Yuming Lu

Thickness effect of La2Zr2O7 single buffers on metallic substrates using pulsed laser deposition for YBa2Cu3O7−δ-coated conductors

In the present work, the single buffer layers of La2Zr2O7 (LZO) with different thicknesses were prepared using pulsed laser deposition (PLD) on highly textured Ni?5?at.%W tapes for YBa2Cu3O7??-coated conductors. The highly textured LZO layers are obtained on the metallic substrates, making the subsequent epitaxial growth of superconducting YBa2Cu3O7?? layers feasible. The superconducting transition temperature increases as the LZO thickness increases up to 240?nm, achieving a value of Tc of more than 92?K with a narrow transition width less than 1?K. Inductive measurement reveals that the critical current density (Jc) reaches 0.77?MA?cm?2 at 77?K in self-field, showing that the single buffer of LZO is comparable to standard buffer architectures such as CeO2/YSZ/Y2O3 or CeO2/LZO. This implies that the LZO as a single buffer layer is promising for the simplification of the preparation process, leading to cost-effective coated conductors.

Magnetic anisotropy of RFe sub 10 Si sub 2 alloys

Magnetocrystalline anisotropy, Curie temperatures, spin reorientation temperatures, and first‐order magnetization processes (FOMP) are reported for RFe10Si2 compounds with R=Y, Gd, Tb, Dy, Ho, Er, and Lu. The saturation magnetizations of them at 1.5 K are determined. The value of the R‐Fe magnetic‐coupling constant JRFe/k, derived by a mean‐field analysis of the Curie temperatures, is about −10 K. A tentative spin‐phase diagram for the RFe10Si2 series is presented.

Flux pinning of stress-induced magnetic inhomogeneity in the bilayers of YBa2Cu3O7−δ/La0.67Sr0.33MnO3−δ

Elaborately designed bilayers consisting of epitaxial YBa2Cu3O7−δ (YBCO) and La0.67Sr0.33MnO3−δ (LSMO) films were fabricated by pulsed laser deposition with respect to the investigation into magnetic-dependent vortex pinning effect. The improvement in the critical current density and a pronounced upward shift in the superconducting irreversibility line based on magnetotransport measurements are observed when compared to the pure YBCO film, suggesting the enhancement in flux pinning in the studied bilayer. It is believed that the improved flux pinning for YBCO arises from the magnetic inhomogeneity of the underlying LSMO. Magnetization measurements show a nonuniform magnetic state in the LSMO film, most probably being macroscopically phase-separated clusters with ferromagnetic (FM) and anti-FM domains, which can be caused by the epitaxial strain due to the lattice mismatch. Such a magnetic disorder is hardly affected by the external magnetic field, unlike the magnetic disorder induced by the domain structu...

Surface texture and interior residual stress variation induced by thickness of YBa2Cu3O7-δ thin films

YBa2Cu3O7-δ (YBCO) thin films with various thicknesses from 80 nm to 2000 nm are prepared on single crystal SrTiO3 by means of pulsed laser deposition technique. While it is hard for the x-ray diffraction to observe the evolutions in epitaxial orientation and interior lattice structure, the atomic force microscope shows the degraded surface morphologies and coalesced particles which arise from more misoriented grains with increasing thin film thickness. A detailed Raman spectrum investigation reveals that the a-axis grains exist predominately at the top surface of the films as the total thickness of the film increases up to 2000 nm. It is also evident that the Raman peak corresponding to the O2+/O3− mode emerges with the redshift first and then the blueshift as the film thickness increases. These Raman shifts suggest that the tensile stress in the (a, b) plane arising from the lattice mismatch between the epitaxial film and substrate may release gradually with increasing the film thickness, while the addi...

Robust high-temperature magnetic pinning induced by proximity in YBa2Cu3O7−δ/La0.67Sr0.33MnO3 hybrids

An elaborately designed bilayer consisting of superconducting YBa2Cu3O7−δ (YBCO) and ferromagnetic La0.67Sr0.33MnO3−δ (LSMO) was prepared on a single crystal LaAlO3 substrate by pulsed laser deposition (PLD), with a view to understanding the mechanism behind the influence of superconductor/ferromagnet proximity on the critical current density, Jc. The present bilayer system shows significant modifications in Jc, as evidenced by the suppressed decay of its temperature dependence, as well as the crossing behavior of the magnetic field dependence of Jc at high temperatures. This indicates that enhanced flux pinning emerges at high temperatures, and it is believed to arise from the special magnetic inhomogeneity, i.e., the ferromagnet/antiferromagnet clusters caused by phase separation due to the epitaxial stress between LSMO and the substrate.

