Energy gaps and phonon structures in tunneling spectra of Bi2Sr2Ca1Cu2O8+x and Bi2Sr2Ca2Cu3O10+y superconductors

Abstract

Polycrystalline Bi-2212 or Bi-2223 systems and single crystals of the Bi-2212 system were investigated at several temperatures by means of the break-junction and point-contact tunneling techniques, respectively. In the case of Bi-2223 we obtained an averaged gap value 2Δ=65±4 meV. The Bi-2212 system yields 2Δ= 45±5 meV in the case of polycrystalline samples and 2Δ=47±5 meV in the case of single crystals. Hence there result BCS-ratios 2Δ/k B T c of 7.3, 6.8 and 6.5, respectively. A dc-Josephson-type supercurrent could be observed in polycrystalline Bi-2212 samples in addition to the usual gap structure. The point-contact tunneling spectra of Bi-2212 single crystals exhibit structures in the second derivative d2I/d V2 which we interpret as inelastic tunneling processes due to electron-phonon scattering most likely in the barrier region. The phonon energies deduced from these structures are in accordance with data obtained from inelastic neutron scattering and Raman spectroscopy. Good agreement is obtained with a lattice dynamical calculation of the partial phonon densities of states of individual atoms in the unit cell.