TcSUH
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Special Seminar
Measurements of Electromagnetic Properties of Materials at Microwave Frequencies
by: Prof. Jerry Krupka
Date: Friday September 15, 2000
Time: 11:00 am – 12:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
- Transmission line measurements.
- General concepts of measurements by resonance methods.
- Measurements of dielectric and magnetic materials in resonant cavities.
- Dielectric resonator techniques.
- Mode dielectric resonators.
- Split cavities and split post dielectric resonators.
- Measurements of ultra-low loss materials using whispering gallery mode resonators.
- Measurements of anisotropic materials.
- Permeability measurements of gyromagnetic and paramagnetic materials.
- Surface resistance measurements of superconductors
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Special Seminar
Discovery of Superconductivity in Quarternary Borocarbides and Related Studies at TIFR, Bombay
by: Prof. R. Nagarajan
Date: Wednesday August 16, 2000
Time: 4:00 pm – 5:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
After the revolutionary discovery of high Tc oxide superconductors, discovery of superconductivity in quaternary borocarbides in 1993 was a landmark in superconductivity. The talk will describe the discovery made at the Tata Institute of Fundamental Research (TIFR), Bombay; and discuss some of the studies conducted before and after the discovery. A summary of the studies on the rare earth series RNi4B (R = Y, rare earth) which preceded the discovery will be given. In this, mention will be made about the narrow domain wall, as narrow as an atomic layer, behaviour in SmNi4B. After describing the discovery of superconductivity in Y-Ni-B-C which led to the subject of quaternary borocarbides, some of the studies on quaternary borocarbides carried out at TIFR and in collaboration will be given. The discussion will include suppression of superconductivity by non magnetic impurities in the magnetic superconductor DyNi2B2C ([Tc] < TN), suppression of superconductivity in the moderate heavy fermion YbNi2B2C, which was synthesized at TIFR. The talk will conclude with the recent work on magnetic properties of Sm-Ru-B-C system in which a magnetic phase exhibits a very large coercive field of about 15T at 5K.
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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).
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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.
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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
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