Critical parameters of disordered nanocrystalline superconducting Chevrel-phase PbMo6S8 -: art. no. 174503

Highly dense structurally disordered nanocrystalline bulk PbMo6S8 samples were fabricated by mechanical milling (MM) and hot isostatic pressing (HIP) at a pressure of 2000 bar and temperature of 800degreesC for 8 h. In spite of the lower superconducting transition temperature (T-C(N)0.95rho=12.3 K), nanocrystalline bulk PbMo6S8 samples were found to have significantly higher resistivity [rho(N)(16 K)=680 muOmega cm] and upper critical field [B-C2(M=0)(0)=110 T] than conventional samples [T-C(N)0.95rho=15.1 K, rho(N)(16 K)=80 muOmega 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)approximate to{[B-C2(JC=0)(T)](n)/21kappa(m)mu(0)d*}b(p)(1-b)(q) where nsimilar to2.35, msimilar to2, psimilar to 1/2, qsimilar to2, kappa 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 microstructure 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.