The temperature and doping dependence of ${\mathrm{Bi}}_{2.1}{\mathrm{Sr}}_{1.9}{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ vacuum tunneling spectra was measured. The tunneling spectra show a clear gaplike feature for almost all the measured temperatures (10–300 K) and doping levels [overdoped ${(T}_c\sim$80 K) to underdoped ${(T}_c\sim$80 K), via optimum-doped ${(T}_c\sim$90 K)]. The superconducting gap structure below $T_c$ develops into a pseudogap above $T_c.$ The model analysis based on the BCS-type density of states showed a sudden increase in the gap value and inelastic scattering rate at $T_c.$

• Received 19 June 1998

DOI:https://doi.org/10.1103/PhysRevB.60.1377