Thermodynamic signatures of diagonal nematicity in RbFe2As2 superconductor.

Abstract

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 $A{\mathrm{Fe}}_2{\mathrm{As}}_2$ ( $A=\mathrm{Rb}$ 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 ${\mathrm{RbFe}}_2{\mathrm{As}}_2$ . 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 $\sim 2.8$ 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 $\sim 50$ K with no evidence of static magnetism. These findings imply that the diagonal nematicity in ${\mathrm{RbFe}}_2{\mathrm{As}}_2$ has a distinct mechanism involving charge degrees of freedom, exhibiting unusual thermodynamic properties of the transition.