Superconductivity and magnetic and transport properties of single-crystalline CaK(Fe1−xCrx)4As4

Abstract

Members of the CaK(Fe1−xCrx)4As4 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 (Tc=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∼0.017 and rising slowly up to 60 K for x∼0.077. The T−x phase diagrams for CaK(Fe1−xTx)4As4 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)4As4 is also studied as a function of magnetic field. A clear change in Hc2′(T)/Tc, where Hc2′(T) is dHc2(T)/dT, at x∼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 Hc1 and Hc2 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.

Source-reported materials — not catalogue approval

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