Nematic-Fluctuation-Driven Superconductivity and Quantum Criticality in Fe-Based Superconductors*

In various iron-based superconductors, superconducting transition temperature T_c is enlarged in the vicinity of the nematic quantum point. At the same time, exotic quantum critical behaviors appear. These observations indicate the dominant roles of the orbital and bond-order fluctuations in the electronic states. Theoretically, nematic fluctuations are induced by the "paramagnon interference mechanism", which is represented by the Aslamazov-Larkin vertex corrections (AL-VCs) [1]. The nematic susceptibility due to the AL-VCs is obtained by the density-wave (DW) equations method [1], while the "pairing interaction due to nematic fluctuations I_nem" is not given by this method.

In order to study the nematic-fluctuation-driven superconductivity in iron-based superconductors, we derive I_nem due to the paramagnon interference mechanism. By applying the “Bethe-Salpeter equation method” developed by the authors, we calculate the precise momentum dependence of I_nem fully microscopically. (Infinite series of AL-VCs are included in the derived  I_nem.) By analyzing the five orbital Hubbard model for Ba122, we derive the nematic-fluctuation-mediated s₊₊ wave high-T_c superconductivity, with the aid of the small amount of impurities. The present theory enables us to study various nematic quantum critical behaviors in various iron-based superconductors.

[1] H. Kontani, R. Tazai, Y. Yamakawa, and S. Onari, Adv. Phys. 70, 355 (2021)