Effect of nominal substitution of transition metals for excess Fe in   Fe_{1+x}Se superconductor
  Taking cue from the increase in the superconducting transition temperature (Tc) of Fe1+xSe via nominal (2 wt%) substitution of Cr instead of excess Fe, we have now extended our study with nominal substitution (<=5 wt%) with other transition metals (Ni, Co, Fe, Mn, Cr, V and Ti) in place of excess iron. The Tc is found to increase (maximum ~11 K) or get suppressed depending on the substituted transition metal. Our studies indicate that the superconducting transition temperature depends on various parameters like the ionic size of the transition metal, its magnetic moment as well as the amount of hexagonal phase present as impurity. 
Anil K Yadav, Santosh Kumar, Anup V Sanchela, Ajay D Thakur and C V   Tomy
cond-mat.supr-con