  Coherence factors are a hallmark of superconductivity as a pair-condensation
phenomenon. When electrons pair, quasi-particles develop an acute sensitivity
to different types of scattering potential, described by the appearance of
coherence factors in the scattering amplitudes. While the effects of coherence
factors are well established in isotropic superconductors, they are much harder
to detect in their anisotropic counterparts, such as high-Tc cuprates. Here we
demonstrate a new approach which highlights the momentum-dependent coherence
factors in Ca2-xNaxCuO2Cl2. Using Fourier-transform scanning tunnelling
spectroscopy to detect quasi-particle interference effects, our experiments
reveal a magnetic-field dependence in quasi-particle scattering which is
sensitive to the sign of the anisotropic gap. This result can be understood in
terms of d-wave coherence factors and it exposes the role of vortices as
quasi-particle scattering centers. We also show that a magnetic field gives
rise to an enlarged gapless region around the gap nodes.
