Features of the energy gap above Tc in Bi2Sr2CaCu2O8+δ as seen by break-junction tunneling

We report tunneling measurements of the energy gap above T-c using as-grown Bi2Sr2CaCu2O8+delta single crystals with T-c=86-89 K, which are in slightly overdoped regions. The measurements were done with break junction for the temperatures mainly above 77 K at which the gap magnitudes of 4 Delta approximate to 100 meV are obtained. We find clear evidence for the gap above T-c. For the most significant data, the gap magnitude decreases with increasing the temperature up to T-c, but it turns to increase to reach approximate to 130 meV at approximate to 120-130 K. The gap feature begins to reduce above this temperature and eventually disappears at the well-defined temperature of T*approximate to 170-190 K The observation of the above characteristics is believed to be a consequence of the clean interface of the break junction, which enables us to probe delicate electronic structures in bulk Bi2Sr2CaCu2O8+delta[S0163-1829(99)02230-4].