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

Y. Mizukami; O. Tanaka; K. Ishida; M. Tsujii; T. Mitsui; S. Kitao; M. Kurokuzu; M. Seto; S. Ishida; A. Iyo; H. Eisaki; K. Hashimoto; T. Shibauchi

arXiv · 2108.13081

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

Y. Mizukami·O. Tanaka·K. Ishida·M. Tsujii·T. Mitsui·S. Kitao·M. Kurokuzu·M. Seto·S. Ishida·A. Iyo·H. Eisaki·K. Hashimoto·T. Shibauchi

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Abstract

Electronic nematic states with broken rotational symmetry often emerge in correlated materials. In most iron-based superconductors, the nematic anisotropy is oriented in the Fe-Fe direction of the iron square lattice. Recently, a novel type of nematicity along the diagonal Fe-As direction has been 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 have provided controversial results regarding the presence of diagonal nematic order. Here we report high-resolution heat capacity measurements under in-plane field rotation in RbFe$_2$As$_2$. While the temperature dependence of specific heat shows no discernible anomaly associated with the nematic transition, the field-angle dependence of specific heat near the superconducting transition (at $\sim 2.8$ K) reveals clear two-fold oscillations within the plane, providing thermodynamic evidence for the diagonal nematicity. Moreover, we find that M\"ossbauer spectroscopy sensitively probes the nematic transition at $\sim 50$ K with no evidence of static magnetism. These results imply that the diagonal nematicity in RbFe$_2$As$_2$ has a unique mechanism involving charge degrees of freedom, having unusual thermodynamic properties of the transition.

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Y. Mizukami, O. Tanaka, K. Ishida, M. Tsujii, T. Mitsui, S. Kitao, M. Kurokuzu, M. Seto, S. Ishida, A. Iyo, H. Eisaki, K. Hashimoto, T. Shibauchi. 2021-08-30. Thermodynamic Signatures of Diagonal Nematicity in RbFe$_2$As$_2$ Superconductor. https://arxiv.org/abs/2108.13081

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