Paramagnetic effect tuned by magnetic pinning in YBa2Cu3O7−δ/La0.67Sr0.33MnO3 heterostructures

In the present heterostructure of YBa2Cu3O7−δ/La0.67Sr0.33MnO3 (YBCO/LSMO), a paramagnetic moment is observed, increasing monotonically with enhancing applied-field strength during the field-cooled (FC) magnetization process. Such a paramagnetic effect, however, emerges only in those samples where manganite layers are magnetically inhomogeneous, probably arising from the flux compression in the FC process. It is supposed that the flux compression occurs as a result of the strong magnetic pinning, caused by the stress-induced ferromagnetic/antiferromagnetic (FM/AFM) phase separation in the constituent layer of LSMO.

Proximity and reversal magnetoresistance behaviour in the oxide heterostructure of cuprate and manganite

In this paper, the oxide bilayer and trilayer consisting of superconducting cuprate and antiferromagnetic manganite are prepared using the technique of pulsed-laser deposition. Magnetic hysteresis loops at different temperatures and magnetic fields show distinct magnetic performances due to the proximity and interplay between Meissner currents in the YBCO layer as well as the magnetic fields emerging in the antiferromagnetic manganite layer. A giant positive magnetoresistance (MR) for the trilayers is clearly demonstrated in the region of mixed vortex states near the onset temperature of the superconducting transition. It is believed that the observed positive MR should be associated with the thermally activated flux dissipation, while the spin-dependent interface scattering due to the existing proximity effect may play a more significant role.

Cost-effective electrodeposition of an oxide buffer for high-temperature superconductor coated conductors

It is demonstrated that electrodeposition is a promising cost-effective technique to grow oxide buffer on metallic tapes. The resultant layer of CeO2 shows the biaxial texture of FWHM = 5.5° as well as a smooth surface of RMS = 2.0 nm, and YBa2Cu3O7−δ coated conductor with such a buffer exhibits the critical current density of 1.85 MA cm−2 at 77 K. Of more interest is that the CeO2 film thickness reaches a high value of more than 200 nm without any cracks, while it is very hard to achieve a thickness of more than 70 nm in the conventional vapor deposition methods employed.

Enhanced conversion efficiency in dye-sensitized solar cells with nanocomposite photoanodes

Nanocomposite architectures consisting of conductive carbon nanotubes (CNTs) and nanocrystalline TiO2 films are prepared as photoanodes of dye-sensitized solar cells (DSCs). It is observed that both the electron transport time and electron lifetime decrease with CNT addition, implying that its incorporation is effective in enhancing the electro-conductivity of mesoscopic TiO2 contacts. To rectify CNT orientations, a parallel electric field is applied during the sample preparation, which gives rise to a significant enhancement in the energy conversion efficiency of resultant DSCs. Electrochemical impedance spectroscopy clarifies the photoelectrochemical processes, with enhanced contribution to the efficient electron transport, arising from the increased CNT orientation.

Low-Resistance and Strong-Adhesion Soldering of Second-Generation High-Temperature Superconductor Tapes Within a Short Time

An effective soldering process of second-generation coated conductors (CCs) with low joint resistance was presented by using home-made equipment in the present study. The benefit of preprocessing, i.e., polishing, was proved with respect to the reduced joint resistances based on the voltage–current curves. It is revealed that the joint resistance of 10-8 Ω with an overlapping length as short as 2 cm can be achieved, while the joint resistances would further decrease with increasing the overlapping area, in agreement with Ohms law. In the case of the overlapping length of 8 cm, the order of nΩ can be achieved for the resistances of solder joints. The heating time in joint fabrication process can be shortened to 1 min without negative effects on joint resistance, speeding up the soldering process largely. The effectiveness was evidenced further by the hard degradation in Ic through the joint part of the studied tapes and the enhanced adhesion between solder layer and Cu layer according to the cross-section observation. Temperature dependence of the joint specimen resistances suggested that the lattice constant of superconducting layer after jointing may be preferable to attain a higher critical temperature than that of original tape. The fragmentation of the superconducting layer and decohesion of the interface between hastelloy/buffer and ceramic layers, not the Cu-solder interface for the joint by external force, implied that the valid soldering was achieved using the present process.