Abstract
The influence of structural defects on superconducting properties of pure Ta was studied by the positron annihilation lifetime spectroscopy (PALS), magnetic and electrical resistivity measurements. The obtained results provide a clear evidence that the type-I superconductivity in Ta may be converted into type-II by introducing structural defects. In particular, PALS spectra measured for Ta samples indicate that two cold-rolled samples contain a high concentration of defects (mainly dislocations and vacancies associated with them). At the same time, the annealed sample is almost free of any defects. Magnetic results show that both cold-rolled samples exhibit type-II superconductivity while after annealing the Ta sample recovers type-I superconductivity. The estimated Ginzburg-Landau parameter κ from the electrical resistivity data reveals that the annealed Ta sample is the type-I superconductor with κ=0.69(1) which is slightly less value than the critical value of κc=1/2. At the same time, both cold-rolled samples are type-II superconductors with κ≈κc.
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