Nonmonotonic pseudogap in high-T-c cuprates

The mechanism of high-temperature superconductivity has not been resolved for so long because the normal state of cuprates, which exhibits enigmatic pseudogap phenomena, is not yet understood. We performed careful temperature- and momentum-resolved photoemission experiments to show that the depletion of the spectral weight in slightly underdoped cuprate superconductor, usually called the "pseudogap," exhibits an unexpected nonmonotonic temperature dependence: decreases linearly approaching T-* at which it reveals a sharp transition but does not vanish and starts to increase gradually again at higher temperature. The low-temperature behavior of the pseudogap is remarkably similar to one of the incommensurate charge ordering gap in the transition-metal dichalcogenides, while the reopening of the gap at room temperature fits the scenario of temperature-driven metal-insulator transition. This observation suggests that two phenomena, the electronic instability to density-wave formation and the entropy-driven metal-to-insulator crossover, may coexist in the normal state of cuprates.