Two distinct electronic sites in the Cu-O plane of the (La,Sr)CuO(4)   pseudogap state
  The pseudogap state is widely regarded as a precursor to high-temperature superconductivity yet remains poorly understood. Using high field (30 T) NMR, we report two electronically distinct oxygen sites within the copper-oxygen (Cu-O) planes of an underdoped cuprate in the pseudogap state. At temperatures well above the bulk superconducting transition, roughly 25% of the oxygen sites evidence superconducting pair formation, the remaining evidence antiferromagnetic ordering on the nearest neighbor Cu sites. Any superconducting fluctuations are slower than the microsecond timescales of the NMR probe, a confirmation that the (nearly) static bimodality of electronic sites in the Cu-O plane - first reported using scanning tunneling microscopy on surfaces - extends throughout the bulk pseudogap state. 
Robert X. Smith, Philip L. Kuhns, Arneil P. Reyes, and Gregory S.   Boebinger
cond-mat.supr-con cond-mat.str-el