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Special Seminar

Normal and Superconducting Properties of Fermi System with Non-Degenerate Component: A Model for High-[Tc] Cuprates

by: Dr. George A. Levin

Date: Wednesday August 19, 1998

Time: 10:30 am – 11:30 am

Location: Houston Science Center – Building 593 — Room 102

Overview

Many of the anomalies of the normal and superconducting state properties of the cuprates and their evolution with doping, from underdoped to optimally to overdoped regime, can be understood within the framework of a two-component model, one of which is conventional degenerate and the other, non-degenerate. The non-degenerate component appears when the Fermi surface of a conventional band is brought by tuning the stoichiometry in the vicinity of the top of another, submerged below the Fermi level, quasi-two-dimensional band. In the underdoped regime the excitations in the submerged band manifest themselves as a pseudogap, evident in the spin susceptibility, specific heat, tunneling conductances, resistivity, and Hall coefficient. The pseudogap vanishes with increasing doping, but the presence of the submerged band strongly affects the properties of the optimally and overdoped systems as well.

Special Seminar

Large Increases in Jc of Textured HTS via Radiation Damage

Dr. Roy  Weinstein

by: Dr. Roy Weinstein

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.

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.

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).

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.

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