Abstract
In fifth-group element superconductors V, Nb, and Ta, the increase in superconducting transition temperature (Tc) was attempted by using both high-pressure torsion (HPT) and additional hydrostatic pressure (HP) compression. The former brings about the grain refinement and strain accumulation in the unit-cell level. The additional compression for severely strained superconductors triggers strengthening intergrain-contact and/or structural deformation in the unit-cell level. The manner of the appearance of the above two effects depends on the kind of elements: First, in V, there is no prominent effect of HPT, comparing to the hydrostatic compression effects on its non-strained material. Next, in Ta, the effect of strengthening intergrain-contact appears at small hydrostatic compression, resulting in temporal increase in Tc. Finally, Nb exhibits prominent increase in Tc by both effects and, in particular, the structural deformation in the unit-cell level promotes the increase in Tc. Thus, the accumulation of residual strain in the level of starting material can be a promising work to manipulate Tc under HP compression.
Fig. 2 Strategy of the present study in the case of Nb.
33) (a) In high-pressure torsion (HPT) for controlling the initial condition, pressurization and revolution are conducted by using two metal anvils. As the revolution number (
N) increases, dislocations are accumulated (a-1), and the grain size largely changes. The small-angle grain boundaries gradually transform to large-angle grain boundaries with increasing
N ((a-2) → (a-3)). Finally due to the balance between generation and annihilation of dislocations, the grain size remains unchanged despite the increase in
N. (b) A few pieces of sample cut from the HPT-processed V, Nb, and Ta are placed in diamond anvil cell (DAC) to conduct the hydrostatic pressure experiment. There, the specimen is pressed in a quasi-hydrostatic pressure condition using the DAC.
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