STM/STS study on underdoped La2-xSrxCuO4

We performed scanning tunneling microscopy/spectroscopy (STM/STS) experiments on superconducting La2-xSrxCuO4 with x = 0.12. We observed atomically resolved STM images, which show nearly square lattice with lattice constant a = ∼4 A although many atomic defects were also observed. We found that the surface atoms form dimers and result in 2 x 1 superstructures. STS spectrum exhibits an asymmetric V-shaped gap with gap size Δ ∼ 100 meV, which is not d-wave superconducting gap but similar to so-called zero-temperature pseudogap (ZTPG). This ZTPG-like spectrum indicates the reduction of doping level around the surface