STM STUDIES OF ELECTRONIC ORDER IN THE UNDERDOPED SURFACE OF YBa₂Cu₃O_y

To study the nanoscale electronic order in high-T_c superconductors, we have performed low-temperature scanning tunneling microscopy (STM) experiments on YBa₂Cu₃O_y, single crystals. The electronic state of the underdoped CuO₂ plane is studied through the BaO surface taking advantage of the cleaved surface, which has an insufficient carrier doping into the topmost CuO₂ plane because of the incomplete carrier reservoir. The STM image measured at low bias-voltage below 20 mV shows the one-dimensional (1D) electronic modulation along the Cu-O bonds. The 1D modulation do not have long-range order and the periodicity varies within the range ~ 2a - 4a depending on the position on the surface. These results indicate that electronic order in underdoped YBa₂Cu₃O_y, differs from the 4a×4a checkerboard structure observed in Bi₂Sr₂CaCu₂O_y and Ca_2-xNa_xCuO₂Cl₂.