Disordered Nanocrystalline Superconducting PbMo₆S₈ with a Very Large Upper Critical Field

Large increases in the upper critical field B_C2(0) are reported in bulk superconductors that demonstrate another novel property for nanocrystalline materials. Disordered nanocrystalline PbMo₆S₈ superconductors were fabricated by mechanical milling and hot isostatic pressing. Conventional PbMo₆S₈ has B_C2(0)∼50  T. The nanocrystalline materials have higher resistivity (ρ_N) and B_C2(0)∼100  T. The disorder produced in these nanocrystalline materials is significantly different from that produced by doping because it increases ρ_N and, hence, B_C2(0) without significantly reducing the electronic density of states or superconducting transition temperature (T_C). Furthermore, the disorder reduces the electron mean-free path to ∼1   nm which is more than an order of magnitude smaller than the grain size and necessary to achieve the unprecedented increase in B_C2(0).