Andreev Reflection studies on the magnetic superconductor RbEuFe₄As₄

In the Eu containing pnictide RbEuFe₄As₄, superconductivity coexists with helical long-range magnetic order of the Eu²⁺ 4f local moments. The superconducting order in RbEuFe₄As₄ sets in below the critical temperature T_{c ~} 37 K, and deep in the superconducting state, below T_{N ~} 15 K, the magnetic phase emerges. Though the superconducting and the magnetic orders exist simultaneously, whether the two orders are coupled or not is still debated. Our Andreev reflection spectroscopy measurements reveal that there is a significant coupling between the two orders. Deep in the magnetically ordered state, Andreev reflection from superconducting RbEuFe₄As₄ experiences a pronounced anomalous suppression that weakens rapidly as the temperature increases up to T_N. Beyond T_N, the anomaly dies out and conventional Andreev reflection is restored.