderings in perovskite manganites.

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