Vortex phase diagram in 12442-type RbCa ₂ Fe ₄ As ₄ F ₂ single crystal revealed by magneto-transport and magnetization measurements

Abstract

Abstract The recently discovered 12442-type ACa 2 Fe 4 As 4 F 2 (A = K, Rb, Cs) compounds are the only iron-based superconductors (IBSs) with double FeAs layers between neighboring insulating layers, analogous to the double CuO 2 layers in some high- T c cuprates. Here, we report the study of vortex phase diagram of RbCa 2 Fe 4 As 4 F 2 single crystal via magneto-transport and magnetization measurements. The resistive transition under magnetic fields shows a foot-like kink at a characteristic temperature, T s , followed by a resistive tail in nearly zero resistivity region. Such behavior is ascribed to a vortex slush transition at T s , below which the vortex state has short-range vortex lattice correlation, and then a second-order transition into the vortex glass phase occurs with further decreasing temperature. Above T s , the Arrhenius plot of resistivity shows two linear regions that are separated by a crossover line T cr ( B ), which is associated with a crossover from collective to plastic pinning or different flux pinning behaviors resulted from different types of defect. In addition, the magnetic hysteresis loops reveal a second magnetization peak (SMP), which is shifted to lower fields with increasing temperature for T < 12 K. However, the SMP unexpectedly moves back to a higher field at T = 12 K, and then gradually turns into a shoulder or kink that moves to higher fields at high temperatures, such anomalous behavior has never been observed in IBSs. According to the magneto-transport and magnetization data, the vortex phase diagram of RbCa 2 Fe 4 As 4 F 2 is finally constructed. Details on the different vortex phase transitions and relevant physical scenarios are given and discussed.