Anisotropic thermodynamic and transport properties of single-crystalline CaKFe4As4

Single-crystalline, single-phase CaKFe4As4 has been grown out of a high-temperature, quaternary melt. Temperature-dependent measurements of x-ray diffraction, anisotropic electrical resistivity, elastoresistivity, thermoelectric power, Hall effect, magnetization, and specific heat, combined with field-dependent measurements of electrical resistivity and field and pressure-dependent measurements of magnetization indicate that CaKFe4As4 is an ordered, stoichiometric, Fe-based superconductor with a superconducting critical temperature, T-c = 35.0 +/- 0.2 K. Other than superconductivity, there is no indication of any other phase transition for 1.8 K <= T <= 300 K. All of these thermodynamic and transport data reveal striking similarities to those found for optimally or slightly overdoped (Ba1-xKx)Fe2As2, suggesting that stoichiometric CaKFe4As4 is intrinsically close to what is referred to as "optimal-doped" on a generalized, Fe-based superconductor, phase diagram. The anisotropic superconducting upper critical field, H-c2(T), of CaKFe4As4 was determined up to 630 kOe. The anisotropy parameter. (T) = H-c2(perpendicular to)/H-c2(parallel to), for H applied perpendicular and parallel to the c axis, decreases from similar or equal to 2.5 at T-c to similar or equal to 1.5 at 25 K, which can be explained by interplay of paramagnetic pair breaking and orbital effects. The slopes of dH(c2)(parallel to)/dT similar or equal to -44 kOe/K and dH(c2)(perpendicular to)/dT similar or equal to -109 kOe/K at T-c yield an electron mass anisotropy of m(perpendicular to)/m(parallel to) similar or equal to 1/6 and short Ginzburg-Landau coherence lengths xi(parallel to) (0) similar or equal to 5.8 angstrom and xi(parallel to) (0) similar or equal to 14.3 angstrom. The value of H-c2(perpendicular to) (0) can be extrapolated to similar or equal to 920 kOe, well above the BCS paramagnetic limit.