This research work focuses on the theoretical investigation of the upper critical magnetic field, ; Ginzburg-Landau coherence length, ; and Ginzburg-Landau penetration depth, , for the two-band iron based superconductors , , and LiFeAs. By employing the phenomenological Ginzburg-Landau (GL) equation for the two-band superconductors , , and LiFeAs, we obtained expressions for the upper critical magnetic field, ; GL coherence length, ; and GL penetration depth, , as a function of temperature and the angular dependency of upper critical magnetic field. By using the experimental values in the obtained expressions, phase diagrams of the upper critical magnetic field parallel, , and perpendicular, , to the symmetry axis (-direction) versus temperature are plotted. We also plotted the phase diagrams of the upper critical magnetic field, versus the angle . Similarly, the phase diagrams of the GL coherence length, , and GL penetration depth, , parallel and perpendicular to the symmetry axis versus temperature are drawn for the superconductors mentioned above. Our findings are in agreement with experimental observations.

中文翻译：

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多波段铁基超导体中上临界磁场（HC2）的理论研究

本研究工作侧重于上临界磁场的理论研究；金茨堡-朗道相干长度，和 Ginzburg-Landau 穿透深度，对于双波段铁基超导体，和 LiFeAs。通过使用双波段超导体 、 和 LiFeAs 的现象学 Ginzburg-Landau (GL) 方程，我们获得了上临界磁场的表达式， ；GL 相干长度, ; 和 GL 穿透深度，作为温度和上临界磁场的角度依赖性的函数。通过在获得的表达式中使用实验值，绘制了平行于 和垂直于对称轴（-方向）与温度的上临界磁场的相图。我们还绘制了上临界磁场与角度 的相图。类似地，绘制了上述超导体的 GL 相干长度 、 和 GL 穿透深度 、 平行和垂直于对称轴与温度的相图。我们的发现与实验观察一致。