# Angle-resolved phase-sensitive determination of the in-plane gap symmetry in YBa2Cu3O7−δ

• An Erratum to this article was published on 01 May 2006

## 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 $d X 2 − Y 2$ 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 $d X 2 − Y 2$+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 YBa2Cu3O7−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 i dx y, is or ip component must be smaller than a few per cent of the $d X 2 − Y 2$ component of the gap.

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## Acknowledgements

We would like to thank D. Blank, A. Brinkman, M. Dekkers, A. Golubov, R. Koch, D. Newns, G. Rijnders, F. Roesthuis, H. Rogalla, H.-J. Smilde and C. Varma for discussions. This work was supported by the Dutch Foundation for Research on Matter (FOM), the Netherlands Organization for Scientific Research (NWO), the Dutch STW NanoNed programme, the European Science Foundation (ESF) PiShift programme, and by the Center for Probing the Nanoscale (CPN), an NSF NSEC, NSF Grant No. PHY-0425897.

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Correspondence to J. R. Kirtley.

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### Competing interests

The authors declare no competing financial interests.

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Kirtley, J., Tsuei, C., Ariando et al. Angle-resolved phase-sensitive determination of the in-plane gap symmetry in YBa2Cu3O7−δ. Nature Phys 2, 190–194 (2006). https://doi.org/10.1038/nphys215

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