Study On Phase Transitions With External Magnetic Field In Two-Band S-Wave Iron-Based Superconductors

In 2008,Japanese Hosono's group discovered the superconducting F-doped LaFeAsO(1111)compound with a phase-transition temperature of 26K,which raised a boom of the scientific research on iron-based superconductivity.For Fe-based high temperature superconductors the critical fields are higher than that of copper oxide compounds,and the co-existence of superconductivity and magnetism makes them of great value in the exploration of high temperature superconducting mechanism and the spintronic applications.Besides the iron-based 1111 systems mentioned above,122,11,111 systems and even more complex superconductor Sr₄Sc₂O₆Fe₂As₂ have also been fabricated.In 2020,Hardy et al.experimentally studied the behavior of typical 11-type two-band superconductor FeSe with strong magnetic field.In a certain range of magnetic field and temperature,assuming a second-order phase transition from the Fulde-Ferrell-Larkin-Ovchinnikov(FFLO)state with spatially modulated order parameter to normal metal,the experimental data of the upper critical field can be better understood.It indirectly indicates that the gapless superconducting Sarma phase is likely to occur between the FFLO and the vortex mixed states.At first,we study the Sarma phase of quasi-two-dimensional two-band s-wave superconductors in external magnetic field based on the two-band Bardeen-Cooper-Schrieffer theory.According to the difference of free energy between the normal state and superconducting state,we can see that the large asymmetry between the two gaps and the non-zero interband exchange interaction in two-band s-wave superconductors are the important conditions for the stability of Sarma phase.Secondly,with the two-band Ginzburg-Landau theory we discuss the upper critical field and related magnetic phase-transition properties of the iron-based superconductor FeSe.All the numerical computations on the upper critical field and London penetration depth are in good agreement with the experimental data over a broad temperature range.Our results thus indicate that FeSe is a two-gap s-wave superconductor.The calculations also show that the anisotropy of effective masses in the band with large(or smaller)gap is about 10(or 2).