Hydrostatic pressure effects on superconducting transition of nanostructured niobium highly strained by high-pressure torsion

Abstract

We study the effects of hydrostatic pressure (HP) compression on the superconducting transition of severely strained Nb samples, whose grain sizes are reduced to the submicrometer level. Engineered granularity by high-pressure torsion (HPT) treatment changes the strength of coupling between submicrometer-scale grains and introduces lattice strain. We attempt to utilize the initially accumulated shear strain in the starting material for increasing the superconducting transition temperature Tc under HP compression. The HP effects on non-strained Nb have already been investigated in the pressure regime over 100 GPa by Struzhkin et al. [Phys. Rev. Lett. 79, 4262 (1997)], and Tc reportedly exhibited an increase from 9.2 to 9.9 K at approximately 10 GPa. (1) Slightly strained Nb in the HPT treatment exhibits the increase in Tc under HP due to the strengthening of the intergrain coupling, so the pressure scale of the pressure response observed by Struzhkin et al. is reduced to approximately one-seventh at the maximum. (2) Prominently strained Nb in the HPT treatment exhibits the increase in Tc under HP due to a reduction in structural symmetry at the unit-cell level: In a Nb sample subjected to HPT (6 GPa, 10 revolutions), Tc exceeds 9.9 K at approximately 2 GPa. According to our first-principle calculations, the reduction in the structural symmetry affords an increase in the density of states at the Fermi energy, thereby yielding a prominent increase in Tc at low pressures.

Hydrostatic Compression Effects on Fifth-Group Element Superconductors V, Nb, and Ta Subjected to High-Pressure Torsion

“…Calculated electronic density of states of the bcc lattice (thin red curves) and the bct ones (thick blue curves). 33) The inset depicts the magnified portion around the Fermi level.…”

“…We have to consider another mechanism for increasing T c for the HPT-Nb (N = 10) materials. According to another work, 33) sharp increase in T c for HPT-Nb (N = 10) using the Apiezon-J grease does not appear in the case of good HP condition using MEW and O 2 . The maximum T c for HPT-Nb (N = 10) using the Apiezon-J oil should be related with the anisotropic contraction of the unit cell.…”

“…Temperature dependence of in-phase ac magnetization mA for HPT-Nb samples with N = 10 using Ap-J as PTM [first run (a), second one (b), and third one (c)] at various pressures 33). The diameter of the sample chamber for the first and second runs was 0.2 mm, and that for the third run was 0.1 mm.…”

“…Pressure dependence of normalized interplanar distance of (110), (200), (211), (220), and (310) planes for ARC (a), AR (b), HPT-Nb (N = 1) (c), and HPT-Nb (N = 10) (d) 33). The broken guide lines in all figures represent the interplanar distance change for ARC (black), AR (blue), and N = 1 (red).…”

“…FC77 was used as PTM in all cases. Pressure dependence of anisotropic lattice deformation using normalized lattice constants a 1 /a cubic , a 2 /a cubic , and a 3 /a cubic for ARC (a), AR (b), HPT-Nb (N = 1) (c), and HPT-Nb (N = 10) (d) 33). a cubic is a lattice constant considering the bcc lattice.…”