Disordered Nanocrystalline Superconducting ${\mathrm{P}\mathrm{b}\mathrm{M}\mathrm{o}}_6{\mathrm{S}}_8$ 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 ${\mathrm{P}\mathrm{b}\mathrm{M}\mathrm{o}}_6{\mathrm{S}}_8$ superconductors were fabricated by mechanical milling and hot isostatic pressing. Conventional ${\mathrm{P}\mathrm{b}\mathrm{M}\mathrm{o}}_6{\mathrm{S}}_8$ has $B_{C2}(0)\sim 50\mathrm{T}$. The nanocrystalline materials have higher resistivity (${\rho }_N$) and $B_{C2}(0)\sim 100\mathrm{T}$. The disorder produced in these nanocrystalline materials is significantly different from that produced by doping because it increases ${\rho }_N$ 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 $\sim 1\mathrm{n}\mathrm{m}$ which is more than an order of magnitude smaller than the grain size and necessary to achieve the unprecedented increase in $B_{C2}(0)$.

Figure 1

XRD patterns of the conventional sample (sample 1 in Table ), the HIP and annealed samples (see samples 2 and 3 in Table ), and the 200 h milled powder. The temperature and time of the final heat treatment are labeled in the figure.

Figure 2

$M\mathrm{\text{−}}H$ curves measured using a commercial vibrating sample magnetometer at different temperatures and magnetic fields up to 12 T. The backgrounds measured at a temperature above $T_C$ were subtracted from the data. The upper critical field $B_{C2}$ was determined by linearly extrapolating the reversible region to the field where magnetization becomes zero. (a) sample 1, (b) sample 3; the linear extrapolation was performed at fields larger than 9 T for all temperatures except for 10.8 K.

Figure 3

Resistive superconducting transitions measured at magnetic fields up to 15 T. (a) Sample 1; (b) sample 3.