We found strong zero bias conductance peak enhancement in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$ by tunneling in the $\mathrm{ab}$-plane direction. Our results are consistent with the recent studies on Andreev reflection between a $d$-wave superconductor and normal metal. We have also observed exotic behaviors of the conductance peak if the counter electrode is a conventional superconductor. If these characteristics are better understood, they can be very useful in studying the pairing properties in high $T_c$ superconductors.

• Received 3 April 1997

DOI:https://doi.org/10.1103/PhysRevLett.80.1296