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Special Seminar

What Problems of Physics and Astrophysics Seem Now to be Especially Important and Interesting?

by: Prof. Vitaly L. Ginzburg

Date: Thursday November 18, 1999

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The author proposes an educational program aimed primarily at widening the scientific horizons of the young generation of physicists. For this purpose, a list of the top problems of the day are given and addressed, although the list is admittedly subjective and unavoidably inexhaustive and limited (one cannot do the impossible).

Special Seminar

Spin Dephasing vs Optical Dephasing

by: Dr. Ming-Wei Wu

Date: Tuesday November 02, 1999

Time: 11:00 am – 12:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

We study the kinetics of spin coherence of optically excited electrons in an undoped insulating ZnSe/Zn1-xCdxSe quantum well under moderate magnetic fields in the Voigt configuration. After clarifying the optical coherence and the spin coherence, we build the kinetic Bloch equations and calculate dephasing and relaxation kinetics of laser-pulse-excited plasma due to statically screened Coulomb scattering and electron hole spin exchange. We find that the Coulomb scattering, which is the main mechanism for the optical dephasing, cannot cause the spin dephasing, and that the electron-hole spin exchange is the main mechanism of the spin decoherence. Moreover, the beat frequency in the Faraday rotation angle is determined mainly by the Zeeman splitting, red shifted by the Coulomb scattering and the electron hole spin exchange. Our numerical results are in agreement with experimental findings. A possible scenario for the contribution of electron-hole spin exchange to the spin dephasing of the n-doped material is also discussed.

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.

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