F. Yang

Superconducting YBa2Cu3O6.8 films on metallic substrates using in situ laser deposition

In situ laser deposition was used to fabricate YBa2Cu3O6.8 (YBCO) films on metallic substrates. With a buffer layer, YBCO films were strongly c‐axis oriented. Tc = 83 K and Jc= 4000 A/cm2 at 77 K were obtained for a 0.5 μm YBCO film with a 0.15 μm yttrium‐stabilized ZrO2 buffer layer. There was negligible interdiffusion occurring at the interfaces. The n value of YBCO films ranged from 1.4 to 1.5.

Critical current density enhancement in YBa2Cu3O6.8 films on buffered metallic substrates

A transition buffer, consisting of subbuffers grown at different temperatures, was used to enhance the crystallinity and the transport properties of YBa2Cu3O6.8 (YBCO) films on metallic substrates. Films were fabricated using in situ laser deposition and were strongly c‐axis oriented. Critical temperature of 87 K and critical current density of 3×104 A/cm2 at 77 K were obtained for a 0.5‐μm‐thick YBCO film with a 0.2 μm transition yttrium‐ stabilized ZrO2 buffer layer. Also, the magnetic field dependence of Jc could be enhanced by using Pt‐coated metallic substrates.A transition buffer, consisting of subbuffers grown at different temperatures, was used to enhance the crystallinity and the transport properties of YBa2Cu3O6.8 (YBCO) films on metallic substrates. Films were fabricated using in situ laser deposition and were strongly c‐axis oriented. Critical temperature of 87 K and critical current density of 3×104 A/cm2 at 77 K were obtained for a 0.5‐μm‐thick YBCO film with a 0.2 μm transition yttrium‐ stabilized ZrO2 buffer layer. Also, the magnetic field dependence of Jc could be enhanced by using Pt‐coated metallic substrates.

Superconducting YBa sub 2 Cu sub 3 O sub 6. 8 films on metallic substrates using in situ laser deposition

In situ laser deposition was used to fabricate YBa2Cu3O6.8 (YBCO) films on metallic substrates. With a buffer layer, YBCO films were strongly c‐axis oriented. Tc = 83 K and Jc= 4000 A/cm2 at 77 K were obtained for a 0.5 μm YBCO film with a 0.15 μm yttrium‐stabilized ZrO2 buffer layer. There was negligible interdiffusion occurring at the interfaces. The n value of YBCO films ranged from 1.4 to 1.5.

In-plane texturing and its effect on critical current densities of YBa2Cu3O7−x thin films grown on polycrystalline substrates

Abstract It is demonstrated that on polycrystalline substrates, a - b plane textures of c -axis oriented YBa 2 Cu 3 O 7− x (YBCO) thin films can be controlled with biaxially aligned yttria-stabilized zirconia (YSZ) buffer layers grown by ion beam-assisted pulsed laser deposition. An X-ray φ scan measurement shows that the in-plane alignment of the a - (or b -) axes in polycrystalline YBCO thin film is determined by the in-plane texture of YSZ buffer layer. The transport critical current density ( J c ) of the YBCO thin film was found to directly correlate with its degree of in-plane alignment characterized by the full-width at half-maximum (FWHM) of YBCO (103)/(013) φ scan profile. For a YBCO thin film with the FWHM of 27°, J c shows an order of magnitude increase in comparison to YBCO thin films with a random in-plane distribution of a - and b -axes. Using a numerical method to describe the distribution of the a - (or b -) axes of YBCO grains, the misorientation angles of the grain boundaries were determined, and the transport critical current densities of polycrystalline YBCO thin films were calculated. The results are in good agreement with the experimental J c data for samples with different degrees of in-plane alignment. It indicates that the reduction of the number of high-angle grain boundaries is the main reason for the improved J c observed in the biaxially aligned YBCO thin films.

Transport properties of YBa2(Cu0.98M0.02)3Oy(M = Fe, Co, Ni) superconducting thin films

Critical current densities of YBa2(Cu0.98M0.02)3Oy (M = Fe, Co, Ni) superconducting thin films made by laser ablation are investigated as a function of temperature and magnetic field. Jc in magnetic field shows that Ni-doping has a large effect on intragranular current while Fe-doping seems to have more influence on intergranular coupling. All three dopants give rise to basically same field-dependence of their volume pinning force Fp at elevated temperature (T = 60 K) except in the low field region (H < 2 T), and Fp can be scaled to a universal function Fp/Fp.max = b(1−b)2. Hall effect measurements show that the effective carrier concentration is lower than that in undoped material.

