Emergent charge-2$e$ carriers well hidden from electronic band structures
Emergence of charge-2$e$ bosonic carriers as tightly bound electrons offer perhaps the simplest route to understand the non-Fermi liquid behaviors widely observed in functional materials. However, such scenarios are often discarded when electronic carriers are observed with well-defined energy-momentum dispersion. Here, using attractive Hubbard model as a representative example, we demonstrate the emergence of such 2$e$-carriers coexisting with residual electronic carriers through determinant quantum Monte Carlo computation of their propagators. Interesting, even already dominant in density, the emergent 2$e$-carriers appear to efficiently elude detection by the electronic spectral function, which still shows good quasi-particles with negligible mass enhancement. Nonetheless, above the superfluid temperature, the presence of 2$e$-carriers is revealed through a pseudogap in the electronic spectral function. Our results exemplify the risk of common practice in discarding strong correlation in materials solely based on observation of clean band structures with weakly enhanced mass. More importantly, our finding provides the microscopic foundation for scenarios of boson-fermion mixed liquid as effective descriptions for some of the correlated functional materials.