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Electronic nematic states with broken rotational symmetry often emerge incorrelated materials. In most iron-based superconductors, the nematicanisotropy is oriented in the Fe-Fe direction of the iron square lattice.Recently, a novel type of nematicity along the diagonal Fe-As direction hasbeen suggested in heavily hole-doped $A$Fe$_2$As$_2$ ($A=$ Rb or Cs). However,the transport studies focusing on the fluctuations of such nematicity haveprovided controversial results regarding the presence of diagonal nematicorder. Here we report high-resolution heat capacity measurements under in-planefield rotation in RbFe$_2$As$_2$. While the temperature dependence of specificheat shows no discernible anomaly associated with the nematic transition, thefield-angle dependence of specific heat near the superconducting transition (at$\sim 2.8$ K) reveals clear two-fold oscillations within the plane, providingthermodynamic evidence for the diagonal nematicity. Moreover, we find thatM\"ossbauer spectroscopy sensitively probes the nematic transition at $\sim 50$K with no evidence of static magnetism. These results imply that the diagonalnematicity in RbFe$_2$As$_2$ has a unique mechanism involving charge degrees offreedom, having unusual thermodynamic properties of the transition.

Thermodynamic Signatures of Diagonal Nematicity in RbFe$_2$As$_2$ Superconductor