S. L. Yan

Cation exchange: A scheme for synthesis of mercury-based high-temperature superconducting epitaxial thin films

A cation-exchange process has been developed for growth of high-quality epitaxial thin films of highly volatile mercury-based high-temperature superconductors. By selecting epitaxial precursor matrices of TlyBa2CaCu2Ox (y=1,2) and annealing them in Hg vapor, c-axis-oriented epitaxial HgBa2CaCu2O6+δ thin films with superior quality were formed through Tl and Hg-cation exchange. At 110 K, these films can carry supercurrent density close to 1 MA/cm2 which is nearly an order of magnitude higher than the best reported previously. This cation-exchange technique also provides a general scheme for synthesis of other volatile compounds with predesigned structure and composition.A cation-exchange process has been developed for growth of high-quality epitaxial thin films of highly volatile mercury-based high-temperature superconductors. By selecting epitaxial precursor matrices of TlyBa2CaCu2Ox (y=1,2) and annealing them in Hg vapor, c-axis-oriented epitaxial HgBa2CaCu2O6+δ thin films with superior quality were formed through Tl and Hg-cation exchange. At 110 K, these films can carry supercurrent density close to 1 MA/cm2 which is nearly an order of magnitude higher than the best reported previously. This cation-exchange technique also provides a general scheme for synthesis of other volatile compounds with predesigned structure and composition.

High critical current density in epitaxial HgBa2CaCu2OX thin films

High quality superconducting HgBa2CaCu2Ox (Hg-1212) thin films have been reproducibly fabricated using cation-exchange method. The thin films have pure Hg-1212 phase and have smooth surface morphology. The superconducting transition temperatures of these films are in the range of 120–124 K. The critical current density Jc is up to 3.2×106 A/cm2 at 77 K and drops only by a factor of 2 at 100 K and self field. At 110 K, a Jc of 7.8×105 A/cm2 has been obtained. X-ray diffraction pole figures show that these films are epitaxially grown on LaAlO3(001) substrates, which is consistent with a χmin of 19% obtained using Rutherford backscattering/channeling analysis.

Stability of Hg-based high-Tc superconducting thin films

Abstract Stability of Hg-based superconducting thin films has been studied in air, acetone, alcohol, water, and thermal cycling. T c , J c , and surface microstructures were analyzed as functions of exposure period. In dry air, fully reacted samples did not show degradation for a period of over one year, while samples which were not fully reacted due to air contamination or other reasons degraded within a few weeks. In humid air, samples were totally degraded within a few minutes. It has been found that the films degraded considerably in alcohol and acetone after they were immersed for a few hours, while this degradation was much severe in water. The mechanism of sample degradation has been discussed as well.

Supercurrents in HgBa2CaCu2O6+δ and TlBa2CaCu2O7 epitaxial thin films

The availability of high-quality epitaxial thin films of HgBa2CaCu2O6+δ (Hg-1212) and TlBa2CaCu2O7 (Tl-1212) with high critical current densities (Jc) has made it possible to examine and compare the Jc of these species. Results reveal that the Jc of 1212 species have very similar temperature behavior at low fields, strongly suggesting that the 30 K shift in critical temperature (Tc) induced by the exchange of Hg and Tl in the 1212 structure is due largely to a change in charge carrier density and/or electronic band structure.

Microstructural evolutions in converting epitaxial Tl2Ba2CaCu2Ox thin films to epitaxial HgBa2CaCu2O6+δ thin films

Superconducting HgBa2CaCu2O6+δ (Hg-1212) thin films were obtained from Tl2Ba2CaCu2Ox (Tl-2212) precursor films using a cation–exchange process. In this process, Tl cations on the precursor lattice were thermally excited and then replaced with Hg cations. The mechanism of such an atomic perturbation process has presented an interesting topic in material research. This work investigated the evolution of the crystalline structure and surface morphology of the film during such a conversion. It has been found that the Hg-1212 films may inherit epitaxy and surface morphology from their Tl2Ba2CaCu2Ox precursor films at the low perturbation energy limit. Although the c-axis lattice constant was reduced from 1.48 nm for Tl-2212 to 1.27 nm for Hg-1212 during the cation exchange, the in-plane texture of the film remains. The inverse conversion from Hg-1212 to Tl-2212 has also been investigated. Hg-1212 is found to be energetically preferred so that a perturbation above a threshold is required to convert it back to T...

Response of superconducting characteristics of Hg-based high-Tc thin films to photolithographic processes

The stability of the properties of Hg-based superconducting thin films has been studied during conventional photolithographic patterning processes. The patterning process was divided into individual steps such as photoresist baking, developing and etching in order to observe the effect of each step separately on the superconducting properties of the samples. After baking of the photoresist, a considerable degradation in the superconducting properties was observed, while samples did not show any visible degradation after they were etched with acid solution.

Passivation effects of polymethyl methacrylate on superconducting HgBa2CaCu2Ox+δ thin films

Abstract Polymethyl methacrylate (PMMA) was investigated as a possible protective layer for HgBa2CaCu2Ox+δ superconducting thin films. Microwave surface resistance (Rs) and DC transport properties were measured on both virgin and PMMA-coated samples before and after they were exposed to certain environments including thermal cycling at high and low vacuums, air and water. At high vacuum near 0.5 mTorr, no degradation was observed on samples after undergoing 20 thermal cycles in the temperature range of 90–290 K. In the case of low vacuum ∼200 mTorr, only the coated samples showed stability after six thermal cycles. When exposed to air, the coated sample did not show any sign of degradation, whereas bare sample severely degraded during the same testing period. However, a long-term direct immersion in water could weaken the adhesion between PMMA and the film surface letting water penetrate. Hence, this polymer may be used as a protective layer for Hg-based superconductors against atmosphere-assisted degradation but not against direct water exposure.

Growth of superconducting Hg-1212 very-thin films

High quality epitaxial HgBa/sub 2/CaCu/sub 2/O/sub 6+/spl delta// (Hg-High 1212) films with thickness less than 100 nm have been successfully synthesized using a cation-exchange process. The films show the superconducting transition up to /spl sim/118 K which is close to the intrinsic value of 124 K for the Hg-1212 phase, and critical current densities up to 1.1/spl times/10/sup 7/A/cm/sup 2/ at 5 K, 1.14/spl times/10/sup 6/A/cm/sup 2/ at 77 K and 2.59/spl times/10/sup 5/A/Vcm/sup 2/ at 100 K in zero field.

Superconducting Thallium Oxide and Mercury Oxide Films

This paper reports on a study to convert electrodeposited Tl-oxide films to Hg-1212 using the cation-exchange process. A two-layer electrodeposition process was used to prepare Tl–Ba–Ca–Cu–O films with Tl-1212 and Tl-2212 stoichiometry. Electrodeposited Tl-films were also deposited with Bi and Sr substituted for the Tl and Ba, respectively and also reacted using cation exchange to attempt formation of Hg-1212. The superconducting transition temperature for the ‘‘non-substituted’’ films improved from 97 to 120 K after the cation-exchange process with a critical current density exceeding 2 � 10 6 at 5 K in zero field. The cation-exchange process for the Bi, Sr substituted films appears to be more difficult with no marked improvement in superconducting properties. 2001 Elsevier Science B.V. All rights reserved.