Two-fluid model of the pseudogap of high-temperature cuprate superconductors based on charge-2e bosons

Starting from the effective low-energy theory of a doped Mott insulator, obtained by exactly integrating out the high-energy scale, we show that the effective carrier density in the underdoped regime agrees with a two-fluid description. Namely, it has distinct temperature-independent and thermally activated components. We identify the thermally activated component as the bound state of a hole and a charge-2e boson, which occurs naturally in the effective theory. The thermally activated unbinding of this state leads to the strange metal and subsequent T-linear resistivity. We find that the doping dependence of the binding energy is in excellent agreement with the experimentally determined pseudogap energy scale in cuprate superconductors.