Visualization of the molecular Jahn-Teller effect in an insulating K4C60 monolayer

We present a low-temperature scanning tunneling microscopy (STM) study of KxC60 monolayers on Au(111) for 3 <= x <= 4. The STM spectrum evolves from one that is characteristic of a metal at x = 3 to one that is characteristic of an insulator at x = 4. This electronic transition is accompanied by a dramatic structural rearrangement of the C-60 molecules. The Jahn-Teller effect, a charge-induced mechanical deformation of molecular structure, is directly visualized in the K4C60 monolayer at the single-molecule level. These results, along with theoretical analyses, provide strong evidence that the transition from metal to insulator in KxC60 monolayers is caused by the Jahn-Teller effect.