Simultaneous quasiparticle and Josephson tunneling in BSCCO-2212 break junctions

Tunneling measurements are reported for superconductor-insulator-superconductor (SIS) break junctions on underdoped, optimally-doped, and overdoped single crystals of Bi~SrzCaCu2013+d (Bi- 2212). The junction I - V characteristics exhibit welldefined quasiparticle current jumps at eV = 2A as well as hysteretic Josephson currents. The quasiparticle branch has been analyzed in the framework of 4 2 - , 2 (&wave) superconductivity and indicates that there is preferential tunneling along the lobe directions of the &wave gap. For overdoped Bi-2212 with T,=62 K, the Josephson current is measured as a function of junction resistance, &, which varied by two orders of magnitude (1 kn to 100 kn). I,& product is proportional to the 0.47 power of I, and displays a maximum of 7.0 mV. When the hole doping is decreased from overdoped (Tc=62 K) to the underdoped regime (T,=70 K), the average I,& product increases as does the quasiparticle gap. The maximum I,& is N 40% of the A/e at each doping level, with a value as high as 25 mV in underdoped Bi-2212.