Direct Observation of Pseudogap in High-T_c Superconductor

We found evidence of a pseudogap opening in the electron density of states (DOS) of a Bi-based high-T_c cuprate. We probed the DOS using a newly developed scanning tunneling microscope that can operate in the temperature range 10-300 K. Using a precision annealing technique for carrier density control, we obtained temperature and doping dependences of DOS and determined the pseudogap phase boundary. Using a theoretical fitting, we evaluated the temperature and doping dependences of the superconducting gap and pseudogap. The two gaps behaved differently upon doping, suggesting that the pseudogap and superconducting gap belong to different calsses of physical quantity.