We propose that the odd-frequency s wave (s^odd wave) superconducting gap function, which is usually unstable in the bulk, naturally emerges at the edge of d wave superconductors. This prediction is based on the surface spin fluctuation pairing mechanism owing to the zero-energy surface Andreev bound state. The interference between bulk and edge gap functions triggers the d+s^odd state, and the generated spin current is a useful signal uncovering the “hidden” odd-frequency gap. In addition, the edge s^odd gap can be determined via the proximity effect on the diffusive normal metal. Furthermore, this study provides a decisive validation of the “Hermite odd-frequency gap function,” which has been an open fundamental challenge to this field.

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