Possible frustrated superconductivity in kagome superconductors

Geometric frustration has long been a subject of enduring interest in condensed matter physics. While geometric frustration traditionally focuses on magnetic systems, little attention is paid to the "frustrated superconductivity" which could arise when the superconducting interaction conflicts with the crystal symmetry. The recently discovered kagome superconductors provide a particular opportunity for studying this due to the fact that the frustrated lattice structure and the interference effect between the three sublattices can facilitate the frustrated superconducting interaction. Here, we propose a theory that supports the frustrated superconducting state, derived from the on-site $s$-wave superconducting pairing in conjunction with the nearest-neighbor pairings hoping and the unique geometrical frustrated lattice structure. In this state, whereas the mutual $2π/3$ difference of the superconducting pairing phase causes the six-fold modulation of the amplitude and breaks the time-reversal symmetry with $4π$ phase changes of the superconducting pairing as one following it around the Fermi surface, it is immune to the impurities without the impurity-induced in-gap states and produces the pronounced Hebel-Slichter peak of the nuclear spin-lattice relaxation rate below $T_{c}$. Notably, the theory also reveals a disorder-induced superconducting pairing transition from the frustrated superconducting state to an isotropic $s$-wave superconducting state without traversing the nodal points, recovering and explaining the behavior found in experiment. This study not only serves as a promising proposal to mediate the divergent or seemingly contradictory experimental outcomes regarding superconducting pairing symmetry, but may also pave the way for advancing investigations into the frustrated superconducting state.