Type-II superconductivity in cold-rolled tantalum

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 coldrolled 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 kappa from the electrical resistivity data reveals that the annealed Ta sample is the type-I superconductor with kappa = 0.69(1) which is slightly less value than the critical value of kappa(c) = 1/root 2. At the same time, both cold-rolled samples are type-II superconductors with kappa approximate to kappa(c). (C) 2020 Elsevier B.V. All rights reserved.