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Bi-Weekly Seminar

Magnetic-Ordering-Related Phonon and Crystal Field Anomalies in Rare Earth Manganites

Prof. Milko N. Iliev

by: Prof. Milko N. Iliev

Date: Friday October 28, 2005

Time: 1:00 pm – 2:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The complex relationships among the lattice distortions, magnetism, and dielectric and transport properties of rare earth manganites RMnO3 (R = rare earth, Y, Sc) with both orthorhombic and hexagonal structure are attracting increasing interest. The role of structural distortions is widely recognized, but there are only a few studies on their variation with R and how this affects the spin-phonon and electron-phonon coupling. The results of recent experiments on the variations with R of the Raman spectra of orthorhombic RMnO3 (R=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y) will be reported. In this series, with decreasing radius rR of R (R=La to Eu), the magnetic transition temperature TN to A-type antiferromagnetic (A-AFM) ordering of Mn3+ decreases from ~140 K to ~40 K. With further decrease of rR (R=Gd to Ho), however, the magnetic structure below TN changes from A-AFM to an incommensurate antiferromagnetic one (IC-AFM) with sine-wave ordering of the Mn3+ moments. It will be shown that the change of magnetic structure correlates with strong mixing of phonon modes involving in-plane Mn-O stretchings and bendings of MnO6 octahedra. The strong spin-phonon coupling is evidenced by phonon softening at T<TN in A-AFN, but not in IC-AFM manganites.Another promising experimental approach&nash;temperature-dependent crystal field IR spectroscopy–will be discussed and the first results on crystal field anomalies near TN in hexagonal RMnO3 (R = Ho, Er, Tm, Yb) will be reported.

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Special Seminar

X-ray Diffraction from Functional Perovskites

Dr. Wolfgang  Donner

by: Dr. Wolfgang Donner

Date: Friday October 14, 2005

Time: 1:00 pm – 2:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Thin epitaxial oxide films that adopt the perovskite structure display a variety of phenomena such as ferroelectricity, ferromagnetism, metal-insulator transitions or oxygen permeability. Their physical properties are strongly affected by defects such as strain gradients, point and line defects or even the presence of a surface. X-ray diffraction experiments, notably carried out using highly brilliant Synchrotron radiation, are able to shed light on those structure-property relations. The talk will present an overview of diffraction experiments on perovskite films, performed by various groups using Synchrotron radiation.

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Special Seminar

Development of Coated Conductors

by: Dr. Dean Peterson

Date: Friday October 07, 2005

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Commercial manufacture of high temperature superconducting wires with appropriate properties and costs for full implementation in electric power applications ultimately depends on overcoming several technical barriers. Many organizations across the world have demonstrated the great potential for production of coated conductor tapes in long lengths with appropriate superconducting properties for practical power usage. This review will summarize technical progress and remaining barriers with a focus on the Ion Beam Assisted Deposition (IBAD) approach to fabricating coated conductors. The most promising techniques for depositing practical superconducting films at high rates will also be presented. Current plans for using coated conductors in power prototype demonstrations will be summarized. Opportunities for collaborative research and development areas are also to be highlighted.

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Bi-Weekly Seminar

First Experimental Test of the Incorrect Assumption that Continuous Columnar Pinning Centers Produce the Highest Jc in Superconductors

