Orthorhombic fulleride (CH3NH2)K3C60 close to Mott-Hubbard instability: Ab initio study

TL;DR

This study uses ab initio methods to explore the electronic and magnetic properties of methylamine-intercalated orthorhombic K3C60, revealing how interactions and distortions influence its Mott-insulating state and low Ne9el temperature.

Contribution

It provides new insights into how methylamine intercalation affects the electronic structure, magnetic interactions, and low-temperature behavior of K3C60, highlighting the role of pseudo-Jahn-Teller distortions and orientational disorder.

Findings

Orthorhombic crystal-field anisotropy drives the system into a Mott-insulating phase.

Interactions with CH3NH2 stabilize specific fullerene distortions and low-spin states.

Disorder and reduced dimensionality may explain the low Ne9el temperature.

Abstract

We study the electronic structure and magnetic interactions in methylamine-intercalated orthorhombic alkali-doped fullerene (CH3NH2)K3C60 within the density functional theory. As in the simpler ammonia intercalated compound (NH3)K3C60, the orthorhombic crystal-field anisotropy \Delta lifts the t1u triple degeneracy at the \Gamma point and drives the system deep into the Mott-insulating phase. However, the computed \Delta and conduction electron bandwidth W cannot alone account for the abnormally low experimental N\'eel temperature, T_N = 11 K of the methylamine compound, compared to the much higher value T_N = 40 K of the ammonia one. Significant interactions between CH3NH2 and C60^{3-} are responsible for the stabilization of particular pseudo-Jahn-Teller fullerene-cage distortions and the ensuing low-spin S = 1/2 state. These interactions also seem to affect the magnetic properties, as interfullerene exchange interactions depend on the relative orientation of pseudo-Jahn-Teller distortions of neighboring C60^{3-} molecules. For the ferro-orientational order of CH3NH2-K^+ groups we find an apparent reduced dimensionality in magnetic exchange interactions, which may explain the suppressed N\'eel temperature. The disorder in exchange interactions caused by orientational disorder of CH3NH2-K^+ groups could further contribute to this suppression.