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

Members of the $\mathrm{CaK}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cr}}_x)}₄{\mathrm{As}}₄$ 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 ${\mathrm{CaKFe}}₄{\mathrm{As}}₄$ (${T}_c=35$ 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 $x\ensuremath{\ge}0.038$. The magnetic transition temperature increases in a roughly linear manner as Cr substitution increases. A temperature-composition ($T\ensuremath{-}x$) phase diagram is constructed, revealing a half-dome of superconductivity with the magnetic transition temperature, ${T}^*$, appearing near 22 K for $x\ensuremath{\sim}0.017$ and rising slowly up to 60 K for $x\ensuremath{\sim}0.077$. The $T\ensuremath{-}x$ phase diagrams for $\mathrm{CaK}{({\mathrm{Fe}}_{1\ensuremath{-}x}{T}_x)}₄{\mathrm{As}}₄$ for $T=\text{Cr}$ and Mn are essentially the same despite the nominally different band filling; this is in marked contrast to $T=\text{Co}$ and Ni series for which the $T\ensuremath{-}x$ 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 $\mathrm{CaK}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cr}}_x)}₄{\mathrm{As}}₄$ is also studied as a function of magnetic field. A clear change in ${H}_c2^{\ensuremath{'}}(T)/{T}_c$, where ${H}_c2^{\ensuremath{'}}(T)$ is $d{H}_c2(T)/dT$, at $x\ensuremath{\sim}0.012$ 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.