We have measured the specific heat of single crystals of superconducting YNi2B2C and TmNi2B2C. Analysis of YNi2B2C data in zero and 5 T magnetic fields indicates modestly strong electron-phonon coupling and a superconducting energy gap Δ(O) = 29 K. Superconductivity coexists with antiferromagnetic order below 1.5 K in TmNi2B2C. An interpretation of very low temperature specific heat indicates substantial magnetic anisotropy, with ferromagnetic planes weakly coupled antiferromagnetically to each other. Two-dimensional ferromagnetic spin waves produce a large linear-in-temperature contribution to the specific heat. We find no compelling evidence for an unconventional pairing mechanism in these systems.
Bibliographical noteFunding Information:
Work at Los Alamos was performed under the auspices of the U.S. Department of Energy. Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. W-7405-ENG-82 and work there was supported by the Director of Energy Research, Office of Basic Energy Sciences. We thank A. Balatsky and Z. Fisk for useful discussions.