Observation of pseudogap in underdoped Bi₂Sr₂CaCu₂O_8+δ by tunneling spectroscopy

We have performed tunneling measurements on underdoped and optimally doped single crystals of Bi₂Sr₂CaCu₂O_8+δ (Bi2212). The tunneling is in the ab-plane, in the maximum gap direction for all of our experiments. Measurement temperature ranges from 4.2 K to well above 100 K. The superconducting gap has been measured for T < T_c. 2Δ/kT_c for underdoped Bi2212 (∼13.4) is significantly larger than that of optimally doped Bi2212 (∼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 underdoped Bi2212 persists with larger gap value at higher temperatures. There are interesting differences in the nature of superconducting gap and pseudogap.