Unconventional Coherence Peak in Cuprate Superconductors

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. In this study, an unconventional coherence peak is identified for the first time using nuclear quadrupole resonance on YBa2Cu4O8, pointing to a distinctive pairing symmetry. The spin-lattice relaxation rate in nuclear quadrupole resonance and nuclear magnetic resonance with nuclear spin I>1/2 comprises the magnetic relaxation rate 1/T1mag, which probes magnetic fluctuations, and the quadrupole relaxation rate 1/T1quad, which probes charge fluctuations. By utilizing Cu63 and Cu65 isotopes, we successfully distinguish 1/T1mag and 1/T1quad of YBa2Cu4O8 and reveal the presence of the coherence peak in 1/T1quad but not in 1/T1mag, in contrast to conventional superconductors. Our finding demonstrates that unconventional superconductors do not exhibit a coherence peak in 1/T1 when the relaxation is due to fluctuations of the hyperfine field. Conversely, a coherence peak is expected when the relaxation is caused by electric field gradient fluctuations, due to the different coherence factors between charge and magnetic fluctuations. Our successful measurements of 1/T1 for the chains of YBa2Cu4O8 suggest that, should the conditions for predominant quadrupole relaxation be satisfied, this phenomenon could provide a novel approach to exploring the unconventional nature of the pairing mechanism in other superconductors. Published by the American Physical Society 2024