THE ENERGY GAP IN THE HIGH-T_{<font>c</font>} COPPER OXIDE SUPERCONDUCTORS

We briefly review high-resolution electron energy loss spectroscopy (HREELS) studies of the low energy excitations in the high-temperature superconductor Bi 2 Sr 2 CaCu 2 O 8 . HREELS has been used to determine the magnitude and temperature dependence of the energy gap (2Δ) for Bi 2 Sr 2 CaCu 2 O 8 single crystals. Below the transition temperature low-energy excitations are detected in the energy loss spectra. Because the energy loss spectra are proportional to the real part of the frequency dependent resistivity, ρ R (ω), the energy gap can be determined directly from the HREEL data. At low temperature 2Δ = 6kT c and Δ develops sharply for T &lt; T c . The constraints that these new results place on potential mechanisms of superconductivity are discussed.