Superconductivity and magnetic and transport properties of single-crystalline CaK(Fe_1-xCr_x)₄As₄

Members of the CaK(Fe1-xCrx)(4)As-4 series have been synthesized by high-temperature solution growth in single-crystalline form and characterized by x-ray diffraction, elemental analysis, and magnetic and transport measurements. The effects of Cr substitution on the superconducting and magnetic ground states of CaKFe4As4 (T-c = 35 K) have been studied. These measurements show that the superconducting transition temperature decreases monotonically and is finally suppressed below 1.8 K as x is increased from 0 to 0.038. For x-values greater than 0.012, signatures of a magnetic transition can be detected in magnetic measurements with the associated features in the transport measurements becoming detectable for x >= 0.038. The magnetic transition temperature increases in a roughly linear manner as Cr substitution increases. A temperature-composition (T-x) phase diagram is constructed, revealing a half-dome of superconductivity with the magnetic transition temperature, T*, appearing near 22 K for x similar to 0.017 and rising slowly up to 60 K for x similar to 0.077. The T-x phase diagrams for CaK(Fe1-xTx)(4)As-4 for T = Cr and Mn are essentially the same despite the nominally different band filling; this is in marked contrast to T = Co and Ni series for which the T-x diagrams scale by a factor of 2, consistent with the different changes in band filling Co and Ni would produce when replacing Fe. Superconductivity of CaK(Fe1-xCrx)(4)As-4 is also studied as a function of magnetic field. A clear change in H-c2'(T)/T-c, where H-c2'(T) is dH(c2)(T)/dT, at x similar to 0.012 is observed and probably is related to a change of the Fermi surface due to magnetic order. Coherence length and the London penetration depths are also calculated based on H-c1 and H-c2 data. Both of them as a function of x show changes near x = 0.012, again consistent with Fermi surface changes associated with the magnetic ordering seen for higher x-values.