Tunneling measurements onBi2CaSr2Cu2O8

Measurements have been made of the differential resistance dV/dI versus voltage of point contacts between polycrystalline samples of the high-temperature superconductor ${\mathrm{Bi}}_2$ ${\mathrm{Ca}}_1$ ${\mathrm{Sr}}_2$ ${\mathrm{Cu}}_2$ ${\mathrm{O}}_8$ and tungsten points. The spectra exhibited tunneling structure on a nearly symmetric background that decreased with increasing bias voltages. The potential difference between two minima in the dV/dI spectra was used to obtain an estimate of 25±3 mV for the gap parameter 2Δ. The zero-bias minimum characteristic of a no-barrier metal-superconductor junction was never observed.