Abstract
The superconducting state is formed by the condensation of Cooper pairs and protected by the superconducting gap. The pairing interaction between the two electrons of a Cooper pair determines the gap function. Thus, it is pivotal to detect the gap structure for understanding the mechanism of superconductivity. In cuprate superconductors, it has been well established that the gap may have a d-wave function. This gap function has an alternative sign change in the momentum space. It is however hard to visualize this sign change. Here we report the measurements of scanning tunneling spectroscopy in Bi2Sr2CaCu2O8+δ and conduct the analysis of phase-referenced quasiparticle interference (QPI). We see the seven basic scattering vectors that connect the octet ends of the banana-shaped contour of Fermi surface. The phase-referenced QPI clearly visualizes the sign change of the d-wave gap. Our results illustrate an effective way for determining the sign change of unconventional superconductors.
Highlights
The superconducting state is formed by the condensation of Cooper pairs and protected by the superconducting gap
We report the experiments and analysis based on the defect-induced bound state (DBS-) quasiparticle interference (QPI) method[19,20] on Bi
The spectra show the V-shaped bottoms near zero bias, which reveals the intrinsic feature of the gap nodes in optimally doped Bi-2212
Summary
The superconducting state is formed by the condensation of Cooper pairs and protected by the superconducting gap. EðkÞ 1⁄4 ε2 þ Δ2ðkÞ with ε the kinetic energy of the quasiparticles counting from the Fermi energy EF If this pairing process can apply to other unconventional superconductors, the sign of the gap Δ(k) would change if the pairing interaction. It has been well documented that the gap has a d-wave form Δ = Δ0cos2θ This basic form of the gap was first observed by experiments of angle resolved photoemission spectroscopy (ARPES) without sign signature[1,2], and later supported by many other experiments, such as thermal conductivity[3], specific heat[4,5], scanning tunneling microscopy (STM)[6,7,8], neutron scattering[9,10], and Raman scattering[11], etc. Some of the techniques mentioned above may involve the sign change of the gap, such as the inelastic neutron scattering and STM measurements, they cannot tell how the gap sign changes explicitly along the Fermi surface in the momentum space
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