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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>A</mml:mi> <mml:msub><mml:mrow><mml:mi>Fe</mml:mi></mml:mrow> <mml:mn>2</mml:mn></mml:msub> <mml:msub><mml:mrow><mml:mi>As</mml:mi></mml:mrow> <mml:mn>2</mml:mn></mml:msub> </mml:math> ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>A</mml:mi> <mml:mo>=</mml:mo> <mml:mrow><mml:mi>Rb</mml:mi></mml:mrow> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub><mml:mrow><mml:mi>RbFe</mml:mi></mml:mrow> <mml:mn>2</mml:mn></mml:msub> <mml:msub><mml:mrow><mml:mi>As</mml:mi></mml:mrow> <mml:mn>2</mml:mn></mml:msub> </mml:math> . 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>∼</mml:mo> <mml:mn>2.8</mml:mn></mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>∼</mml:mo> <mml:mn>50</mml:mn></mml:math> K with no evidence of static magnetism. These findings imply that the diagonal nematicity in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub><mml:mrow><mml:mi>RbFe</mml:mi></mml:mrow> <mml:mn>2</mml:mn></mml:msub> <mml:msub><mml:mrow><mml:mi>As</mml:mi></mml:mrow> <mml:mn>2</mml:mn></mml:msub> </mml:math> has a distinct mechanism involving charge degrees of freedom, exhibiting unusual thermodynamic properties of the transition.