Unconventional Coherence Peak in Cuprate Superconductors

Abstract

The Hebel-Slichter coherence peak, observed in the spin-lattice relaxation rate $1/T_1$ just below the critical temperature $T_{\mathrm{c}}$, 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 $1/T_1^{\mathrm{mag}}$, which probes magnetic fluctuations, and the quadrupole relaxation rate $1/T_1^{\mathrm{quad}}$, which probes charge fluctuations. By utilizing ${}^{63}$Cu and ${}^{65}$Cu isotopes, we successfully distinguish $1/T_1^{\mathrm{mag}}$ and $1/T_1^{\mathrm{quad}}$ of YBa${}_2$Cu${}_4$O${}_8$ and reveal the presence of the coherence peak in $1/T_1^{\mathrm{quad}}$ but not in $1/T_1^{\mathrm{mag}}$, 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.

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Cited by in corpus (1)

• Superconducting Coherence Peak in Near-Field Radiative Heat Transfer