B.W. Kang

Epitaxial growth of HgBa2CaCu2O6+δ thin films on SrTiO3 substrates

Abstract High quality c -axis oriented HgBa 2 CaCu 2 O 6+δ thin films have been grown epitaxially on SrTiO 3 substrates with the a -axis of the film aligned with the (100)-axis of the substrates. The film-substrate interface quality can be greatly improved by using a fast temperature ramping Hg-vapor annealing process (FTRA). Zero-resistance T c values above 120 K are routinely obtained for FTRA processed HgBa 2 CaCu 2 O 6+δ thin films.

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.

Minimization of detrimental effect of air in HgBa2CaCu2O6+δ thin film processing

We have studied the effects of halogenizing HgBa2CaCu2O6+δ (Hg-1212) thin films and have found that the addition of chlorine/fluorine significantly enhances (1) the reproducibility and stability of Hg-1212 thin films, (2) the phase purity of Hg-1212 to over 90%, and (3) the magnetic critical current density (Jc,mag) both at zero field and in-field. A possible mechanism of preventing contamination of precursor materials by chlorine/fluorine doping is proposed.

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.

Lithium-doping-assisted growth of HgBa2Ca2Cu3O8+δ superconducting phase in bulks and thin films

Abstract The superconducting HgBa 2 Ca 2 Cu 3 O 8+ δ (Hg-1223) phase has been successfully formed at temperatures as low as 720°C, by the addition of Li at a level of 0.1 to 0.4 per unit cell, thus effectively lowering the processing temperature by more than 100°C. Data indicate that Li most likely forms a flux which catalyzes the reaction of constituents and the formation of superconducting domains at low temperatures. There are indications that the Li may also help improve the onset of superconductivity by locally perturbing the electronic structure as an interstitial. Applying the process to thin-film fabrication has successfully produced c -oriented film with 73% Hg-1223 phase purity at a processing temperature of 800°C and 0.2 Li doping level.

Effect of sodium doping on the oxygen distribution of Hg-1223 superconductors

Abstract Low-temperature oxygen annealing was performed on Na-doped and undoped Hg-1223 phases. It is argued that Na could perturb the electronic band structure of the material by changing the distribution of oxygen in the Hg–O layer, which then could affect the superconductivity of the material during the annealing process. It has been found that T c can be increased by more than 20 K using an annealing temperature of 300°C for 10 h, while annealing above 350°C leads to loss of superconductivity due to Hg loss. Normal state- ρ tends to decrease with increasing annealing temperatures up to 400°C. In addition, flattening in the normal state- ρ curves of doped samples indicated an increasingly insulating behavior.

Mixed-State Hall Effect in HgBa 2 CaCu 2 O 6+δ Thin Films with Columnar Defects

The mixed state Hall effect has been studied in high quality HgBa2CaCu2O6+δ thin films while the pinning strength is modified by the irraddition of columnar defects. It has been shown that the pinning strength can be significantly improved by columnar defects. The sign reversal in Hall resistivity is found to be nearly independent of pinning while the scaling behavior between Hall resistivity (ρxy) and longitudinal resistivity (ρxx) is influenced by pinning. The exponent β in the scaling law βxy ∼ βxyβ decrease from 1.8 ± 0.1 to 1.5 ± 0.1 as pinning is enhanced. This results suggest that the sign reversal and the scaling are unrelated, and pinning is not the mechanism of the sign reversal.