Charge 2e Boson Underlies Two - Fluid Model of the Pseudogap in Cuprate Superconductors

Starting from the effective low energy theory of a doped Mott insulator, 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.