Anisotropic phase diagram and strong coupling effects in Ba1-xKxFe2As2 from specific-heat measurements

We present a thermodynamic study of the phase diagram of single-crystal Ba1-xKxFe2As2 using specific-heat measurements. In zero-magnetic field a clear step in the heat capacity of Delta C/T-c=0.1 J/mol K-2 is observed at T-c approximate to 34.6 K for a sample with x=0.4. This material is characterized by extraordinarily high slopes of the upper critical field of mu(0)partial derivative H-c2(c)/partial derivative T=-6.5 T/K and mu(0)partial derivative H-c2(ab)/partial derivative T=-17.4 T/K and a surprisingly low anisotropy of Gamma similar to 2.6 near T-c. A consequence of the large field scale is the effective suppression of superconducting fluctuations. Using thermodynamic relations we determine Ginzburg-Landau parameters of kappa(c)similar to 100 and kappa(ab)similar to 260 identifying Ba1-xKxFe2As2 as extreme type II. The large value of the normalized discontinuity of the slopes of the specific heat at T-c, (T-c/Delta C)Delta(dC/dT)(Tc)similar to 6, indicates strong-coupling effects in Ba1-xKxFe2As2.