Low energy spectroscopic mapping study on (Ca,Na)$_2$CuO$_2$Cl$_2$

Spatial variations of the low energy ($<$50~meV) local density of states (LDOS) spectra of (Ca,Na)$_2$CuO$_2$Cl$_2$ ($T_c\sim$ 25~K) have been studied using scanning tunneling microscopy/spectroscopy (STM/STS) in the temperature range between 0.4~K and 20~K. Previous STM/STS studies revealed the checkerboard-like glassy electronic order which is associated with a V-shaped pseudogap $\sim150$~mV.~[1] We have found another small gap ($\sim10$~mV) with LDOS peaks at the gap edges. The gap-edge peak is spatially inhomogeneous and tends to disappear at elevated temperatures, suggesting that the small gap is related to superconductivity. [1]~T. Hanaguri {\it et al.}, Nature {\bf 430}, 1001 (2004).