Kekulé-Kitaev model: Linear and nonlinear responses and the effect of magnetic fields

The Kekul\'e-Kitaev model, an extension of the Kitaev model, exhibits quantum spin liquid properties, which has an exact solution through Kitaev parton construction. In this study, we calculate the dynamical spin structure factor as a linear response and the third-order magnetic susceptibility as a nonlinear response using two-dimensional coherent spectroscopy for the Kekul\'e-Kitaev model. Our results reveal that the few-matter fermion excitations approximation provides reliable results for both linear and nonlinear responses. Notably, while the Kekul\'e-Kitaev model shows linear and nonlinear responses qualitatively similar to the Kitaev model, it displays distinct behavior under a weak uniform and staggered magnetic field. Specifically, the Kekul\'e-Kitaev model does not present a non-Abelian phase under a uniform magnetic field, while such phase appears in the presence of a staggered magnetic field. Interestingly, within the non-Abelian phase, signals originating from two nonadjacent fluxes in the nonlinear response are stronger than signals from other flux excitations. Furthermore, we demonstrate that the ground state of the Kekul\'e-Kitaev model at isotropic coupling is mapped, through unitary spin rotations, to an excited state of the Kitaev model with a uniform flux configuration.