Critical parameters of disordered nanocrystalline superconducting Chevrel-phase PbMo_6S_8

摘要

Highly dense structurally disordered nanocrystalline bulk PbMo_6S_8 samples were fabricated by mechanical milling (MM) and hot isostatic pressing (HIP) at a pressure of 2000 bar and temperature of 800℃ for 8 h. In spite of the lower superconducting transition temperature (T_C~(0.95ρ_N)=12.3 K), nanocrystalline bulk PbMo_6S_8 samples were found to have significantly higher resistivity [ρ_N(16 K) = 680 μΩcm] and upper critical field [B_(C2)~(M=0) (0) = 110T] than conventional samples [T_C~(0.95ρ_N)=15.1 K, ρ_N,(16K)=80 μΩ cm, and B_(C2)~(M=0)(0) = 45 T, respectively; Phys. Rev. Lett. 91, 027002 (2003)]. The microstructural evolution during MM and HIP and the critical current density (J_C) are presented in this paper. J_C of the nanocrystalline bulk samples increased by a factor of more than 3 for high magnetic fields up to 12 T compared to the conventional sample. The scaling analysis is consistent with a grain-boundary pinning mechanism where F_p ≈{[B_(C2)~(J_C=0)(T)]~n/21κ~mμ_0d~*}b~p(1 -b)~q where n～2.35, m～2,p～1/2, q～2, κ is the Ginzburg-Landau constant (calculated from reversible magnetization measurements), and d~* is the grain size (derived from x-ray diffraction analysis). Despite the pinning framework, the underlying science that determines J_C challenges the standard flux pinning paradigm that separates intrinsic and extrinsic properties, since the disorder and micro-structure of these nanocrystalline materials are on a sufficiently short length scale as to increase both the density of (extrinsic) pinning sites and the (intrinsic) upper critical field.