TcSUH
Warning: Undefined variable $events_postname in /home1/tcsuh570/public_html/events_select2.php on line 105
Special Seminar
Large Increases in Jc of Textured HTS via Radiation Damage
Date: Friday July 31, 1998
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Jc is limited by motion of magnetic fluxoids. Pinning these fluxoids in place greatly increases Jc. Successful pinning centers have been produced by chemical, mechanical, and radiation techniques. The radiation methods which have been used include high Z ion beams, protons, neutrons, fusion of 6Li with neutrons followed by 7Li decay, and fission of U by neutrons (the U/n method).
I will address the U/n method and results to date. A detailed study of YBCO is reported, plus introductory studies of NdBCO, SmBCO, and BiSCCO. In YBCO increases in Jc by factors of 15-40 are obtained. In BiSCCO, increases in Jc by a factor of 20 are seen (in the high field regime), the irreversibility is approximately doubled, and the anisotropy is decreased by a factor of 8.
Back to TcSUH News & Events
Special Seminar
Status and Perspectives of HTS Superconducting Electronics
by: Dr. Alex Braginski
Date: Thursday July 30, 1998
Time: 11:00 am – 12:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
The purpose of this talk is to give a realistic assessment of the status of high-temperature superconducting (HTS) electronics R&D and of the market impact of HTS's. Cryogenics, with issues of handling, reliability, and cost, remain the prime obstacle to acceptance by potential users. The advent of HTS's stimulated exaggerated and unrealistic hopes for quick and major developments, including market penetration. Indeed, technical developments have been most impressive and I will highlight the most pertinent of these. However, realistic time scales for complete development cycles (including industrially viable prototypes) can be longer than a decade. For any market, an attractive cost/benefit ratio must be convincingly demonstrated first. Only (1) HTS filter systems integrated with cryocoolers for cellular telecommunication and (2) HTS SQUID magnetometers have been developed to the point of potential or existing commercial availability. Even here, further R&D activity is required, e.g. for better product acceptance. In the case of (1), economic factors will define the market future and size within the next few years, with relatively firm prospects for satellite implementation and good possibilities for terrestrial base stations. In the case of (2), the development of new applications, especially in medical diagnostics, nondestructive materials evaluation, geophysical exploration, etc., might cause growing user acceptance. Prototypes of precision measuring HTS instruments (e.g. programmable voltage standards or D/A converters) might also emerge soon. LTS and HTS digital signal processors and switches might represent the biggest long-term future, e.g. in telecommunication, but only after semiconductor electronics hit their intrinsic physical barriers. Also, the VLSI/ULSI level must first be attained by superconducting digital (e.g. RSFQ) circuit technology. Within the next decade, all of the above and other electronic applications might conquer market niches, some of them sizable.
Back to TcSUH News & Events
Special Seminar
Hydrogen Induced Cleaving of 6H-SiC and Si
by: Dr. O. W. Holland
Date: Tuesday June 02, 1998
Time: 2:00 pm – 3:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
The hydrogen cleaving process can transfer thin layers from a bulk SiC wafer onto an alternate substrate. It consists of H implantation followed by wafer bonding and annealing [1]. The ability to synthesize thin films of SiC on inexpensive substrates will substantially reduce the cost of fabricating SiC-based integrated circuits.
Results describing both the physical and chemical behavior of H implanted into 6H-SiC, as well as a comparison with results in Si, will be presented. Physical characterization was done using RBS channeling, NRA, optical microscopy, and AFM, while IR was used to determine the atomic bonding of the implanted H. Hydrogen and residual damage profiles were determined over an extended range of annealing temperatures.
We found a dose of 3x1016 H/cm2 to be near the minimum dose for forming blisters, the initial indicators of the cleaving process. Blisters first appear after annealing at 850 [deg]C, where significant changes in the H profile are observed. The depth of the craters left after exfoliation is shown to match the range of the implanted hydrogen. While much of the physical characterization indicates a behavior similar to that in Si, scaled with the appropriate temperatures, IR measurements suggest something much different. Si-H bonding, considered to be critical in affecting the cleaving process in Si, is surprisingly not observed in SiC. The reasons for and implications of this observation will be discussed. These results will provide the underpinning for the SiC cleavage process, and its possible application to SiC technology.
[1] L. Di Cioccio et al., Mat. Sci. Eng. B46, 349 (1997).
Back to TcSUH News & Events
Special Seminar
Quantum Fluctuations and Phase Transition in SrTiO3 Thin Films
by: Prof. Xiaoxing Xi
Date: Friday May 15, 1998
Time: 2:00 pm – 3:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Perovskite oxide SrTiO3 is an exceptional material in which quantum fluctuations play a central role. As established by Moller and Burkard, quantum fluctuations suppress the ferroelectric ordering in SrTiO3, leading to a quantum paraelectric ground state at low temperature. The possibility of a coherent quantum state proposed by Moller et al. has prompted numerous theoretical and experimental studies on quantum phase transition in this system. In this talk I will present our measurement of the complex dielectric permittivity as a function of temperature in high-quality SrTiO3 thin films deposited by pulsed laser deposition. We found that a peak in the real part of the dielectric constant and a low temperature loss peak, both previously suggested as indications of a quantum phase transition, showed markedly distinct thickness and electric field dependence. This behavior is qualitatively different from that in the SrTiO3 single crystals, and is consistent with the 2D Ising model in transverse field if one assumes a stronger ferroelectric coupling with respect to the quantum fluctuations in the thin films.
Back to TcSUH News & Events
Special Seminar
Optical Response of Thin Metallic and HTSC Films
by: Prof. Josef Humlicek
Date: Monday May 04, 1998
Time: 11:00 am – 12:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
We discuss the optical (mainly infrared) response of conducting thin films on suitable substrates, such as TiN on silicon and YBa2Cu3O7 on MgO and SrTiO3, focusing on the unique features of the film samples compared to bulk materials. Special emphasis is placed on possible inhomogeneities of the free-carrier polarizability in the growth direction. We also discuss the peculiarities of the highly anisotropic spectra of HTSC films in the TO and LO phonon range of the c-axis response. The emphasized experimental technique is Fourier-transform ellipsometry and polarized reflectance at oblique incidence.
Back to TcSUH News & Events