Anisotropic phase diagram and strong coupling effects in Ba 1-x K x Fe 2 As 2

We present a thermodynamic study of the phase diagram of single-crystal ${\\text{Ba}}_{1\\ensuremath{-}x}{\\text{K}}_{x}{\\text{Fe}}_{2}{\\text{As}}_{2}$ using specific-heat measurements. In zero-magnetic field a clear step in the heat capacity of $\\ensuremath{\\Delta}C/{T}_{c}=0.1\\text{ }\\text{J}/\\text{mol}\\text{ }{\\text{K}}^{2}$ is observed at ${T}_{c}\\ensuremath{\\approx}34.6\\text{ }\\text{K}$ for a sample with $x=0.4$. This material is characterized by extraordinarily high slopes of the upper critical field of ${\\ensuremath{\\mu}}_{0}\\ensuremath{\\partial}{H}_{c2}^{c}/\\ensuremath{\\partial}T=\\ensuremath{-}6.5\\text{ }\\text{T}/\\text{K}$ and ${\\ensuremath{\\mu}}_{0}\\ensuremath{\\partial}{H}_{c2}^{ab}/\\ensuremath{\\partial}T=\\ensuremath{-}17.4\\text{ }\\text{T}/\\text{K}$ and a surprisingly low anisotropy of $\\ensuremath{\\Gamma}\\ensuremath{\\sim}2.6$ 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 ${\\ensuremath{\\kappa}}_{c}\\ensuremath{\\sim}100$ and ${\\ensuremath{\\kappa}}_{ab}\\ensuremath{\\sim}260$ identifying ${\\text{Ba}}_{1\\ensuremath{-}x}{\\text{K}}_{x}{\\text{Fe}}_{2}{\\text{As}}_{2}$ as extreme type II. The large value of the normalized discontinuity of the slopes of the specific heat at ${T}_{c}$, $({T}_{c}/\\ensuremath{\\Delta}C)\\ensuremath{\\Delta}{(dC/dT)}_{{T}_{c}}\\ensuremath{\\sim}6$, indicates strong-coupling effects in ${\\text{Ba}}_{1\\ensuremath{-}x}{\\text{K}}_{x}{\\text{Fe}}_{2}{\\text{As}}_{2}$.