Combined effects of Sr substitution and pressure on the ground states in   CaFe$_2$As$_2$
  We present a detailed study of the combined effects of Sr substitution and hydrostatic pressure on the ground-state properties of CaFe$_2$As$_2$. Measurements of the electrical resistance and magnetic susceptibility, both at ambient and finite pressure $P$ $\leq$ 2$\,$GPa, were performed on Ca$_{1-x}$Sr$_x$Fe$_2$As$_2$ single crystals grown out of Sn flux. We find that upon Sr substitution the ranges of stability of both the structural-magnetic transition and the transition into the non-magnetic collapsed tetragonal phase are shifted to higher pressure levels with the latter moving at a higher rate. This suggests the possibility of separating the two phase lines, which intersect already at elevated temperatures for $x$ = 0 and low Sr concentration levels. For $x$ = 0.177 we find strong evidence that both phases remain separated down to lowest temperature and that a zero-resistance state emerges in this intermediate pressure window. This observation indicates that Sr-substitution combined with hydrostatic pressure provides another route for stabilizing superconductivity in CaFe$_2$As$_2$. Our results are consistent with the notion that (i) preserving the fluctuations associated with the structural-magnetic transition to low temperatures is vital for superconductivity to form in this material and that (ii) the non-magnetic collapsed tetragonal phase is detrimental for superconductivity. 
S. Kn\"oner, E. Gati, S. K\"ohler, B. Wolf, U. Tutsch, S. Ran, M. S.   Torikachvili, S. L. Bud'ko, P. C. Canfield, and M. Lang
cond-mat.supr-con