Unraveling the Pseudo Gap Phase in Cuprate Superconductors: Experimental Signature, Theoretical Frameworks, and Its Role in High Temperature Superconductivity

Abstract

Among the various mysteries in cuprate high temperature superconductors, the pseudogap (PG) phase stands out for the difficulty in pinning down its origin and its close connection to unconventional superconductivity. It appears above the superconducting transition temperature ​, where part of the low energy spectral weight becomes depleted and several competing or intertwined orders such as charge and pair density waves, nematicity, and spin fluctuations begin to develop. A persistent challenge lies in the systematic discrepancies revealed by different experimental probes, as transport measurements locate the critical doping near , spectroscopic studies around , and symmetry sensitive techniques close to . These variations reflect the distinct sensitivities of each probe to correlation length scales and electronic coherence rather than experimental inconsistency. This review brings together evidence from angle resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy or spectroscopy (STM or STS), nuclear magnetic resonance (NMR), resonant X ray scattering (RXS), and optical conductivity, showing that the pseudogap is a spatially heterogeneous, symmetry breaking electronic state whose onset temperature decreases roughly linearly with doping and terminates sharply at . Three complementary theoretical frameworks, namely quantum criticality, Mott physics, and intertwined orders, collectively describe these observations. Experiments showing the abrupt disappearance of nematic order and a logarithmic rise in the electronic specific heat coefficient suggest that the pseudogap terminates at a quantum critical point. This transition appears to separate a correlation dominated pseudogapped metal from a coherent Fermi liquid phase rather than occurring through a gradual crossover. The doping level identified from transport data aligns with optimal superconductivity, implying that the recovery of long range phase coherence rather than the complete removal of pseudogap features is what ultimately enhances . Unresolved questions include reconciling probe dependent boundaries through systematic cross technique studies on identical crystals and developing correlation length resolved probes to distinguish spatial scales of electronic reconstruction. A major theoretical challenge remains to unify competing frameworks and to elucidate how the pseudogap terminates and coherence emerges at , which represents a key step toward a microscopic theory of high temperature superconductivity in cuprates.