Nuclear relaxation in the superconducting state of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$: Evidence for an s+id state

We study the relationship between the symmetry of the pairing state and the NMR measurements in the superconducting state of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$. We find that among various pairing states including s-wave, d-wave, and s+id states, only the s+id state is in qualitative agreement with both the Knight shift and 1/${\mathit{T}}_1$ measurements. This conclusion is further supported by the agreement between the calculated result using the s+id state and the experimental data on the ratio of the nuclear relaxation rates of Cu(2) and O(2,3) nuclei which cannot be fit by either s-wave or d-wave states.