Abrupt Onset of Second Energy Gap at Superconducting Transition of Underdoped Bi2212

TL;DR

This study provides direct evidence of a superconducting energy gap opening at Tc in underdoped Bi2212, supporting the two-gap scenario in high-Tc cuprates and revealing a BCS-like gap near the nodal direction.

Contribution

First direct observation of a superconducting gap with BCS-like temperature dependence in underdoped Bi2212, clarifying the relationship between pseudogap and superconducting gap.

Findings

Superconducting gap opens at Tc near the nodal direction.

The gap exhibits BCS-like temperature dependence.

Distinct pseudogap observed in the antinodal region.

Abstract

The superconducting gap - an energy scale tied to the superconducting phenomena-opens on the Fermi surface at the superconducting transition temperature (TC) in conventional BCS superconductors. Quite differently, in underdoped high-TC superconducting cuprates, a pseudogap, whose relation to the superconducting gap remains a mystery, develops well above TC. Whether the pseudogap is a distinct phenomenon or the incoherent continuation of the superconducting gap above TC is one of the central questions in high-TC research. While some experimental evidence suggests they 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 Bi2212 in the momentum space region overlooked in previous measurements. Near the diagonal of Cu-O bond direction (nodal direction), we found a gap which opens at TC and exhibits a canonical (BCS-like) temperature dependence accompanied by the appearance of the so-called Bogoliubov quasiparticles, 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. The emerging two-gap phenomenon points to a picture of richer quantum configurations in high temperature superconductors.