Reversible Tuning of the Collapsed Tetragonal Phase Transition in CaFe2As2 by Separate Control of Chemical Pressure and Electron Doping

Single crystals of Ca(Fe1-xRux)2As2 (0<x<0.065) and Ca1-yLay(Fe0.973Ru0.027)2As2 (0<y<0.2) have been synthesized and studied with respect to their structural, electronic and magnetic properties. The partial substitution of Fe by Ru induces a decrease of the c-axis constant leading for x<0.023 to a suppression of the coupled magnetic and structural (tetragonal to orthorhombic) transitions. At x_cr=0.023 a first order transition to a collapsed tetragonal (CT) phase is found, which behaves like a Fermi liquid and which is stabilized by further increase of x. The absence of superconductivity near x_cr is consistent with truly hydrostatic pressure experiments on undoped CaFe2As2. Starting in the CT regime at x=0.027 we investigate the additional effect of electron doping by partial replacement of Ca by La. Most remarkably, with increasing y the CT phase transition is destabilized and the system is tuned back into a tetragonal ground state at y>0.08. This effect is ascribed to a weakening of interlayer As-As bonds by electron doping. Upon further electron doping filamentary superconductivity with Tc of 41 K at y=0.2 is observed.