Thermodynamic evidence for pressure-induced bulk superconductivity in the Fe-As pnictide superconductor CaFe ₂As ₂

We report on specific-heat and resistivity experiments performed in parallel in a Bridgman-type pressure cell in order to investigate the nature of pressure-induced superconductivity in the iron pnictide compound CaFe ₂As ₂. The presence of a pronounced specific-heat anomaly at Tc reveals the bulk nature of the superconducting state. The thermodynamic transition temperature differs dramatically from the onset of the resistive transition. Our data indicate that superconductivity occurs in the vicinity of a crystallographic phase transition. We discuss the discrepancy between the two methods as caused by strain-induced superconducting precursors formed above the bulk thermodynamic transition due to the vicinity of the structural instability.