Phonon Dynamics and Multipolar Isomorphic Transition in β-Pyrochlore KOs2O6
We investigate with a microscopic model anharmonic K-cation oscillation observed by neutron experiments in β-pyrochlore superconductor KOs 2 O 6 , which also shows a mysterious first-order structural transition at T p = 7.5 K. We have identified a set of microscopic model parameters that successfully reproduce the observed temperature dependence and the superconducting transition temperature. Considering changes in the parameters at T p , we can explain puzzling experimental results about electron–phonon coupling and neutron data. Our analysis demonstrates that the first-order transition is multipolar transition driven by the octupolar component of K-cation oscillations. The octupole moment does not change the symmetry and is characteristic to noncentrosymmetric K-cation potential.