SPECIFIC-HEAT OF SINGLE-CRYSTAL YNI2B2C AND TMNI2B2C SUPERCONDUCTORS

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 DELTA(0) = 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.