Angle-resolved phase-sensitive determination of the in-plane gap symmetry in YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7‑δ</SUB>

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• Tsuei, C. C.
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• Ariando
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• Verwijs, C. J. M.
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• Harkema, S.
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• Hilgenkamp, H.

Abstract

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 dX2‑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 dX2‑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 id_xy, is or ip component must be smaller than a few per cent of the dX2‑Y2 component of the gap.