Characteristics of impurity-induced pseudogap

•We have studied characteristics of the pseudogap states of Co substituted Bi2212 crystals used by STM/STS.•The pseudogap of Co 4% samples have temperature dependence.•We observed a disappearance of a 4a periodic modulation and a development of 1D modulation in the DOS.•An intimate relation between the DOS modulation and the pseudogap is confirmed. We have performed STM/STS measurements on a single crystal of Bi2.1Sr1.9Ca (Cu1−xCox) 2O8+δ (Co-Bi2212), to reveal impurity effects on the pseudogap in cuprate high-Tc superconductors. We report a drastic change in the temperature dependence of a pseudogap and in the density of states (DOS) modulation with a 4a period, in a certain doping range. In the Co 4% substituted samples, the pseudogap gradually closed like a gap of a BCS superconductor for slightly overdoped and overdoped regime, while their low temperature values were enhanced due to impurity. In addition, a disappearance of a 4a periodic modulation and a development of new modulation were observed in the DOS spatial distribution. These results indicate an intimate relation between the DOS modulation and the pseudogap, and qualitative difference in the impurity enhanced pseudogap and conventional one.