Superconductivity in amorphous actinide alloys

Superconductivity has been studied in amorphous binary alloys of actinides containing Co, Fe, and Ni. Amorphous thorium alloys have critical temperatures of up to 3.8 K and electronic properties similar to other transition-metal amorphous alloys. By contrast, amorphous uranium alloys have critical temperatures below 1.0 K and exhibit two unusual electronic properties: (1) The temperature coefficient of resistivity is quite large and negative (≲-1.3×${10}^{\mathrm{-}4}$/K) which is generally not found in other low resistivity (<90μΩ cm) amorphous alloys containing transition metals; (2) the upper critical-field gradient is the highest observed in bulk amorphous superconductors, up to 44 kOe/K. Based on the resistivity and critical-field gradient we estimate the renormalized density of states to be two to three times that of 4d and 5d amorphous superconductors. The possibility of strong spin fluctuation in these materials is discussed.