Abstract
This overview describes the progressive results of the superconducting critical temperature in bulk nanostructured metals (niobium, vanadium and tantalum) processed by high-pressure torsion (HPT). Bulk nanostructured superconductors provide a new route to control superconducting property, because ultrafine-grain structures with a high density of grain boundaries, dislocations, and other crystalline defects modify the superconducting order parameter. The critical temperature Tc in Nb increases with the evolution of grain refinement owing to the quantum confinement of electrons in ultrafine grains. In V and Ta, however, Tc decreases at a certain HPT revolution number (i.e. at certain strain levels). The different behaviour of Tc in the three materials is explained by the competition effect between the quantum size effect and disorder effect; these effects are characterized by the parameters of grain size, electron mean free path, and superconducting coherence length.
Fig. 3 Critical temperature
Tc as a function of HPT revolution numbers
N for HPT-Nb. Three samples were measured for each
N. The broken line indicates
Tc for
N = 0 and for a single crystal of Nb.
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