Superconducting energy gap and antiferromagnetic spin fluctuations in the superconductor YNi₂B₂C: an NMR study

Abstract

Pulsed NMR studies of ¹¹B and ⁸⁹Y have been carried out in the recently discovered quaternary borocarbide superconductor YNi₂B₂C (T_c=15.5 K). These results are as follows: (a) Superconducting state: Though there is only one crystallographic B site in this structure, we have observed two ¹¹B resonances in the superconducting state. One signal comes from a superconducting region, and the other from a normal-metal region. As for the former signal, the nuclear relaxation rate, T₁^-1 decreases drastically with further decrease of temperature with no appreciable enhancement just below T_c, and it starts to saturate below 6 K. (b) Normal state: (T₁T)^-1 increases as temperature decreases. We interpret this temperature dependence as arising due to two-dimensional or three-dimensional antiferromagnetic spin fluctuations (structure of YNi₂B₂C is tetragonal and highly anisotropic).