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Lattice Ginzburg-Landau Model of a Ferromagnetic p-wave Pairing Phase in Superconducting Materials and an Inhomogeneous Coexisting State

We study the interplay of the ferromagnetic (FM) state and the p-wave superconducting (SC) state observed in several materials such as UCoGe and URhGe in a totally nonperturbative manner. To this end, we introduce a lattice Ginzburg-Landau model that is a genuine generalization of the phenomenological Ginzburg-Landau theory proposed previously in the continuum and also a counterpart of the lattice gauge-Higgs model for the s-wave SC transition, and study it numerically by Monte-Carlo simulations. The obtained phase diagram has qualitatively the same structure as that of UCoGe in the region where the two transition temperatrures satisfy $T_{\rm FM}>T_{\rm SC}$. For $T_{\rm FM}/T_{\rm SC} < 0.7$, we find that the coexisting region of FM and SC orders appears only near the surface of the lattice, which describes an inhomogeneous FMSC coexisting state.