Specific heat and phase diagrams of single crystal iron pnictide superconductors

Abstract

We present specific heat measurements on single crystals of the pnictide superconductors NdFeAsO 1− x F x and Ba 1− x K x Fe 2As 2. Low-temperature measurements on Ba 1− x K x Fe 2As 2 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 T c of 47 K and 34.6 K, respectively, proving bulk superconductivity. They are characterized by a low superconducting anisotropy near T c of Γ ∼ 4 for NdFeAsO 1− x F x and 2.6 for Ba 1− x K x Fe 2As 2, which is promising for potential applications. We observe extraordinarily high upper critical field slopes of μ 0 ∂ H c 2 c / ∂ T = - 6.5 T / K and μ 0 ∂ H c 2 ab / ∂ T = - 17.4 T / K for Ba 1− x K x Fe 2As 2 which points to the emergence of paramagnetic limiting effects at low temperatures. A thermodynamic analysis reveals that this material is extreme type-II with κ c ∼ 100 and κ ab ∼ 260.