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Anisotropic superconductivity and magnetism in single-crystal RbEuFe{sub 4}As{sub 4}.


Smylie, M.; Willa, K.; Bao, J.-K.; Ryan, K.; Islam, Z. ; Claus, H.; Simsek, Y.; Diao, Z.; Rydh, A.; Koshelev, A.; Kwok, W.-K.; Chung, D.; Kanatzidis, M.; Welp, U.


We investigate the anisotropic superconducting and magnetic properties of single-crystal RbEuFe{sub 4}As{sub 4} using magnetotransport and magnetization measurements. We determine a magnetic ordering temperature of the Eu-moments of T_m = 15 K and a superconducting transition temperature of T_c = 36.8 K. The supeconducting phase diagram is characterized by high upper critical field slopes of -70 kG/K and -42 kG/K for in-plane and out-of-plane fields, respectively, and a surprisingly low superconducting anisotropy of Γ = 1.7. Ginzburg-Landau parameters of κ_c∼67 and κ_ab∼108 indicate extreme type-II behavior. These superconducting properties are in line with those commonly seen in optimally doped Fe-based superconductors. In contrast, Eu-magnetism is quasi-two dimensional as evidenced by highly anisotropic in-plane and out-of-plane exchange constants of 0.6 K and < 0.04 K. A consequence of the quasi-2D nature of the Eu-magnetism are strong magnetic fluctuation effects, a large suppression of the magnetic ordering temperature as compared to the Curie-Weiss temperature, and a cusp-like anomaly in the specific heat devoid of any singularity. Magnetization curves reveal a clear magnetic easy-plane anisotropy with in-plane and out-of-plane saturation fields of 2 kG and 4 kG.



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