TEMPERATURE-DEPENDENCE OF THE ENERGY-GAP IN BI2SR2CACU2O8 SUPERCONDUCTORS BY HIGH-RESOLUTION ELECTRON-ENERGY-LOSS SPECTROSCOPY

High-resolution electron-energy-loss spectroscopy has been used to determine the magnitude and temperature dependence of the energy gap for oxygen-annealed Bi2Sr2CaCu2O8 single crystals. Below T(c), low-energy excitations arc detected in the energy-loss spectra. These spectra are proportional to the frequency-dependent resistivity, rho(omega), and thus the energy gap can be determined directly from the onset of rho(omega). We find that at low temperature 2-DELTA = 6kT(c) and that DELTA- develops sharply for T < T(c). The similarity of these results to the predictions of models that consider spin interactions suggests that higher T(c)'s may be obtainable in this material.