Anomalous Second Magnetization Peak in 12442-Type RbCa_(2)Fe_(4)As_(4)F_(2)Superconductors

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_(c)~31 K and similar self-field critical current density J_(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_(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 Rb Fe_(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 lowtemperature superconductors,revealing the possible universal origin related to the E-P phase transition.

Critical Current Density,Vortex Pinning,and Phase Diagram in the NaCl-Type Superconductors InTe_(1-x)Se_(x)(x=0,0.1,0.2)

Research of vortex properties in type-Ⅱsuperconductors is of great importance for potential applications and fundamental physics.Here,we present a comprehensive study of the critical current density Jc,vortex pinning,and phase diagram of Na Cl-type In Te_(1-x)Se_(x)(x=0,0.1,0.2)superconductors synthesized by high-pressure technique.Our studies reveal that the values of Jc calculated by the Bean model exceed 10^(4)A/cm^(2)in the In Te_(1-x)Se_(x)system,signifying good potential for applications.The magnetic hysteresis loops(MHLs)show an asymmetric characteristic at various degrees,which is associated with the surface barrier.Intriguingly,a rare phenomenon in which the second magnetization peak in the MHLs occurs only in the field-descending branch is detected in In Te_(0.9)Se_(0.1).Such an anomalous behavior has not been observed before and can be described by considering the respective roles of the surface barrier and bulk pinning in the field-ascending and field-descending branches.By analyzing the pinning force density versus reduced field,the pinning mechanisms are studied in detail in the framework of the Dew-Hughes model.Finally,combining the results of resistivity and magnetization measurements,the vortex phase diagrams are constructed and discussed.

Thermally activated flux flow,vortex-glass phase transition and the mixed-state Hall effect in 112-type iron pnictide superconductors

The transport properties in the mixed state of high-quality Ca_(0.8)La_(0.2)Fe_(0.98)Co_(0.02)As_2single crystal,a newly discovered 112-type iron pnictide superconductor,are comprehensively studied by magneto-resistivity measurement.The field-dependent activation energy,U_0,is derived in the framework of thermally activated flux flow(TAFF)theory,yielding a power law dependence U_0～H~αwith a crossover at a magnetic field around 2 T in both H⊥ab and H//ab,which is ascribed to the different pinning mechanisms.Moreover,we have clearly observed the vortex phase transition from vortex-glass to vortex-liquid according to the vortex-glass model,and vortex phase diagrams are constructed for both H⊥ab and H//ab.Finally,the results of mixed-state Hall effect show that no sign reversal of transverse resistivityρ_(xy)(H)is detected,indicating that the Hall component arising from the vortex flow is in theories or experiments previously reported on some high-T_ccuprates.

Intrinsic superconducting transport properties of ultra-thin Fe_(1+y)Te_(0:6)Se_(0:4) microbridges

We investigated the superconducting properties of Fe_(1+y)Te_(0:6)Se_(0:4) single-crystalline microbridges with a width of 4 m and thicknesses ranging from 20.8 to 136.2 nm. The temperature-dependent in-plane resistance of the bridges exhibited a type of metalinsulator transition in the normal state. The critical current density(J_c) of the microbridge with a thickness of 136.2 nm was82.3 kA/cm^2 at 3K and reached 105 kA/cm^2 after extrapolation to T = 0 K. The current versus voltage characteristics of the microbridges showed a Josephson-like behavior with an obvious hysteresis. These results demonstrate the potential application of ultra-thin Fe-based microbridges in superconducting electronic devices such as bolometric detectors.