Electronic specific heat of Ba1-xKxFe2As2 from 2 to 380 K

Abstract

Using a high-resolution differential technique we have determined the electronic specific heat coefficient γ(T)=Cel/T of Ba1-xK xFe2As2 with x=0 to 1.0, at temperatures (T) from 2 K to 380 K and in magnetic fields H=0 to 13 T. In the normal state γn(x,T) increases strongly with x at low temperature, compatible with a mass renormalization ∼12 at x=1, and decreases weakly with x at high temperature. A superconducting transition is seen in all samples from x=0.2 to 1, with transition temperatures and condensation energies peaking sharply at x=0.4. Superconducting coherence lengths ξab∼20 Å and ξc∼3 Å are estimated from an analysis of Gaussian fluctuations. For many dopings we see features in the H and T dependencies of γs(T,H) in the superconducting state that suggest superconducting gaps in three distinct bands. A broad "knee" and a sharp mean-field-like peak are typical of two coupled gaps. However, several samples show a shoulder above the sharp peak with an abrupt onset at Tc,s and a T dependence γs(T)â√1-T/ Tc,s. We provide strong evidence that the shoulder is not due to doping inhomogeneity and suggest it is a distinct gap with an unconventional T dependence Δs(T)â(1-T /Tc,s )0.75 near Tc,s. We estimate band fractions and T=0 gaps from three-band α-model fits to our data and compare the x dependencies of the band fractions with spectroscopic studies of the Fermi surface. © 2013 American Physical Society.