William R. Meier

Anisotropic thermodynamic and transport properties of single-crystalline CaKFe4As4

We grew single-crystalline, single-phase CaKFe4As4 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, Tc=35.0±0.2 K. Other than superconductivity, there is no indication of any other phase transition for 1.8K≤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. Furthermore, the anisotropic superconducting upper critical field, Hc2(T), of CaKFe4As4 was determined up to 630 kOe. The anisotropy parameter γ(T)=H

Nodeless multiband superconductivity in stoichiometric single-crystalline CaKFe4As4

⊥\atop{c2}

Magnetic fluctuations and superconducting properties of CaKFe4As4 studied by As75 NMR

/H

X-Ray diffraction on large single crystals using a powder diffractometer

∥\atop{c2}

Influence of multiband sign-changing superconductivity on vortex cores and vortex pinning in stoichiometric high- Tc CaKFe4As4

, for H applied perpendicular and parallel to the c axis, decreases from ≃2.5 at Tc to ≃1.5 at 25 K, which can be explained by interplay of paramagnetic pair breaking and orbital effects. The slopes of dH