Analysis of magnetic and quadrupole resonance in cuprates

The Hebel-Slichter coherence peak, observed in the spin-lattice relaxation rate 1/T1 just below the critical temperature Tc, serves as a crucial experimental validation of the Bardeen-Cooper-Schrieffer pairing symmetry in conventional superconductors. However, no coherence peak in 1/T1 has been observed in unconventional superconductors like cuprates. Recently, we identified a coherence peak for the first time using nuclear quadrupole resonance on YBa2Cu4O8, pointing to a distinctive pairing symmetry. In this work, we carry out a systematic investigation of the coherence peak in unconventional superconductors as they transition into the superconducting phase at Tc. Using d-wave cuprates as an example, we reveal the presence of a coherence peak below Tc in the charge channel. The nuclear quadrupole relaxation rate is shown to be an effective method for detecting this unconventional coherence peak, with the superconducting coherence factor playing a pivotal role in its emergence.