Single Crystal Growth and Study of the Ferromagnetic Superconductor RbEuFe₄As₄

RbEuFe₄As₄ exhibits both superconducting order in the FeAs layers and ferromagnetic order in the Eu layers, providing a good platform to study the interaction and microscopic coexistence of these two traditionally incompatible orders. Growing high-quality RbEuFe₄As₄ single crystals is essential to investigate these phenomena at a deeper level. Here we report the successful growth of the RbEuFe₄As₄ single crystals with millimeter-size dimensions using a RbAs flux. The high-quality crystals were characterized via resistivity, magnetization, and heat capacity measurements. Single crystal X-ray diffraction data refinements reveal almost regular FeAs₄ tetrahedra (As1–Fe–As1 = 108.60(7)°, As2–Fe–As2 = 109.01(8)°, and As1–Fe–As2 = 109.81(2)°) in RbEuFe₄As₄, providing structural support for the highest superconducting transition temperature (T_c = 36.8 K) among all known AAeFe₄As₄ (A = K, Rb, Cs; Ae = Ca, Sr, Eu) compounds. Our flux method using RbAs can also be employed to grow other desired transition metal compounds targeting the same crystal structure.