Albert Agcaoili Gapud

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.

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.

Fabrication of c‐oriented HgBa2Ca2Cu3O8+δ superconducting thin films

Superconducting Hg‐based cuprate thin films have been fabricated on (100) SrTiO3 substrate using rf sputtering and post‐Hg‐vapor annealing. These films are dominated by c‐axis‐oriented Hg‐1223 phase as indicated by x‐ray diffraction and SQUID measurements. Using four‐probe technique, the Tc,onset was found to be 130–132 K and Jc was up to 8.5×104 A/cm2 at 77 K and zero field.

Fabrication and physical properties of large-area HgBa2CaCu2O6 superconducting films

Epitaxial c -axis-orientated HgBa2 CaCu2 O6 (Hg-1212) superconducting thin films with their dimensions as large as ½ inch × ½ inch have been recently achieved using a cation-exchange process. Uniformity of the film was studied by slicing samples into eight pieces, on which uniform zero-resistance Tc values above 120 K were observed. In self-field, Jc values of the film range from 14.6 × 106 to 23.7 × 106 A cm-2 at 5 K, 1.77 × 106 to 3.85 × 106 A cm-2 at 77 K and 0.54 × 106 to 1.38 × 106 A cm-2 at 100 K. The scattering of Jc is found to be within 44% across the film and could be smaller with improvement of processing.

Synthesis of Hg-1223 superconductors using a cation-exchange process

Abstract The newly developed cation-exchange process was applied to convert Tl 2 Ba 2 Ca 2 Cu 3 O 10 (Tl-2223) superconductors to HgBa 2 Ca 2 Cu 3 O 8+ δ (Hg-1223) superconductors via Tl–Hg cation-exchange in Hg-vapor at processing temperatures ranging from 700 to 800°C for 6–24 h. Oxygen annealing process at 350°C for 10 h was followed to optimize the content of oxygen in the Hg-1223 samples. The Tl-2223 phase has been successfully converted to Hg-1223 phase with a negligible amount of non-superconducting impurity phase. T c values up to 135 K have been obtained in Hg-1223, which are ∼15 K higher than that of the original Tl-2223 compounds. Since the cation-exchange process is a kinetic process, it is very promising for fabrication of high-quality Hg-1223 superconductor at large scales.

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.

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.

Superconductivity in Lithium- and Sodium-Doped Mercury-Based Cuprates

The effect of Na- and Li-doping in Hg-based cuprates has been studied. It has been found that superconductivity can be achieved in samples containing small amount of Na or Li. While little change is found in the crystalline structure, the formed superconducting phase have the onset Tc typically above 140 K and zero-resistance Tc up to 136K. More interestingly, the processing temperature for the doped Hg-1223 can be reduced significantly to near 700 °C which is promising for thin films growth on many technologically compatible substrates.

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.