Unconventional Coherence Peak in Cuprate Superconductors

The Hebel-Slichter coherence peak, observed in the spin-lattice relaxation rate 1 / T 1 just below the critical temperature T c , serves as a crucial experimental validation of the Bardeen-Cooper-Schrieffer pairing symmetry in conventional superconductors. However, no coherence peak in 1 / T 1 has b...

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Veröffentlicht in:Physical review. X 2024-12, Vol.14 (4), p.041072, Article 041072
Hauptverfasser: Li, Zheng, Mu, Chao, Li, Pengfei, Wu, Wei, Hu, Jiangping, Xiang, Tao, Jiang, Kun, Luo, Jianlin
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Sprache:eng
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Zusammenfassung:The Hebel-Slichter coherence peak, observed in the spin-lattice relaxation rate 1 / T 1 just below the critical temperature T c , serves as a crucial experimental validation of the Bardeen-Cooper-Schrieffer pairing symmetry in conventional superconductors. However, no coherence peak in 1 / T 1 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 YBa 2 Cu 4 O 8 , 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 / T 1 mag , which probes magnetic fluctuations, and the quadrupole relaxation rate 1 / T 1 quad , which probes charge fluctuations. By utilizing Cu 63 and Cu 65 isotopes, we successfully distinguish 1 / T 1 mag and 1 / T 1 quad of YBa 2 Cu 4 O 8 and reveal the presence of the coherence peak in 1 / T 1 quad but not in 1 / T 1 mag , in contrast to conventional superconductors. Our finding demonstrates that unconventional superconductors do not exhibit a coherence peak in 1 / T 1 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 / T 1 for the chains of YBa 2 Cu 4 O 8 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.
ISSN:2160-3308
2160-3308
DOI:10.1103/PhysRevX.14.041072