Thermodynamic signatures of diagonal nematicity in RbFe2As2 superconductor

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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 AFe2As2 (A=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 RbFe2As2. 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 ∼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 ∼50 K with no evidence of static magnetism. These findings imply that the diagonal nematicity in RbFe2As2 has a distinct mechanism involving charge degrees of freedom, exhibiting unusual thermodynamic properties of the transition.
Funding Information
  • Grants-in-Aid for Scientific Research (JP22H00105, JP21H01793, JP20H02600, JP20K21139, JP19H00649, JP18H05227, JP18KK0375)
  • Japan Society for the Promotion of Science
  • Sumitomo Foundation

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