Specific heat investigation for line nodes in heavily overdoped B a_{1 x}K_{x}F e_{2}A s_{2}
Abstract
Previous research has found that the pairing symmetry in the ironbased superconductor B a_{1 x}K_{x}F e_{2}A s_{2} changes from nodeless s wave near optimally doped, x ≈0.4 0.55 and T_{c}>30 K , to nodal (either d wave or s wave) at the pure end point, x =1 and T_{c}<4 K . Intense theoretical interest has been focused on this possibility of changing pairing symmetry, where in the transition region both order parameters would be present and timereversal symmetry would be broken. Here we report specific heat measurements in zero and applied magnetic fields down to 0.4 K of three individual single crystals, free of lowtemperature magnetic anomalies, of heavily overdoped B a_{1 x}K_{x}F e_{2}A s_{2} ,x =0 .91 , 0.88, and 0.81. The values for T^{c mid} are 5.6, 7.2, and 13 K and for H_{c 2}≈4.5 , 6, and 20 T, respectively. The data can be analyzed in a twogap scenario, Δ_{2}/Δ_{1}≈4 , with the magnetic field dependence of γ (=C /T as T →0 ) showing an anisotropic "S shaped" behavior vs H , with the suppression of the lower gap by 1 T and γ ≈H^{1 /2} overall. Although such a nonlinear γ vs H is consistent with deep minima or nodes in the gap structure, it is not clear evidence for one, or both, of the gaps being nodal in these overdoped samples. Thus, following the established theoretical analysis of the specific heat of d wave cuprate superconductors containing line nodes, we present the specific heat normalized by H^{1 /2} plotted vs T /H^{1 /2} of these heavily overdoped B a_{1 x}K_{x}F e_{2}A s_{2} samples which—thanks to the absence of magnetic impurities in our sample—convincingly shows the expected scaling for line node behavior for the larger gap for all three compositions. There is, however, no clear observation of the nodal behavior C ∝α T^{2} in zero field at low temperatures, with α ≤2 mJ /mol K^{3} being consistent with the data. This, together with the scaling, leaves open the possibility of extreme anisotropy in a nodeless larger gap, Δ_{2}, such that the scaling works for fields above 0.250.5 T (0.20.4 K in temperature units), where this is an estimate for the size of the deep minima in the Δ_{2}∼20 25 K gap. Therefore, the location of the change from nodeless to nodal gaps between optimally doped and heavily overdoped B a_{1 x}K_{x}F e_{2}A s_{2} based on the present work may be closer to the KF e_{2}A s_{2} end point than x = 0.91 .
 Publication:

Physical Review B
 Pub Date:
 June 2015
 DOI:
 10.1103/PhysRevB.91.214506
 arXiv:
 arXiv:1506.00058
 Bibcode:
 2015PhRvB..91u4506K
 Keywords:

 74.25.Bt;
 74.25.Ha;
 74.70.Xa;
 Thermodynamic properties;
 Magnetic properties;
 Condensed Matter  Superconductivity
 EPrint:
 accepted by Phys. Rev. B