The Effect of Manganese Doping on the Superconducting Transition Temperature and the Upper Critical Field of Tin-Molybdenum

We have measured the superconducting transition temperature, T(,c), and the upper critical field, H(,c2)(T), of manganese-doped SnMo(,6)S(,8)(Mn(,x)Sn(,1-x)Mo(,6)S(,8)). These samples had reproducible superconducting properties. They were characterized by x-ray powder diffractometry, optical microscopy, electron microscopy, and electron microprobe work. The transition temperature decreased slowly with increasing manganese content until x = 0.06. It then increased until x = 0.10, after which it fell slowly again. Manganese would normally have strong pair-breaking effects on the quasiparticles, but our results provide an exception. X-ray Absorbtion Near-Edge Structure (XANES) measurements showed that the manganese exists as Mn('+6) in our compound. Six of its seven valence electrons are transfered to the Mo(,6)S(,8) clusters of the compound. As Mn('+6) has only one valence electron in the 3-d orbital we expect it to be only slightly magnetic. This helps to explain the low rate of T(,c) depression. T(,c) for these compounds is dependent on the number of electrons available to the molybdenum atoms, and it peaks when this number is optimized. The peak in the T(,c) curve is explained by considering the extra electrons transfered to the molybdenum as the manganese doping is increased. H(,c2)(T) curves upward from 0 to 15 kOe and is linear from 15 to 30 kOe. The curvature of the low-field section increases as the manganese doping is raised from x = 0.0 to x = 0.06. Above x = 0.06 the curvature does not change. The slope of the linear section does not vary with manganese content.