Superconductivity and phase diagrams of CaK(Fe1−xMnx)4As4 single crystals

Abstract

In order to study the effects of Mn substitution on the superconducting and magnetic ground state of CaKFe4As4 (Tc=35 K), members of the CaK(Fe1−xMnx)4As4 series have been synthesized by high-temperature solution growth in single-crystalline form and characterized by elemental analysis, thermodynamic, and transport measurements. 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 0.036. For x values greater than 0.016, signatures of a magnetic transition can be detected in both thermodynamic and transport measurements in which kinklike features allow for the determination of the transition temperature T* that increases as Mn 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 26 K for x∼0.017 and rising slowly up to 33 K for x∼0.036. In addition to the creation of the T−x phase diagram for CaK(Fe1−xMnx)4As4, specific-heat data are used to track the jump in specific heat at Tc; the CaK(Fe1−xMnx)4As4 data do not follow the scaling of ΔCp with Tc3 as many of the other Fe-based superconducting systems do. These data suggest that, as magnetic pair breaking is present, the jump in Cp for a given Tc is reduced. Elastoresistivity coefficients 2m66 and m11−m12 as a function of temperature are also measured. 2m66 and m11−m12 are qualitatively similar to CaK(Fe1−xNix)4As4. This may indicate that the magnetic order in Mn-substituted system may be still the same as CaK(Fe1−xNix)4As4. Superconductivity of CaK(Fe1−xMnx)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∼0.015 is observed and probably is related to change of the Fermi surface due to magnetic order. Coherence lengths and the London penetration depths are also calculated based on Hc1 and Hc2 data. Coherence lengths as the function of x also show the changes near x=0.015, 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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