Angle-resolved phase-sensitive determination of the in-plane gap symmetry in YBa₂Cu₃O_7-δ

Understanding the nature of the ground state and its low-lying excitations in the copper oxide superconductors is a prerequisite for determining the origin of high-temperature superconductivity. A superconducting order parameter (that is, the energy gap) with a predominantly d_x²_-y² symmetry is well-established. However, various deviations from a pure d-wave pair state, such as the possibility of Cooper pairing with broken time-reversal symmetry or an admixed d_x²_-y²+s pair state, have been theoretically predicted and actively sought in numerous experimental studies. Here, we present an angle-resolved phase-sensitive technique for accurately determining the in-plane pairing symmetry, and demonstrate this technique in optimally doped YBa₂Cu₃O_7-δ. We find that the gap along the b-axis (Cu–O chain) direction is at least 20% larger than that along the a-axis direction, and that any imaginary idxy, is or ip component must be smaller than a few per cent of the d_x²_-y² component of the gap.