A partial, momentum-dependent suppression of low-energy electronic spectral weight observed in underdoped cuprates above the superconducting transition. It is not simply a full insulating band gap, and its observation alone does not establish either preformed pairs or a unique competing order. A spectral suppression and a thermodynamic phase boundary are different pieces of evidence. [ref]
The pseudogap: friend or foe of high Tc? by Michael R. Norman and others. 2005 perspective: phenomenology and competing interpretations
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