George H. Lemon

Fabrication and chemical composition of RF magnetron sputtered Tl-Ca-Ba-Cu-O high Tc superconducting thin films

High‐temperature superconducing Tl‐Ca‐Ba‐Cu‐O (TlCaBaCuO) thin films were fabricated by rf magnetron sputtering on strontium titanate (SrTiO3) substrates. Thin films of 0.5–0.7‐μm thickness were deposited by pure argon sputtering from a single composite powder target of Tl2Ca2Ba2Cu3Ox at an rf power of 250 W and a pressure of 5 mTorr. As‐deposited thin films were sintered and annealed in a thallium‐rich ambient to obtain superconductivity with a zero resistance temperature (Tc0) at 107 K. X‐ray diffraction results showed highly c‐axis oriented films with Tl2Ca2Ba2Cu3Ox (2223) and Tl2Ca1Ba2Cu2Ox (2122) phases present. Auger electron spectroscopy survey and depth profiles were performed to determine the compositional uniformity and impurity contents of the thin films. X ray photoelectron spectroscopy high‐resolution spectra were obtained at the surface, in the bulk, and near the interface with the substrate. Our XPS results support two possible mechanisms for the creation of holes in the TlCaBaCuO compound:...

A general purpose cold finger using a vibration‐free mounted He closed‐cycle cryostata)

A method for mounting a He closed‐cycle cryostat which consists of an exchange gas envelope around the cold head to cool an independently supported sample mount as in model DE202 with a DMX‐20 interface from APD Cryogenics, Inc. is described. No detectable vibration of the sample mount is observed as evaluated using 57Fe Mossbauer spectroscopy and a piezoelectric accelerometer. Using a 25 μm thick α‐Fe foil a linewidth of 0.231(3) mm/s at 300 K with the refrigerator on and the same linewidth with the refrigerator off is observed. The easy optical access afforded by such a cold finger makes it an economical general purpose laboratory tool for performing low‐T spectroscopic investigations, such as microwave, optical, γ‐ray, x‐ray and neutron‐scattering measurements. Other applications include electrical transport, SIMS, RBS, and rare‐gas matrix isolation.

Charge transfer in rare earth-trichloride graphite intercalation compounds

Abstract We report the results of 151Eu Mossbauer and optical reflectivity measurements on intercalation compounds formed either by the reaction of graphite with EuCl3, or with EuCl3: AlCl3 mixtures, in the presence of Cl2. Results on graphite flakes and highly-oriented-pyrolytic graphite (HOPG) indicate the presence of a single narrow Mossbauer line characteristic of Eu3+, which we identify with intercalated EuCl3. No evidence for Eu2+ was found in the Mossbauer spectra. Since Eu3+ is the most easily reduced of the rare earth ions, we conclude that no reduction of any rare earth trichlorides takes place under similar conditions of intercalation. The optical reflectance spectrum of the HOPGEuCl3Cl2 exhibits a sharp Drude edge at ∼ 1.2 eV consistent with metallic conduction and a moderate charge transfer. These results taken collectively suggest that the enhanced metallic conductivity in rare earth chloride intercalation compounds is associated with charge transfer from the graphite to the co-intercalated Cl2 forming Cl- and is consistent with measured stoichiometries CnEuCl3+δ, where δ≅0.2.

Elastic properties and Mossbauer spectra of Fe1+xNb3−xSe10

Abstract We have measured 57 Fe Mossbauer spectra, Youngs modulus, and internal friction of Fe 1+x Nb 3−x Se 10 as functions of temperature. Two quadrupole doublets observed in the Mossbauer spectrum at 300K coalesce into one doublet near 170K, suggesting either that the disorder in the cation sites present at room temperature is removed or that there is an ordering of the distorted octahedral Se cages. A broad minimum in modulus is observed at 50K, with a large peak in internal friction below this at 30K. These are attributed to changes in the charge density wave domain configuration caused by the increasing potential of the competing superlattice.

Molecular structure and crystallization behavior of chalcogenide glasses

Isothermal crystallization of ternary (Ge,S,), -X(Sn,S,), glasses is examined over the glass forming range 0 < x < 0.6. Time-temperature-transformation (TIT) curves are obtained using scanning calorimetry. The results show that both nucleation and growth processes dominate crystallization of the glasses in the range 0 < .Y < 0.25, but only growth processes are found to occur once x 2 0.25. The occurrence of this threshold composition correlates quantitatively with changes in glass molecular structure, recently probed by Raman scattering and Mossbauer spectroscopy.

Deposition and patterning of Tl-Ca-Ba-Cu-O superconducting thin films

High temperature superconducting Tl-Ca-Ba-Cu-0 thin films were sputter deposited from a single composite powder target on SrTiO3 and LaAlO3 substrates, in an rf magnetron sputtering system. The as-deposited thin films were sintered and annealed in an excess Tl partial pressure to obtain superconductivity. The superconducting films were characterized by scanning electron microscope (SEM), x-ray diffraction (XRD) and resistance vs temperature (R vs T) measurements. Fine line features as small as 25 jim were obtained in as-deposited thin films using standard photolithography and wet chemical etching in a weak acid. From the XRD and SEM results, the annealed thin films were found to be highly c-axis oriented, with smooth platelets of 2-10 pm in size. The R vs T measurements showed zero resistance at temepratures as high as 107 K on SrTiO3 and 103 K on LaA1O3.

Fabrication and patterning of Tl-Ca-Ba-Cu-O superconducting thin films on LaAlO3 substrates

High critical transition temperature (high Tc) superconducting thin films of Tl-Ca-Ba-Cu-O (TlCaBaCuO) were fabricated on LaAlO3 substrates in an rf magnetron sputtering system. As-deposited thin films were sintered and annealed in a thallium rich ambient to obtain superconductivity with a zero resistance critical temperature (Tc) at 103 K. X-ray diffraction (XRD) results showed that the thin films were highly c-axis oriented with both Tl2Ca2Ba2Cu3O (2223) and Tl2CalBa2Cu2Ox (2122) phases present. A wet chemical etching process was used for patterning the as-deposited TlCaBaCuO thin films. Linewidths as small as 25 m were patterned using standard photolithography and wet chemical etching techniques. A ring resonator designed for a center frequency of 30 GHz has been fabricated.