TcSUH
Warning: Undefined variable $events_postname in /home1/tcsuh570/public_html/events_select2.php on line 105
Special Seminar
Phase-Coherent Quantum Phenomena in Charge and Spin Density Waves
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.
Back to TcSUH News & Events
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.
Back to TcSUH News & Events
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.
Back to TcSUH News & Events
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.
Back to TcSUH News & Events
Special Seminar
Synthesis of New Organic-Inorganic Molecular Composites - A Route to Novel Functional Materials
Date: Friday April 16, 1999
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
The syntheses of low-dimensional organic-inorganic hybrid materials with novel magnetic, electronic and optical properties represent new directions in solid state chemistry. These are motivated by the notion that complex systems consisting of organic and inorganic components have great potential for the creation of functional materials utilizing the wide variety of properties associated with each component. Due to the limitations presented by differences in synthesis conditions for organic molecules and inorganic solid state materials, formation of crystalline hybrid compounds by self-assembly of molecular and ionic components provides new challenges in chemical synthesis. The preparation of these materials is essentially via self-assembly of the organic and inorganic moieties into crystalline hybrid compounds. Self-assembly techniques take advantage of weak intermolecular interactions to create more complex crystal structures between organic and inorganic moieties while preserving the unique characteristics of the individual components. Studies on crystalline hybrid compounds will allow important structure property relationships among low dimensional self-assembled structures to be established. Our current research focuses on the synthesis and characterization of low dimensional crystalline organic-inorganic metal (Au, Sn, Pb, Bi) iodides with the possibility of incorporating interesting properties associated with each of the organic and inorganic moieties.
Back to TcSUH News & Events