  The superconducting state is achieved by the condensation of Cooper pairs and
is protected by the superconducting gap. The pairing interaction between the
two electrons of a Cooper pair determines the superconducting gap function.
Thus, it is very pivotal to detect the gap structure for understanding the
mechanism of superconductivity. In cuprate superconductors, it has been well
established that the superconducting gap may have a d-wave function {\Delta} =
{\Delta}_0cos2{\theta}. This gap function has an alternative sign change by
every pi/2 in the momentum space when the in-plane azimuthal angle theta is
scanned. It is very hard to visualize this sign change. Early experiments for
recommending or proving this d-wave gap function were accomplished by the
specially designed phase sensitive measurements based on the Josephson effect.
Here we report the measurements of scanning tunneling spectroscopy in one of
the model cuprate system Bi2Sr2CaCu2O8+{\delta} and conduct the analysis of
phase-referenced quasiparticle interference (QPI). Due to the unique
quasiparticle excitations in the superconducting state of cuprate, we have seen
the seven basic scattering vectors that connect each pair of the terminals of
the banana-shaped contour of constant quasiparticle energy (CCE). The
phase-referenced QPI clearly visualizes the sign change of the d-wave gap. Our
results illustrate a very effective way for determining the sign change of
unconventional superconductors.
