Theory of the Metal-Paramagnetic Mott-Jahn-Teller Insulator Transition in A_4C_{60}

TL;DR

This paper investigates the insulating state of A_4C_{60} using theoretical methods, revealing a Mott-Jahn-Teller insulator state characterized by intra-molecular effects and aligning well with experimental gap measurements.

Contribution

It demonstrates that the Mott-Jahn-Teller insulator state in A_4C_{60} can be accurately modeled with dynamical mean-field theory, resolving discrepancies with density functional predictions.

Findings

Density functional calculations predict metallic state, contradicting experiments.

Dynamical mean-field theory predicts a paramagnetic Mott-Jahn-Teller insulator.

Experimental gaps match intra-molecular ion values, supporting the model.

Abstract

We study the unconventional insulating state in A_4C_{60} with a variety of approaches, including density functional calculations and dynamical mean-field theory. While the former predicts a metallic state, in disagreement with experiment, the latter yields a (paramagnetic) Mott-Jahn-Teller insulator. In that state, conduction between molecules is blocked by on-site Coulomb repulsion, magnetism is suppressed by intra-molecular Jahn-Teller effect, and important excitations (such as optical and spin gap) should be essentially intra-molecular. Experimental gaps of 0.5 eV and 0.1 eV respectively compare well with molecular ion values, in agreement with this picture.