Giant tunnelling anisotropy in the high-T_c superconductor Bi₂Sr₂CaCu₂O₈

TUNNELLING spectroscopy has been one of the most fruitful methods in the study of superconductors^1,2. Excellent agreement has been obtained between theory and experiment, and even the fine details of the tunnelling spectra for conventional, low-transition-temperature (low-T_c) superconductors have been explained in terms of electron-phonon interactions. The low-T_c materials are generally isotropic enough for accurate measurements to be made on polycrystalline specimens; in contrast, it has been difficult to obtain reliable and reproducible tunnelling data for the highly anisotropic high-T_c materials³. We have overcome these difficulties by performing break-junction tunnelling measurements⁴ on extremely thin single crystals, and show here that the tunnelling spectra of Bi₂Sr₂CaCu₂O₈ are indeed highly anisotropic. In the superconducting state, for electrons tunnelling parallel to the copper oxide planes, there are no electronic states at the Fermi level. In the normal state the tunnelling conductance is nearly independent of the d.c. bias voltage. Tunnelling perpendicular to the copper oxide planes was found to be qualitatively different from that parallel to the planes, and we suggest that electron scattering processes play an important role here.