R. L. S. Emergo

Thickness dependence of superconducting critical current densityin vicinal YBa2Cu3O7−δ thick films

In YBa2Cu3O7−δ (YBCO) thick films, the superconducting critical current density (Jc) decreases with increasing film thickness (t). The mechanisms responsible for this Jc–t behavior remain unclear. To probe the correlation between the film microstructure and Jc-thickness behavior, we have deposited YBCO thick films up to 3.0μm in thickness on flat and surface-miscut (100) SrTiO3 substrates with 5°, 10°, and 15° vicinal angles. The microstructures of the YBCO films were found to evolve differently on flat and miscut substrates, resulting in different Jc–t behaviors. Surprisingly the small miscut angles of 5°–10° were favorable to obtain higher Jc and smaller Jc reduction at larger film thickness.

Control of BaZrO3 nanorod alignment in YBa2Cu3O7−x thin films by microstructural modulation

The alignment of BaZrO3-nanorods (BZO-NRs) in YBa2Cu3O7−x was studied using vicinal substrates to modulate the microstructure. As the vicinal angle was increased to 10°, the angular splay of BZO-NRs increased. Correspondingly, the vortex pinning along the c-axis increased slightly, a possible consequence of enhanced vortex entanglement. Up to 10°, an increasing density of planar defects was observed, while at ∼20° an orthogonal reorientation of the BZO-NRs along the a-b planes occurs. This suggests that the modulated microstructure introduces a competing effect on the BZO-NR formation and alignment, and beyond a critical vicinal angle, c-axis alignment is no longer favorable for BZO-NRs.

Tuning porosity of YBa2Cu3O7−δ vicinal films by insertion of Y2BaCuO5 nanoparticles

High critical current density (Jc) is the most critical specification for high-temperature-superconductor-coated conductors as required by numerous electric power-related applications. This has motivated an intensive research effort on the effects of microstructure on Jc. By growing YBa2Cu3O7−δ (YBCO) films at a small vicinal angle [R. L. S. Emergo, J. Z. Wu, T. Aytug, and D. K. Christen, Appl. Phys. Lett. 85, 618 (2004)] we have recently obtained a highly porous structure in these films accompanied with a significantly enhanced Jc. This result raises a challenging question on whether the porosity can be tailored in YBCO films to allow a higher Jc. In this study, we have explored the insertion of Y2BaCuO5 (211) nanoparticles in vicinal YBCO thick films to alter the strain at the nanometer scale; a nearly doubled pore density was obtained. A further improved Jc as the consequence of the enhanced pore density in these films suggests a direct correlation between microstructure and Jc and projects an even hig...

Eliminating thickness dependence of critical current density in YBa2Cu3O7-x films with aligned BaZrO3 nanorods

The thickness dependence of critical current density (Jc) has been investigated in YBa2Cu3O7−x films with BaZrO3 (BZO) nanorods (YBCO/BZO) aligned along the film normal. With the presence of linear defects in the YBCO/BZO films, the thickness dependence of Jc becomes negligible at self field and weak applied magnetic field (H), suggesting these linear defects playing a key role to the elimination of the thickness dependence of Jc. The effect of temperature (T) and H on Jc thickness dependence have been investigated and comparisons have been made between YBCO and YBCO/BZO samples. Since BZO nanorods alignment is greatly influenced by growth temperature, the quantitative difference in terms of nanorod’s density and average length is reflected on Jc angular dependence of H and Jc thickness dependence as thermally assisted flux motion (TAFM) becomes important above a threshold H determined by the BZO nanorod density. With further increasing H, a monotonically increasing Jc vs. thickness trend was observed in YBCO/BZO films, in contrast to an opposite trend when collective pinning is dominant. This result suggests the thickness dependence of Jc is dictated by the microstructure and hence pinning mechanism in YBCO films.The thickness dependence of critical current density (Jc) has been investigated in YBa2Cu3O7−x films with BaZrO3 (BZO) nanorods (YBCO/BZO) aligned along the film normal. With the presence of linear defects in the YBCO/BZO films, the thickness dependence of Jc becomes negligible at self field and weak applied magnetic field (H), suggesting these linear defects playing a key role to the elimination of the thickness dependence of Jc. The effect of temperature (T) and H on Jc thickness dependence have been investigated and comparisons have been made between YBCO and YBCO/BZO samples. Since BZO nanorods alignment is greatly influenced by growth temperature, the quantitative difference in terms of nanorod’s density and average length is reflected on Jc angular dependence of H and Jc thickness dependence as thermally assisted flux motion (TAFM) becomes important above a threshold H determined by the BZO nanorod density. With further increasing H, a monotonically increasing Jc vs. thickness trend was observed in ...

