Strange metals and strange insulators with coherent Fermi surface

In a conventional metallic (insulating) phase, resistivity increases (decreases) with temperature, and a coherent Fermi surface exists (does not exist). In a ‘strange’ metal, the Fermi surface is partially or fully incoherent (non-Fermi liquid), and the resistivity increases linearly with temperature. Here we introduce two opposite concepts. (i) We introduce a ‘strange metal’ state where the resistivity features non-Fermi-liquid behavior, but the Fermi-surface is coherent and analytic. (ii) We also introduce insulating-like transport properties, even in the presence of coherent Fermi-surface. Both results are obtained analytically, as well as numerically within the momentum-resolved density-fluctuation (MRDF) theory in a single band Hubbard model. We calculate self-energy due to itinerant-localized density fluctuations (self-consistently) and find their generic strong momentum-dependence. We demonstrate that both the above results are a manifestation of the momentum-dependence of the self-energy, but at different values of the Hubbard interaction. The results highlight the importance of the non-local self-energy effects in correlated materials, giving unusual results as seen in various experiments.