We investigate the magnetic phase diagram near the upper critical field of ${\mathrm{KFe}}_2{\mathrm{As}}_2$ by magnetic torque and specific heat experiments using a high-resolution piezorotary positioner to precisely control the parallel alignment of the magnetic field with respect to the FeAs layers. We observe a clear double transition when the field is strictly aligned in the plane and a characteristic upturn of the upper critical field line, which goes far beyond the Pauli limit at 4.8 T. This provides firm evidence that a Fulde-Ferrell-Larkin-Ovchinnikov state exists in this iron-based ${\mathrm{KFe}}_2{\mathrm{As}}_2$ superconductor.

中文翻译：

---

KFe2As2超导体中Fulde-Ferrell-Larkin-Ovchinnikov态的热力学证据

我们研究了磁场的上临界场附近的磁相图 ${\mathrm{钾铁}}_{\mathrm{2个}}{\mathrm{作为}}_{\mathrm{2个}}$通过磁转矩和比热实验，使用高分辨率压电旋转定位器来精确控制磁场相对于FeAs层的平行排列。当场严格在平面上对齐时，我们观察到明显的双重过渡，并且上临界场线具有特征性的向上倾斜，该上升趋势远远超出了4.8 T的Pauli极限。这提供了有力的证据，表明Fulde-Ferrell-Larkin-Ovchinnikov铁基存在${\mathrm{钾铁}}_{\mathrm{2个}}{\mathrm{作为}}_{\mathrm{2个}}$ 超导体。