Phonon-induced quadrupolar ordering of the magnetic superconductor TmNi₂B₂C

We present synchrotron x-ray diffraction studies revealing that the lattice of thulium borocarbide is distorted below T(Q)similar or equal to 13.5 K at zero field. T-Q increases and the amplitude of the displacements is drastically enhanced by a factor of 10 at 60 kOe when a magnetic field is applied along [100]. The distortion occurs at the same wave vector as the antiferromagnetic ordering induced by the a-axis field. A model is presented that accounts for the properties of the quadrupolar phase and explains the peculiar behavior of the antiferromagnetic ordering previously observed in this compound.