When a sinusoidal drive current I0 cos ωt flows in a small coil close to the surface of a superconducting film, third-harmonic voltage V3 cos(3ωt+θ3) is induced in the coil if the film causes a nonlinear response. We have developed an approximate theoretical method yielding the relationships among I0, V3, and θ3, thus providing the scientific basis for a widely used inductive method for measuring the critical current density Jc in large-area superconducting films. Our results show that V3 is near zero when I0 is smaller than a threshold value Ic0∝Jcd, where d is the film thickness. When I0>Ic0, on the other hand, the third-harmonic voltage is expressed as V3 exp(−iθ3)=ωIc0G(I0/Ic0), where G(x) is a scaling function determined by the configuration of the coil. We demonstrate the scaling law of V3/Ic0 vs I0/Ic0 in a YBa2Cu3O7−δ film.
The magnetic field angle dependence of the critical current density J c (H, θ) was measured in epitaxial YBa 2 Cu 3 O 7−δ (YBCO) thin films with strong flux pinning (J c > 25 GA m −2 at 77 K). The YBCO films were classified into two categories: (1) films that showed J c (θ ) peaks around H ab with the shape of a stratovolcano (i.e., like Mount Fuji) and ( 2) films that showed high, broad J c (θ ) peaks centered at H c in addition to less prominent H ab peaks. Transmission electron microscope observations revealed that the films in category 1 contained a high density of very small precipitates, most of which were less than 7 nm, and that the films in category 2 contained a high density of precipitates whose typical diameters ranged from 5 to 25 nm. The J c (H, θ) data were analyzed based on the angular-dependent coherence length ξ(θ) within an anisotropic Ginzburg-Landau approximation. The pinning of the films in category 1 can be described by a direct summation of the core pinning interaction that is due to small point defects whose diameters are less than 2ξ . The high, broad J c (θ ) peaks centered at H c in the films in category 2 were due to a high density of larger precipitates, and they can also be explained by a similar analysis for spherical pinning centers whose diameters are larger than 2ξ .
We report an up-to-4-fold enhancement in the in-magnetic-field critical current density at 77 K of epitaxial YBa 2 Cu 3 O 7 films on CeO 2 -buffered SrTiO 3 substrates by 3-MeV Au 2þ irradiation. This indicates that irradiation using an industrially practical ion beam, which generally has kinetic energy less than 5 MeV, can provide a substantial increase in the in-field current performance of high-temperature superconductor films. Transmission electron microscopy results show that pointlike defects smaller than 6 nm in diameter were created in the films by the irradiation. V C 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4769836]One of the most significant hurdles in commercialization of electric-power devices using high-temperature superconductors (HTSC) is degraded critical-current-density (J c ) in magnetic fields. 1 In 2004, Macmanus-Driscoll et al. addressed this challenge by precipitating BaZrO 3 in YBa 2 Cu 3 O 7 (YBCO) films, 2 which work as artificial pinning centers (APCs) of vortices. 3 However, precise control of the secondary phase in shape 4 and alignment 5 has recently been recognized as vital to maximize J c and requires highly sophisticated synthesis techniques. Post-growth ion irradiation 6-12 is an easier route for such microstructure engineering, since it can control the properties of irradiation defects (size, shape, density, and alignment) by external parameters (ion mass, energy, fluence, and incident angle) without altering the growth conditions of a target material. 8,11 The following two reasons explain mainly why the irradiation approach to APC introduction has not received much attention during the recent development of second-generation HTSC wires, which consist of an epitaxial HTSC film on a metallic tape. The first reason is that previously observed J c improvements were substantial only in the bulk material but not in films, 1 which are a naturally pinning-center-rich form of the material. 13 The second is that measurable improvement in films was attained through the use of extremely high-energy ions (from hundreds MeV to GeV) generated by industrially impractical accelerators. 7,10-12 Indeed, previous ion-irradiation experiments for improving the current properties of HTSC heavily focused on the high-ion-energy range called electronic-stopping regime (>100 MeV), where incident ions lose their kinetic energy by the electronic excitation of target atoms. In that regime, continuous columnar defects can be fabricated along ion tracks 8,9,11 in perfect accordance with a long-accepted "consensus" that a cylindrical APC is most effective for increasing J c . 7,14 Conversely, despite its industrial affinity, ion irradiation in the nuclear-stopping regime (<5 MeV), where ions are decelerated by elastic collisions, has lacked an extensive investigation in the context of in-field-J c enhancement, 15-17 based on an assumption that expected point-like defects 9,11 are not as effective as continuous ones.In this letter, we propose reconsideration of these trends in the study...
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