Thermal expansion of CaFe$_2$As$_2$: effect of annealing and cobalt doping*

Careful choice of Co concentration and annealing/quenching temperature in the Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ series allows for tuning the ground state of the from orthorhombic-antiferromagnetic to superconducting to collapsed tetragonal [1].In this talk temperature-dependent, $c$-axis, thermal expansion measurements on several sets of Co-doped CaFe$_2$As$_2$ single crystals that were subjected to a variety of annealing conditions will be presented. These samples were chosen to cover all salient regions of the 3D $x - T_{anneal} - T$ phase diagram. The thermal expansion signatures of different types of phase transitions observed in these series will be discussed and comparison with the other measurements will be made.\\[4pt] [1] S. Ran, {\it et al.}, PRB {\bf 85}, 224528 (2012).