RbFe₂As₂ has the 3d^5.5 electronic configuration, which corresponds to the most heavily hole-doped system among 122-type iron-based superconductors. In this system, a large mass enhancement has been observed, and the importance of the vicinity to the Mott insulator phase at 3d⁵ configuration has been suggested [1]. Recently, we have observed nematic fluctuations along the Fe-As direction, 45° tilted from the nematic direction usually found in other iron-based superconductors. Although the two-fold symmetry along the Fe-As direction has also been suggested from STM measurements [2], the bulk evidence for nematic ordering has not been reported. Here, we perform field-angle-resolved specific heat measurements in RbFe₂As₂ single crystals by using a high-resolution long-relaxation calorimeter. We choose tiny crystals of about 10 μg to avoid cancellation effect of multiple nematic domains. We find clear two-fold oscillations as a function of in-plane field angle near the superconducting transition. This provides bulk evidence for long-range nematic ordering along the Fe-As direction in RbFe₂As₂.
[1] L. de’ Medici et al., PRL 112, 177001 (2014).
[2] X. Liu et al., arXiv:1803.07304.