Novel Orientational Ordering and Reentrant Metallicity in KxC60 Monolayers for 3 <= x <= 5

TL;DR

This study reveals complex phase transitions and unique orientational orderings in potassium-doped C60 monolayers, showing reentrant metallic behavior and novel structures driven by electron kinetic energy optimization.

Contribution

It uncovers new orientational structures and phase transitions in KxC60 monolayers, highlighting the role of electron kinetic energy in driving these phenomena.

Findings

Reentrant metal-insulator-metal phase transitions

Discovery of a pinwheel-like 7-molecule unit cell in insulating K4+dC60

Orientational ordering driven by electron orbital overlap mechanisms

Abstract

We have performed local STM studies on potassium-doped C60 (KxC60) monolayers over a wide regime of the phase diagram. As K content increases from x = 3 to 5, KxC60 monolayers undergo metal-insulator-metal reentrant phase transitions and exhibit a variety of novel orientational orderings. The most striking new structure has a pinwheel-like 7-molecule unit cell in insulating K4+dC60. We propose that the driving mechanism for the orientational ordering in KxC60 is the lowering of electron kinetic energy through maximization of the overlap of neighboring molecular orbitals over the entire doping range x = 3 to 5. In the insulating and metallic phases this gives rise to orbital versions of the superexchange and double-exchange interactions respectively.