By applying strain to iron-arsenide based superconductors, researchers were able to study the interplay between magnetic states, the tetragonal phase, the orthorhombic phase, and the onset of superconductivity of these materials. The Ca(Fe1-xCox)2As2 series is exceptionally pressure sensitive and crystals of these materials expand differently in each direction as temperature is applied. By rigidly adhering one side of the crystals to silica and taking advantage of the differences in their thermal expansion, in situ measurements of the lattice parameters, resistivity, magnetic susceptibility and Mossbauer spectroscopy could all be performed. During cooling, the basal plane’s contraction is fixed to the silica compressing the basal plane while the c-axis expands with respect to the freestanding state due to the Poisson effect. At the phase transition, some of transformed parts relieve this strain giving a well-defined phase coexistence. The combined structural and transport measurements provide a means to map out the ‘strained’ phase transitions. The gradual magneto-structural transition is shown to be a consequence of applying strain rather than stress. The strain is shown to induce a magneto-structural phase transition in originally paramagnetic samples and superconductivity in previously non-superconducting ones.
Phase diagram of Ca(Fe1-xCox)2As2 in the free (black) and strained (red) state.
Effect of Biaxial Strain on the Phase Transitions of Ca(Fe1-xCox)2As2