Strong anharmonicity of Pr thermal vibration in heavy fermion superconductor PrOs₄ Sb₁₂

In order to clarify the detailed structure of PrOs₄ Sb₁₂, single crystal neutron diffraction experiments were carried out and obtained data were analyzed by means of the maximum entropy method (MEM). A space- and time-averaged nuclear density distribution reflecting thermal vibration can be obtained through the MEM analysis. The result unveils a widely spread Pr distribution in the Sb cage at room temperature. The Pr density has a almost flat distribution in the real space centered at the Sb cage, which reaches roughly 0.58 Å in the full width at half maximum (FWHM). These features are far from the simple harmonic model, namely, the present result suggests the strong anharmonicity of the Pr potential in the Sb cage.