Abrupt onset of a second energy gap at the superconducting transition of underdoped Bi2212

摘要

The superconducting gap—an energy scale tied to the superconducting phenomena—opens on the Fermi surface at the superconducting transition temperature (T_c) in conventional BCS superconductors. In underdoped high- T_c superconducting copper oxides, a pseudogap (whose relation to the superconducting gap remains a mystery) develops well above T_c (refs 1, 2). Whether the pseudogap is a distinct phenomenon or the incoherent continuation of the superconducting gap above T_c is one of the central questions in high-T_c research. Although some experimental evidence suggests that the two gaps are distinct, this issue is still under intense debate. A crucial piece of evidence to firmly establish this two-gap picture is still missing: a direct and unambiguous observation of a single-particle gap tied to the superconducting transition as function of temperature. Here we report the discovery of such an energy gap in underdoped Bi_2Sr_2CaCu_2O_(8+δ) in the momentum space region overlooked in previous measurements. Near the diagonal of Cu-O bond direction (nodal direction), we found a gap that opens at T_c and has a canonical (BCS-like) temperature dependence accompanied by the appearance of the so-called Bogoliubov quasi-particles, a classical signature of superconductivity. This is in sharp contrast to the pseudogap near the Cu-O bond direction (antinodal region) measured in earlier experiments.