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

Abstract

New tunneling data are reported in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ which show quasiparticle excitation gaps, Δ, reaching values as high as 60 meV for underdoped crystals with $T_c=70\mathrm{}\mathrm{K}$. These energy gaps are nearly 3 times larger than those of overdoped crystals with similar $T_c$. Despite the large differences in gap magnitude, the tunneling spectra display qualitatively similar characteristics over the entire doping range. Detailed examination of the spectra, including the Josephson $I_c{R}_n$ product measured in break junctions, indicates that these energy gaps are predominantly of superconducting origin.