Effect of dislocations and twin boundaries on critical fields in superconducting niobium

The behavior of niobium single crystals containing dislocations or twin boundaries created by plastic deformation at 4.2 K is studied in a magnetic field. An increase is observed in the upper critical field of superconductivity due to an increase in the Ginzburg–Landau parameter for a crystal with dislocations and to the emergence of localized twinning plane superconductivity (TPS) for a crystal with twin boundaries. Considerably higher values of the critical TPS fields in deformation twins in comparison with the twin boundaries in bicrystals {I. N. Khlyustikov and M. S. Khaikin, Zh. Eksp. Teor. Fiz. 89, 1846 (1985) [Sov. Phys. JETP 62, 1065 (1985)]}, as well as a profound effect of heating of strained samples on the TPS critical parameters, can be attributed to the high internal stresses induced in the vicinity of twin boundaries by plastic deformation.