Specific heat and phase diagrams of single crystal iron pnictide superconductors

We present specific heat measurements on single crystals of the pnictide superconductors NdFeAsO1−xFx and Ba1−xKxFe2As2. Low-temperature measurements on Ba1−xKxFe2As2 reveal that the Sommerfeld coefficient of the quasiparticle specific heat increases linearly with applied magnetic field. This is the signature of a fully gapped superconducting state as is expected for instance in the extended s-wave scenario. The large value of the normal state Sommerfeld coefficient indicates a large electronic density of states and significant mass enhancements. We determine the phase diagram using an entropy conserving construction, which does not require the choice of a resistivity criterion to define the transition temperature. Both materials display clear mean-field steps in the specific heat at Tc of 47 K and 34.6 K, respectively, proving bulk superconductivity. They are characterized by a low superconducting anisotropy near Tc of Γ ∼ 4 for NdFeAsO1−xFx and 2.6 for Ba1−xKxFe2As2, which is promising for potential applications. We observe extraordinarily high upper critical field slopes of μ0∂Hc2c/∂T=-6.5T/K and μ0∂Hc2ab/∂T=-17.4T/K for Ba1−xKxFe2As2 which points to the emergence of paramagnetic limiting effects at low temperatures. A thermodynamic analysis reveals that this material is extreme type-II with κc ∼ 100 and κab ∼ 260.