  We unveil chaotic behavior hidden in the energy spectrum of a Jahn-Teller ion
vibrating in a cubic anharmonic potential as a typical model for rattling in
cage-structure materials. When we evaluate the nearest-neighbor level-spacing
distribution $P(s)$ of eigenenergies of the present oscillator system, we
observe the transition of $P(s)$ from the Poisson to the Wigner distribution
with the increase of cubic anharmonicity, showing the occurrence of chaos in
the anharmonic Jahn-Teller vibration. The energy scale of the chaotic region is
specified from the analysis of $P(s)$ and we discuss a possible way to observe
chaotic behavior in the experiment of specific heat. It is an intriguing
possibility that chaos in nonlinear physics could be detected by a standard
experiment in condensed matter physics.
