STM/STS Study on 4a X 4a Electronic Charge Order and Inhomogeneous   Pairing Gap in Superconducting Bi2Sr2CaCu2O8+d
  We performed STM/STS measurements on underdoped Bi2212 crystals with doping levels p ~ 0.11, ~ 0.13 and ~ 0.14 to examine the nature of the nondispersive 4a X 4a charge order in the superconducting state at T << Tc. The charge order appears conspicuously within the pairing gap, and low doping tends to favor the charge order. We point out the possibility that the 4a X 4a charge order will be dynamical in itself, and pinned down over regions with effective pinning centers. The pinned 4a X 4a charge order is closely related to the spatially inhomogeneous pairing gap structure, which has often been reported in STS measurements on high-Tc cuprates. 
A. Hashimoto, N. Momono, M. Oda, M. Ido