Anomalous Second Magnetization Peak in 12442-Type RbCa$_2$Fe$_4$As$_4$F$_2$ Superconductors
Xiaolei Yi1,2†, Xiangzhuo Xing1,3†*, Yan Meng1,4, Nan Zhou1,5, Chunlei Wang2, Yue Sun1*, and Zhixiang Shi1*
1School of Physics, Southeast University, Nanjing 211189, China 2College of Physics and Electronic Engineering, Xinyang Normal University, Xinyang 464000, China 3School of Physics and Physical Engineering, Qufu Normal University, Qufu 273165, China 4School of Physical Science and Intelligent Engineering, Jining University, Qufu 273155, China 5Key Laboratory of Materials Physics, Institute of Solid State Physics, HFIPS, Chinese Academy of Sciences, Hefei 230031, China
Abstract：The second magnetization peak (SMP) appears in most superconductors and is crucial for the understanding of vortex physics as well as the application. Although it is well known that the SMP is related to the type and quantity of disorder/defects, the mechanism has not been universally understood. We selected three stoichiometric superconducting RbCa$_2$Fe$_4$As$_4$F$_2$ single crystals with identical superconducting critical temperature $T_{\rm c} \sim 31$ K and similar self-field critical current density $J_{\rm c}$, but with different amounts of disorder/defects, to study the SMP effect. It is found that only the sample S2 with moderate disorder/defects shows significant SMP effect. The evolution of the normalized pinning force density $f_{\rm p}$ demonstrates that the dominant pinning mechanism changes from the weak pinning at low temperatures to strong pinning at high temperatures. The microstructure study for sample S2 reveals some expanded Ca$_2$F$_2$ layers and dislocation defects in RbFe$_2$As$_2$ layers. The normalized magnetic relaxation results indicate that the SMP is strongly associated with the elastic to plastic (E-P) vortex transition. As temperature increases, the SMP gradually evolves into a step-like shape and then becomes a sharp peak near the irreversibility field similar to what is usually observed in low-temperature superconductors. Our findings connect the low field SMP of high-temperature superconductors and the high field peak of low-temperature superconductors, revealing the possible universal origin related to the E-P phase transition.
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2023, Vol. 40 
