Observation of pseudogap in underdoped Bi2Sr2CaCu2O8+delta by tunneling spectroscopy

We have performed tunneling measurements on underdoped and optimally doped single crystals of Bi2Sr2CaCu2O8+delta (Bi2212). The tunneling is in the ab -plane, in the maximum gap direction for all of our experiments. Measuremen t temperature ranges from 4.2 K to well above 100 K. The superconducting ga p has been measured for T < T-c. 2 Delta/kT(c) for underdoped Bi2212 (simil ar to 13.4) is significantly larger than that of optimally doped Bi2212 (si milar to 8.8). As the temperature is raised above T-c, the gap vanishes for the optimally doped sample. In contrary, the gap like feature for the unde rdoped Bi2212 persists with larger gap value at higher temperatures. There are interesting differences in the nature of superconducting gap and pseudo gap.