Curtis R. Fincher

Perpendicular magnetic anisotropy and magneto‐optical Kerr effect of vapor‐deposited Co/Pt‐layered structures

We prepared by vapor deposition at room temperature thin (500 A) Co/Pt multilayers or layered structures directly onto glass or Si substrates. They show a preferential magnetization perpendicular to the film plane for Co thicknesses below 12 A and a 100% perpendicular remanence for Co thicknesses below 4.5 A. The magnetic anisotropy can be explained by an interface contribution to the anisotropy. We also investigated the magneto‐optical (MO) polar Kerr effect of these multilayers. Because of their excellent magnetic properties and their potentially high oxidation and corrosion resistance, these Co/Pt‐layered structures are very promising candidates for MO recording. The Kerr rotation θk at λ=820 nm for a 35×(4.0 A Co+12.7 A Pt)‐layered structure, which has 100% magnetic remanence, is modest (−0.12°), but the reflectivity R is high (70%), which results in a respectable figure of merit Rθ2k. Furthermore, the Kerr effect increases towards shorter wavelengths and thus favors future higher‐density recording.

Laser ablation and the production of polymer films

The formation of high-quality thin films of polytetrafluoroethylene (PTFE) is important in many applications ranging from material reinforcement to molecular electronics. Laser ablation, a technique widely used to deposit a variety of inorganic materials, can also be used as a simple and highly versatile method for forming thin polymer films. The data presented show that PTFE films can be produced on various supports by the evaporation of a solid PTFE target with a pulsed ultraviolet laser. The composition of the ablation plume suggests that PTFE ablation and subsequent film formation occur by way of a laser-induced pyrolitic decomposition with subsequent repolymerization. The polymer films produced by this method are composed of amorphous and highly crystalline regions, the latter being predominantly in a chain-folded configuration with the molecular axis aligned parallel to the substrate surface.

Contact resistance in organic thin film transistors

This letter reports on the unexpected dependence of contact resistance on the dielectric layer for pentacene thin film transistors with printed organic conducting electrodes. While the intrinsic mobility is weakly reliant on the dielectric, the contact resistance does vary considerably with dielectric layer. We show that while morphological changes are not apparent, contact resistances vary by an order of magnitude. This result suggests that the barrier to charge injection may depend not only on interactions at the complex triple interface but also on the details of the electronic structure at the semiconductor/dielectric interface.

Laser micromanipulation in the mouse embryo: a novel approach to zona drilling

OBJECTIVE To introduce the use of excimer lasers for penetration of the zona pellucida for micromanipulation purposes. DESIGN Cryopreserved two-cell mouse embryos were thawed and exposed to the 248-nm line of a krypton fluoride excimer laser (Lambda Physik EMG 202, Goettingen, Germany) creating a 2 to 4-micron opening in the zona pellucida. SETTING The Laser Ablation Laboratory at DuPont and the in Vitro Fertilization Laboratory at The Medical Center. INTERVENTIONS The embryos were exposed in either phosphate-buffered solution (PBS) or modified human tubal fluid (HTF) with the laser power varying from 1 to 2 J/cm2 and cultured in Hams F-10 medium (GIBCO, Grand Island, NY) with 0.4% bovine serum albumin. MAIN OUTCOME MEASURES The outcome of each experiment was measured by blastocyst formation of laser-exposed embryos as compared with a set of unexposed control embryos handled in a similar fashion. RESULTS Successful laser penetration of the zona pellucida was achieved using the 248-nm line of a krypton fluoride excimer laser. A higher blastocyst formation was found for embryos exposed in PBS. The higher optical absorption of the modified HTF partially inhibited embryo development. The blastocyst statistics increased 2.5-fold times by reducing the exposure of the embryos to ablation by-products. CONCLUSIONS The use of a krypton fluoride excimer laser was introduced as a new method to open the zona pellucida of two-cell mouse embryos without interrupting blastocyst formation.