Transport properties of silver-doped Y-Ba-Cu-O thin films

Transport properties of dilute silver‐doped Y‐Ba‐Cu‐O thin films prepared by a laser ablation technique have been investigated. The Hall coefficient and critical current density were measured as a function of temperature and in magnetic field up to 1.0 T. Silver doping is shown to result in significant changes in the temperature‐ and magnetic‐field dependence of the transport properties of these films. The Hall coefficient in silver‐doped films shows a clear decrease from that in silver‐free films, consistent with an increase in the hole concentration caused by the silver impurities. This result lends a direct support to the observation that dilute silver dopants behave as acceptors but without appreciable influence on the transition temperature. The magnetic‐field dependence of the normalized critical current density Jc(H)/Jc(0) is enhanced by silver doping, mainly due to a decrease in the effective weak‐link area at the grain boundaries.

Control of the in-plane alignment and the critical current of polycrystalline YBa2Cu3O7−x thin films

Abstract Biaxially aligned yttria-stabilized-zirconia (YSZ) buffer layers are prepared on polycrystalline substrates using ion-beam-assisted laser deposition. The effect of directional ion bombardment on in-plane texruring of YSZ layer is investigated by four-circle X-ray diffractometer. Without the assisting ion beam, uniaxially aligned YSZ layers are obtained at room temperature, exhibiting a preferred orientation of the grains with the [001] axis perpendicular to the substrate surface. With the assisting ion beam, the in-plane texture of YSZ layer is altered as the (110) plane tends to align in the direction of the incident ion beam. The X-ray φ scan of (111) poles shows that the in-plane alignment of the YSZ layer improves as the ion beam voltage increases in the range 200–450 V. The improvement of YSZ in plane alignment is also observed as the layer thickness increases. On the biaxially aligned YSZ buffer layers, superconducting YBa 2 Cu 3 O 7 − x thin films are prepared and their transport critical current densities are found to be directly related to the in-plane alignment of the YBCO thin films. Using a numerical method to model the thin film as a matrix of two dimensional grains, a correlation is established between the line shape of X-ray φ scan and the measured critical current density of polycrystalline YBa 2 Cu 3 O 7 − x thin film.

Weak link dominated anisotropy of critical current density in polycrystalline Y1Ba2Cu3O7−x thin films

The c‐axis oriented Y1Ba2Cu3O7−x thin films with different thicknesses ranging from 1000 to 4000 A have been fabricated on polycrystalline yttria‐stabilized zirconia substrates by pulse laser deposition. The thickness of the film has noticeable influence on the behavior of transport critical current densities at low magnetic field. With the thickness being reduced from 4000 to 1000 A, the Jc(H) anisotropy changes from Jc(H∥c) ≳ Jc(H⊥c) to Jc(H∥c) < Jc(H⊥c)in the region of field less than 2 kOe. It reveals that the geometric nature of grains and grain boundaries which form Josephson weak links determines Jc(H) behavior of polycrystalline Y1Ba2Cu3O7−x thin films in the magnetic field up to several kOe.

Transport measurements of YBa2(Cu0.98M0.02)3Oy (M=Fe,Co,Ni) superconducting thin films

Transport properties of YBa2(Cu0.98M0.02)3Oy (M=Fe, Co, Ni) superconducting thin films are studied and compared with the results measured in doped bulk materials. Critical current density as a function of magnetic field shows that Ni‐doping has a larger effect on intragranular current while Fe‐doping seems to have more influence on the intergranular coupling. All three doped samples show the same field‐dependence on their volume pinning force Fp except in low field region. Fp can be scaled to a universal function Fp/Fp,max=b(1−b)2 indicating that similar pinning mechanism is involved in all these samples. Finally, Hall effect measurements show that the effective carrier concentration is decreased in these doped films.

Texturing of expitaxial Y‐Ba‐Cu‐O thin films on crystalline and polycrystalline substrates

The degree of texturing of the grains in epitaxial Y‐Ba‐Cu‐O films was studied by x‐ray measurements. These high quality laser deposited films were all aligned with the c‐axis perpendicular to the substrate surface, with a spread angle ranging from 0.3° to 4°, depending on the substrate quality. It was found that films on SiTiO3(100) and LaAlO3(100) were totally aligned with the a‐axis either parallel or pependicular to the [100] direction of substrate. On MgO(100) there was a small fraction of the grains in the 〈110〉 direction. For yttria‐stabilized‐zirconia, the a‐axes of the YBCO grains were distributed in the 〈100〉 of the substrate. These observations are consistent with the degree of lattice mismatch in the various directions. For a polycrystalline substrate such as hastelloy, the in‐plane texturing was found to be completely random, consistent with the random nature of the substrate. The correlation of these texturing results with the Jc values was found to be excellent.