Hydrostatic-pressure tuning of magnetic, nonmagnetic and superconducting   states in annealed Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$
  We report on measurements of the magnetic susceptibility and electrical resistance under He-gas pressure on single crystals of Ca(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$. We find that for properly heat-treated crystals with modest Co-concentration, $x$ = 0.028, the salient ground states associated with iron-arsenide superconductors, i.e., orthorhombic/antiferromagnetic (o/afm), superconducting, and nonmagnetic collapsed-tetragonal (cT) states can be accessed all in one sample with reasonably small and truly hydrostatic pressure. This is possible owing to the extreme sensitivity of the o/afm (for $T \leq$ $T_{s,N}$) and superconducting ($T \leq T_c$) states against variation of pressure, disclosing pressure coefficients of d$T_{s,N}$/d$P$ = -(1100 $\pm$ 50) K/GPa and d$T_{c}$/d$P$ = -(60 $\pm$ 3) K/GPa, respectively. Systematic investigations of the various phase transitions and ground states via pressure tuning revealed no coexistence of bulk superconductivity (sc) with the o/afm state which we link to the strongly first-order character of the corresponding structural/magnetic transition in this compound. Our results, together with literature results, indicate that preserving fluctuations associated with the o/afm transition to low enough temperatures is vital for sc to form. 
E. Gati, S. K\"ohler, D. Guterding, B. Wolf, S. Kn\"oner, S. Ran, S.   L. Bud'ko, P. C. Canfield, and M. Lang
cond-mat.supr-con cond-mat.str-el