Predominantly Superconducting Origin of Large Energy Gaps in Underdoped Bi2Sr2CaCu2O8−δ from Tunneling Spectroscopy

New tunneling data are reported in underdoped Bi2Sr2CaCu2O8−δ using superconductor-insulator-superconductor break junctions. Energy gaps, ∆, of 51±2, 54±2 and 57±3 meV are observed for three crystals with Tc=77, 74, and 70 K respectively. These energy gaps are nearly three times larger than for overdoped crystals with similar Tc. Detailed examination of tunneling spectra over a wide doping range from underdoped to overdoped, including the Josephson IcRn product, indicate that these energy gaps are predominantly of superconducting origin. PACS numbers: 74.50.+r, 74.72.Hs, 74.25.Dw, 74.62.Dh Typeset using REVTEX