M. Malmivirta

Dopant Diameter Dependence of

In YBCO films doped with artificial pinning centers, such as BaZrO₃ nanorods or BaCeO₃ nanodots, the critical current density, J_c(B || c), is usually described with the form J_c ~ B^-α even though the shape of the J_c(B)-curve does not really allow this. In the field region just above the low field plateau, the shape of the Jc(B)-curve (in log-log scale) is rounded and not straight as in undoped films. The exponent α is found to decrease from 0.5 to around 0.2 in BaZrO₃ doped films and to 0.4 in BaCeO₃ doped films. The incompatibility with the curved data and the linear fit has lead to publication of α-parameters which are not comparable to each other due to different fitting limits. In this paper we show that it is better to use the Dew-Hughes pinning force F_p(B) = F_p0(B/B_irr)^p(1 - B/B_irr)^q to describe the field dependence, where α ≈ 1 - p. We also show that the p and the roundness of the curve depend on the diameter of the pinning centers, but not, e.g., temperature or dopant concentration. This is shown from measurements of differently doped thin YBCO films and from large scale Ginzburg-Landau simulations. The result should have been expected since the diameters of the dopants are roughly the same size as the coherence length and it has been shown earlier that pinning centers much smaller than the coherence length lead to α = 0.5 and those much larger lead to α = 1.

J_{\textrm{c}}(B)

The flux pinning properties of the high temperature superconductor YBa2Cu3O7−δ (YBCO) have been conventionally improved by creating both columnar and dot-like pinning centres into the YBCO matrix. To study the effects of differently doped multilayer structures on pinning, several samples consisting of a multiple number of individually BaZrO3 (BZO) and BaCeO3 (BCO) doped YBCO layers were fabricated. In the YBCO matrix, BZO forms columnar and BCO dot-like defects. The multilayer structure improves pinning capability throughout the whole angular range, giving rise to a high critical current density, Jc. However, the BZO doped monolayer reference still has the most isotropic Jc. Even though BZO forms nanorods, in this work the samples with multiple thin layers do not exhibit a c axis peak in the angular dependence of Jc. The angular dependencies and the approximately correct magnitude of Jc were also verified using a molecular dynamics simulation.

in Doped YBCO Films

We measured the resistivity of pulsed-laser-deposited BaCeO3 (BCO)-doped YBCO thin films containing spherical BCO particles in fields up to 30 T. The average diameter of the particles depends on the dopant concentration being below 4 nm in all the samples. Raised values of the upper critical field, Bc2, were observed in all the samples. Additionally, the parameter γ, describing the electron mass anisotropy, decreased from 6.2 in the undoped sample to 3.1 in the 8 wt.% BCO-doped sample. These results can be explained by the increased number of defects decreasing the mean free path of electrons and thus lowering the coherence length, which in turn increases Bc2.

Enhanced flux pinning in YBCO multilayer films with BCO nanodots and segmented BZO nanorods

The angular dependencies of the critical current of BaCeO3 (BCO) doped YBa2Cu3O6+x (YBCO) thin films grown with pulsed laser deposition (PLD) on SrTiO3 (001) were systematically investigated. The BCO concentration in the YBCO matrix was varied between 0-8 wt.%. Transmission electron microscopy confirmed that the 4% BCO containing sample has point-like, partly agglomerated BCO particles with diameter of 2.5 nm. The dopant degrades the crystal quality of YBCO but improves its in-field current carrying performance especially at low temperatures. The BCO addition affects the angular dependence of the critical current by broadening the B||ab-axis peak of YCBO but no c-axis peak is seen, which is contrary to BaZrO3 doped films made by PLD. Although the particles are point-like, it is found that a model featuring only isotropic pinning centers is not sufficient to describe the angular dependence of the samples where B||ab-axis of YCBO.

Dirty limit scattering behind the decreased anisotropy of doped YBa2Cu3O7-δ thin films

The angular dependencies of the critical current of BaCeO3 (BCO) doped YBa2Cu3O6+x (YBCO) thin films grown with pulsed laser deposition (PLD) on SrTiO3 (001) were systematically investigated. The BCO concentration in the YBCO matrix was varied between 0-8 wt.%. Transmission electron microscopy confirmed that the 4% BCO containing sample has point-like, partly agglomerated BCO particles with diameter of 2.5 nm. The dopant degrades the crystal quality of YBCO but improves its in-field current carrying performance especially at low temperatures. The BCO addition affects the angular dependence of the critical current by broadening the B||ab-axis peak of YCBO but no c-axis peak is seen, which is contrary to BaZrO3 doped films made by PLD. Although the particles are point-like, it is found that a model featuring only isotropic pinning centers is not sufficient to describe the angular dependence of the samples where B||ab-axis of YCBO.

The angular dependence of the critical current of BaCeO3 doped YBa2Cu3O6+x thin films

The angular dependencies of the critical current of BaCeO3 (BCO) doped YBa2Cu3O6+x (YBCO) thin films grown with pulsed laser deposition (PLD) on SrTiO3 (001) were systematically investigated. The BCO concentration in the YBCO matrix was varied between 0-8 wt.%. Transmission electron microscopy confirmed that the 4% BCO containing sample has point-like, partly agglomerated BCO particles with diameter of 2.5 nm. The dopant degrades the crystal quality of YBCO but improves its in-field current carrying performance especially at low temperatures. The BCO addition affects the angular dependence of the critical current by broadening the B||ab-axis peak of YCBO but no c-axis peak is seen, which is contrary to BaZrO3 doped films made by PLD. Although the particles are point-like, it is found that a model featuring only isotropic pinning centers is not sufficient to describe the angular dependence of the samples where B||ab-axis of YCBO.

The Angular Dependence of the Critical Current of

The effect of BaZrO3 (BZO) dopant concentration (0-9 wt-%) on magnetically obtained irreversibility field Birr and upper critical field Bc2 is investigated in YBa2Cu3O6+x (YBCO) thin films prepared by pulsed laser deposition on SrTiO3 (001) (STO) substrates. It is found that the magnetic determination of Birr and Bc2 works well. The results show that BZO-doping increases Bc2 in all concentrations, implying a change in the coherence length and band structure of the superconductor.

\hbox{BaCeO}_{3}

The effect of BaZrO3 (BZO) concentration on Birr and Bc2 in YBa2Cu3O6+x (YBCO) thin films was investigated with resistive measurements. It was found that in field out-of-plane configuration both Birr and Bc2 decrease with the highest BZO concentration of 9 wt% whereas the other samples show no significant change. The angular dependence of Birr of the highest BZO concentration sample has a pronounced c-axis peak that is typical for BZO-doped YBCO but the others do not exhibit that clear peak. On the other hand, Bc2 obeys the Blatter scaling law and the Blatter scaling parameter γ levels off to 3 − 3.5 above few wt% of BZO.

Doped

The effect of BaCeO3 (BCO) dopant concentration on magnetically obtained irreversibility field Birr and upper critical field Bc2 is investigated in YBa2Cu3O6+x (YBCO) thin films prepared by pulsed laser deposition on SrTiO3 (001) substrates. The unchanged values of Birr and Bc2 are connected to the structural properties where BCO has been observed to form non-elongated and randomly distributed particles within the whole concentration range of 0 – 10 wt.%. This indicates a different type of flux pinning mechanism for BCO-doped YBCO than for the widely studied BaZrO3-doped YBCO where increased Birr and Bc2 values were observed.