by: Dr. Alberto Gandini

Date: Friday September 30, 2005

Time: 1:00 pm – 2:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The improvement of critical current in HTS by optimization of pinning center morphology has been a crucial area for research since the HTS's discovery. In the past decade it has been stated in numerous papers that the optimum pinning centers are provided by continuous columnar defects. This conventional wisdom has never been questioned, nor experimentally tested. This has led several researchers to believe that the highest Jc could only be achieved by means of continuous columnar defects. Columnar defects have been assumed to provide the highest Jc because theoretically they have been shown to maximize the pinning potential. However, pinning theory completely neglects that, as the pinning center density increases, the current percolation is reduced, and hence Jc decreases. Recently we argued that percolation has a larger effect on Jc than previously expected. We proposed that discontinuous pinning centers, which reduce the loss of current percolation, would result in a higher Jc. An experiment was performed to directly compare continuous and discontinuous pinning, using high-energy ions. We now present the surprising experimental result that, in clear contrast with the conventional belief, Jc for discontinuous pinning is much higher than for continuous. This experiment indicates that the superior percolation achieved by discontinuous pinning outweighs the decrease in pinning potential. Record Jc ~ 300 KA/cm2 at 77 K was achieved in melt-textured YBCO for pinning which was 67% discontinuous. This work stands as the first experimental test of the postulate that continuous columnar pinning centers produce the highest Jc, and shows that the postulate is incorrect.

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Special Seminar

The Shadow of a Carbon Nanotube

Dr. John C. Wolfe

by: Dr. John C. Wolfe

Date: Thursday August 18, 2005

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The features of ion beam proximity lithography include sub-nanometer limits for diffraction, penumbra, and resist scattering. Further, by using atoms, which have essentially the same interaction with resist and mask materials as ions, the technology becomes immune to fixed and mobile charge, either in the mask or on the wafer. Thus, atom beam lithography (ABL) seems ideal for prototyping and later manufacturing nanoscale integrated circuits. The fundamental challenge is to fabricate a stencil mask with atomically smooth sidewalls in a membrane that is thick enough (e.g. 0.5 μm) to stop the atoms. We have shown that a scattering mask can bypass this issue. A thin evaporated film was used to shrink the openings of a conventional stencil mask and, while not thick enough to actually stop the particles, enough were scattered into the walls of the mask to generate very good exposure contrast. This approach gives the possibility of using very thin, potentially self-assembled, scattering layer structures to form complex, high density masks. An important open question is how thick these features need to be for faithful atom beam replication. In this talk, we report the ability of a carbon nanotube, 18 nm in diameter, to cast a well-defined shadow in a broad beam of energetic (30 keV) helium atoms. When imaged in resist and engraved into silicon dioxide, the projected replica retains the natural smoothness of the nanotube and shows, for the first time that ultra-thin, self-assembled structures can be used as masks in nanoscale printing.

Experiments were carried out using a 30 keV atom beam proximity lithography system. Briefly, a beam of helium atoms, generated by charge transfer scattering in the extraction region of a duoplasmatron ion source, drifts through a 10 M long tube, and impinges on a stencil mask where the transmitted beamlets transfer the mask pattern to the substrate. The mask was prepared by sprinkling a dry nanotube powder onto a 3 μm thick silicon stencil mask with 1 μm wide openings. The mask was clamped, with 5 μm thick cleaved mica spacers, to a silicon substrate coated with 44 nm thick thermal SiO2 and 50 nm thick PMMA resist. After exposure, the PMMA was developed and the resist pattern transferred into the oxide to a depth of 37 nm by CHF3-RIE. Atoms incident upon the thick regions of the mask are absorbed while scattering generates image contrast for the nanotube. Experimental data will be presented, showing a 20 nm wide nanotube suspended over an opening in the stencil mask, with its image after resist removal, engraved into oxide. Also shown is linewidth versus exposure dose, normalized to the critical dose, of a different tube, 18 nm in diameter. Since printed and nominal linewidth are generally equal at twice the critical dose, the difference between these values, about 4.6 nm, is a measure of pattern bias, perhaps an artifact of metrology, resist development, and/or etching. After subtracting the bias from the measured data, a threshold development model with a blur of 5 nm (FWHM) describes the experiment reasonably well. Thus, the ultimate resolution may be near 4 nm. This result shows that the edges of a nanotube, just a few atomic layers thick, generate enough scattering to be printed. I will report experiments to better understand the pattern bias issue and to determine the resolution limit.

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