Strong nanopore pinning enhances Jc in YBa2Cu3O7−δ films

Transport critical current density (Jc) has been studied in YBa2Cu3O7−δ (YBCO) thin films doped with nanopores as pins on magnetic vortices. The density of the nanopores in the range of 5±3 pores/μm2 corresponds to an accommodation field Hm∼4.1–16.6 mT. High Jc up to 8.3 MA/cm2 has been observed on these porous YBCO films at 77 K and self-field. A close correlation between Jc and the magnetic pinning potential of the nanopores has been demonstrated below Hm, suggesting that nanopores are strong pins on the magnetic vortices.

Controlling BaZrO3 nanostructure orientation in YBa2Cu3O films for a three-dimensional pinning landscape

The orientation phase diagram of self-assembled BaZrO3 (BZO) nanostructures in c-oriented YBa2Cu3O (YBCO) films on flat and vicinal SrTiO3 substrates was studied experimentally with different dopant concentrations and vicinal angles and theoretically using a micromechanical model based on the theory of elasticity. The organized BZO nanostructure configuration was found to be tunable, between c-axis to ab-plane alignment, by the dopant concentration in the YBCO film matrix strained via lattice mismatched substrates. The correlation between the local strain caused by the BZO doping and the global strain on the matrix provides a unique approach for controllable growth of dopant nanostructure landscapes. In particular, a mixed phase of the c-axis-aligned nanorods and the ab-plane-aligned planar nanostructures can be obtained, leading to a three-dimensional pinning landscape with single impurity doping and much improved J c in almost all directions of applied magnetic field.

Impact of edge-barrier pinning in superconducting thin films

It has been suggested that edge-barrier pinning might cause the critical current density (Jc) in bridged superconducting films to increase. Subsequent work indicated that this edge-barrier effect does not impact bridges larger than 1 μm. However, we provide a theoretical assessment with supporting experimental data suggesting edge-barrier pinning can significantly enhance Jc for bridges of a few microns or even tens of microns thus skewing any comparisons among institutions. As such, when reporting flux pinning and superconductor processing improvements for Jc comparisons, the width of the sample has to be taken into consideration as is currently done with film thickness.

Fabrication and characterization of two-pole X-band HgBa2CaCu2O6+δ microstrip filters

HgBa2CaCu2O6+δ (Hg-1212) two-pole X-band microstrip filters of 5% 3dB bandwidth have been fabricated and evaluated. The Hg-1212 filters exhibited a 449±3MHz bandwidth and a center frequency of 10.86±0.02GHz below the superconducting temperature Tc of 121K with low insertion loss of 0.70±0.04dB at 110K. This performance represents the best so far achieved in superconductor filters at above 100K. The better performance of the Hg-1212 filter at higher temperatures, as compared to its YBa2Cu3O7 and Cu counterparts at 77K, was attributed mainly to its higher Tc, which makes Hg-1212 a promising alternative material for passive microwave devices.

Anisotropy of the resistivity and critical current density of porous vicinal YBa2Cu3O7−δ films

The anisotropy of electrical transport properties of YBa2Cu3O7−δ (YBCO) films containing high density nanopores grown on 5°, 10° and 20° vicinal substrates was investigated. For porous films in the normal state, the resistivity in the longitudinal direction is comparable to that of reference YBCO films without nanopores. In addition, the ρc/ρab ratios derived from the resistivities in longitudinal and transverse directions of the porous samples fall in the same range as that of twinned single crystal YBCO. For the superconducting state, the pore surfaces provide strong magnetic pins to magnetic vortices, which leads to significantly increased critical current density Jc along the longitudinal direction as compared to that of the reference YBCO films. This strong isotropic pore surface pinning also increased the Jc along the transverse direction as compared to that of nonporous vicinal YBCO films. This result suggests that the effect of nanopores on the electronic structure of YBCO is minimal and the strong pore surface pinning may provide an effective way to enhance Jc after optimization.

Magnetic flux pinning enhancement in HgBa2CaCu2O6+δ films on vicinal substrates

High-temperature superconducting HgBa2CaCu2O6+δ films were fabricated on the vicinal surfaces of 4°-miscut SrTiO3 single crystal substrates in a cation-exchange process, with the purpose to induce additional growth defects via a step-flow growth mode on the miscut substrates. The critical current densities (Jc) of these films were measured using a standard four-probe method with magnetic fields applied perpendicular to the film surface. The vicinal Hg-1212 films indeed showed higher Jc’s in magnetic fields and irreversibility fields (Hirr) than that on 0°-cutSrTiO3. Hirr at 77K is 2.1T for the film grown 0°-cut SrTiO3, and is enhanced to 2.7T for the one grown on 4°-miscut SrTiO3. If optimized so that the shape and the density of the defects can be controlled experimentally, this may be a promising way of improve magnetic flux pinning for many practical applications related to coated conductors.