Emergent XY-nematic fluctuations in heavily-hole doped iron-based superconductors

Electronic nematicity, rotational symmetry broken state originating from electronic mechanism, has been identified in several strong correlated systems. However, in contrast to its liquid crystal counterparts, the director of electronic nematic domains cannot point in an arbitrary direction (XY- nematic), but is confined to specific directions via non-negligible coupling to the lattice (Ising-nematic). Here we report the elastoresistance measurements in Ba_1-xRb_xFe₂As₂ and CsFe₂As₂, which can evaluate the nematic fluctuations from the change in the resistance induced by uniaxial strain. In RbFe₂As₂ and CsFe₂As₂, we found that B_1g nematic fluctuations is more significant than B_2g nematic fluctuations, providing the evidence for the nematicity along Fe-As direction in these compounds, 45 degree rotated from the unusual nematicity along Fe-Fe direction in the underdoped region. Furthermore, for the intermediate doping between these two distinct Ising-nematic phase, B_1g and B_2g nematic fluctuations exhibit identical Curie-Weiss behaviors, revealing that nearly XY-nematic fluctuation regime. This observation opens up a new route to realizing the novel electronic liquid crystal state in iron-based superconductors.