The interplay of electron doping and chemical pressure in   Ba(Fe(1-y)Coy)2(As(1-x)Px)2
  The effects of internal chemical pressure on electron doped iron arsenide superconductors are studied in the series Ba(Fe(1-y)Coy)2(As(1-x)Px)2. Combinations of both dopants induce superconductivity also in such areas where only one would not suffice, and can likewise move the system into an overdoped state, while no higher critical temperature than 31 K in BaFe2(As(1-x)Px)2 was found. The phase diagram gives no evidence of holes in BaFe2(As(1-x)Px)2 as suggested by recent photoemission experiments. Chemical and physical pressure act similarly in Ba(Fe(1-y)Coy)2(As(1-x)Px)2, but our data reveal that the most important control parameter is the length of the Fe-As bond and not the unit cell volume. This emphasizes that differences between chemical and physical pressure which manifest oneself as the non-linear reduction of the Fe-As distance in BaFe2(As(1-x)Px)2 are strongly linked to the superconducting properties also in the Co doped compounds. 
Veronika Zinth, Dirk Johrendt
cond-mat.supr-con