Similar Energy Features in Bi$_2$Sr$_2$CaCu$_2$O$_8$ $_+$ $_\delta$ and Pb

Abstract

A direct experimental demonstration of the BCS theory in conventional superconductor was realized by the tunneling experiment on Pb in which the phonon spectra were deduced from the fine energy features observed in the electron density of states. This experiment directly illustrated that the electron pairing is mediated by the phonon. However, the success of this experiment was directly related to the improvement of the experimental resolution such that the fine energy features from electron–phonon coupling were finally observed. In high-temperature cuprate superconductors, experimental efforts have also been spent in looking for these energy features. Considering its d wave character, momentum- dependent electron density of states is needed. By using high-resolution laser-based angle-resolved photoemission, we have measured the electron density of states along the underlying Fermi surface of the high-temperature Bi$_2$Sr$_2$CaCu$_2$O$_8$ $_+$ $_\delta$ superconductor. Fine energy features are observed in the momentum-dependent electron density of states, which are similar to those observed in Pb by tunneling. The first observation of those energy features and characterization of their momentum dependence by ARPES will prove key information in unraveling the pairing mechanism in the d wave cuprate superconductors.