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    Effect of laser irradiation on activation energy, irreversibility field and upper critical magnetic field of bulk MgB2 superconductor
    (Springer, 2016) Erdem, O.; Yanmaz, E.
    In this study, the role of laser irradiation on the upper critical field (H-c2), the irreversibility field (H-irr) and the activation energy (U-0) of bulk MgB2 superconductor was first time investigated with the aid of magnetoresistivity measurements conducted at different applied magnetic fields in a range of 0-15 kOe as a function of temperature from 25 to 40 K. For this aim, a disk shaped MgB2 superconductor was produced and cut into two pieces. One of the pieces was irradiated under high vacuum conditions by using an Nd:YVO4 laser. The obtained results showed an increase in the critical temperature (T-c) by about 1 K after irradiation. Also, the values of H-c2(0) were respectively found to be around 143 and 151 kOe while the values of H-irr(0) were about 122 and 130 kOe for the reference and the irradiated samples. Using the thermally activated flux flow model the dependence of U-0 on magnetic field was determined. The maximum U-0 values of 0.43 and 0.83 eV in zero magnetic field were respectively obtained for the reference and the irradiated samples. Also, the magnetic field dependence of the critical current density J(c)(B) values at different temperatures were slightly increased after the irradiation. These results point out that the superconducting properties of bulk MgB2 can be improved via laser irradiation.
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    Effect of laser irradiation on superconducting properties of bulk MgB2 sintered at different temperatures
    (Springer, 2017) Erdem, O.; Yanmaz, E.
    The surfaces of MgB2 bulk samples produced by a pellet/closed tube method at two different sintering temperatures of 650 and 850 A degrees C, after hot pressing at 200 A degrees C, were irradiated with the same irradiation dose by using an Nd:YVO4 laser in order to study the possible potentiality of laser irradiation to improve pinning performances and critical current density of MgB2 superconductor. The measurements showed that the magnetic field dependence of the critical current density values of irradiated sample sintered at 650 A degrees C slightly increased with a narrowing in superconducting transition region as compared to the reference sample sintered at same temperature. However, irradiated sample sintered at 850 A degrees C showed a decrease in pinning performance and similar critical temperature values as compared to the corresponding reference sample. From these results it can be said that the same laser irradiation dose affects superconducting properties of bulk MgB2 in different ways depending on sintering temperature of the superconductor.
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    Enhanced Pinning Properties of Laser-Irradiated Bulk MgB2 Superconductors
    (Springer, 2017) Erdem, O.; Yanmaz, E.
    In order to investigate the effect of laser irradiation on the superconducting properties of MgB2, a pellet-shaped sample was produced and cut into two pieces. The dot-shaped molten patterns, with the distance of 100 A mu m between them, were formed on the surface of one of the pieces by using an Nd:YVO4 laser. The grains of the irradiated sample were significantly grown due to the laser-induced sintering. The modified surface appeared to have dense and compacted grains as a result of increasing interconnectivity among the superconducting grains. The critical current densities at 5 K in a field of 3 T were found to be 5.38 x 10(4) and 8.96 x 10(4) A cm(-2) for the reference and the irradiated samples, respectively. The increment of critical current density for the irradiated sample is presumably connected with the changing of pinning mechanism after the laser irradiation. For the irradiated sample which showed a higher value of critical current density at 5 K under high fields, Delta kappa pinning was obtained as a dominant pinning mechanism, while the normal point pinning was dominant for the reference one. These results point out that the superconducting properties of bulk MgB2 can be changed and improved via laser irradiation in a short time.