LASER INDUCED UNZIPPING : A THERMAL ROUTE TO POLYMER ABLATION

The data presented here show that polytetrafluoroethylene (PTFE) and polymethylmethacrylate (PMMA) can be ablated by the evaporation of solid polymer targets with a pulsed ultraviolet laser. In situ measurements of the plume composition show that the products emitted under laser irradiation are primarily monomer and other species resulting from energetic collisions within the plasma. The similarities between the ablative and pyrolisis mass spectra suggest that ablation of PTFE and PMMA occur through a laser induced pyrolitic decomposition.

Scanning tunneling microscopy of the surface topography and surface etching of nanoscale structures on the high‐temperature superconductors

We report on the use of the scanning tunneling microscope (STM) for etching both single crystal and thin film, YBa2Cu3O7−x. Nanoscale features can be generated with the STM by ablation of atoms rastered by the microscope tip. The etching process can be controlled to remove layers of material which are multiples of the c axis (12 A). Various geometric features have been fabricated ranging from fine lines to square etch pits. The STM has also been used to study the growth mechanism and surface topography of thin films of YBa2Cu3O7−x produced by in situ laser deposition (Jc typically 1×106 A cm−2 at 77 K). Step features equal to the c axis of the material can be readily identified which form pinnacles or chip like morphologies at the surface. These surfaces can also be etched with the STM to reveal a more continuous substructure consistent with the high Jc’s observed.

Extraordinary behaviour of the Y1−xPrxBa2Cu3O7−δ system

Investigations of Y1−xMxBa2Cu3O7−δ (M=Ce, Th)c-axis oriented thin film specimens show that the rate of depression ofTc withx is larger for M=Th, than for M=Ce and Pr, and suggest that Ce, like Th, is tetravalent in this compound. Hall effect measurements on Y1−xPrxBa2Cu3O7−δ single crystals reveal aT2 dependence of the cotangent of the Hall angle in the normal state and a “negative Hall anomaly” belowTc in the superconducting state, in agreement with recent reports. Our research shows that the depth, Δ, of the negative Hall signal scales withT/Tc and that the maximum value of Δ decreases linearly withx and vanishes atx≈0.24. Magnetoresistance measurements on Y1−xPrxBa2Cu3O7−δ single crystals indicate that the irreversibility lineH(T*) obeys a universal scaling relation characterized by anm=3/2 power law nearTc, with a crossover to a more rapid temperature dependence of belowT/Tc≈0.6, similar to that observed for polycrystalline specimens.

Light Diffraction Studies of the Banded Texture in Hydroxypropyl Cellulose

Abstract In most liquid crystal polymers when a thin sample is sheared the polymer exhibits a periodic pattern or texture. While this phenomena has been widely noted, a complete and satisfactory explanation is not yet in hand. We present the results of a study of the dynamics of the formation of the banded texture in liquid crystalline polymers after shear. The data were collected by the diffraction of light from the texture. This approach directly provides the key parameters to characterize the texture without the ambiguity that is often involved with microscopy. It was found that the overall nature of the modulation that gives rise to the texture appears to not vary with time except in magnitude. Both thewavelength and the coherence length of the modulation remain fixed. While both shear rate and thickness of the sample have little effect upon the texture, concentration of the polymer has a very large effect.

Fabrication of Tl-Ba-Ca-Cu-O films by annealing rf-sputtered Ba-Ca-Cu-O films in thallium oxide vapors

Thin, superconducting Tl‐Ba‐Ca‐Cu‐O films were grown using a two‐step fabrication process which involved the growth of Ba‐Ca‐Cu‐O films by reactive magnetron sputtering followed by ex situ high‐temperature crystallization and thallination under thallium oxide vapors. Films were thallinated with both Tl2O3 and Tl2Ba2CuO6 as the source for thallium oxide vapors. Highly oriented films were obtained with the c axis perpendicular to the surface of the film. Best films had a Tc (R=0) of 104 K and a critical current density of 5×104 A/cm2 at 77 K.

Macro-structural Deformation of Kevlar® 40 Aramid Fibers

Abstract Kevlar® 49 “pleating” has been monitored, via measurement of the corresponding light diffraction intensity while stressidg a single filament. The structure responds to stress by opening up, with most of the diffracted intensity disappearing at about 60% of the breaking stress. The pleating is restored upon relaxation.