Thermodynamic signatures of diagonal nematicity in RbFe₂As₂ superconductor

Electronic nematic states, characterized by broken rotational symmetry, are prevalent in correlated materials. In most iron-based superconductors, the nematic anisotropy aligns with the Fe-Fe direction of the iron square lattice. However, recent investigations propose a unique form of nematicity oriented along the diagonal Fe-As direction in heavily hole-doped $$ ( $$ or Cs). Yet, the transport studies focusing on the fluctuations of such nematicity yield conflicting outcomes regarding the presence and orientation of the nematic fluctuations. Here, we report high-resolution heat capacity measurements conducted under in-plane field rotation in $$ . While no discernible anomaly associated with the nematic transition is found in the temperature dependence of specific heat, the field-angle dependence near the superconducting transition (at $$ K) reveals clear 2-fold oscillations within the plane, providing thermodynamic evidence for the presence of diagonal nematicity. Moreover, we find that Mössbauer spectroscopy sensitively probes the nematic transition at $$ K with no evidence of static magnetism. These findings imply that the diagonal nematicity in $$ has a distinct mechanism involving charge degrees of freedom, exhibiting unusual thermodynamic properties of the transition.