Coherent heavy charge carriers in an organic conductor near the bandwidth-controlled Mott transition

TL;DR

This study investigates the behavior of charge carriers near the bandwidth-controlled Mott transition in organic conductors, revealing unexpected sensitivity of effective mass to bandwidth changes while maintaining a large coherent Fermi surface.

Contribution

It provides the first direct experimental verification of the quasiparticle effective mass behavior and Fermi surface coherence near the Mott transition in organic materials.

Findings

Mass renormalization is more sensitive to bandwidth changes than predicted.

A large coherent Fermi surface persists near the transition.

Mass enhancement is not accompanied by increased scattering.

Abstract

The physics of the Mott metal-insulator transition (MIT) has attracted huge interest in the last decades. However, despite broad efforts, some key theoretical predictions are still lacking experimental confirmation. In particular, it is not clear whether the large coherent Fermi surface survives in immediate proximity to the bandwidth-controlled first-order MIT. A quantitative experimental verification of the predicted behavior of the quasiparticle effective mass, renormalized by many-body interactions, is also missing. Here we address these issues by employing organic $\kappa$-type salts as exemplary quasi-two-dimensional bandwidth-controlled Mott insulators and gaining direct access to their charge carrier properties via magnetic quantum oscillations. We trace the evolution of the effective cyclotron mass as the conduction bandwidth is tuned very close to the MIT by means of precisely controlled external pressure. We find that the sensitivity of the mass renormalization to tiny changes of the bandwidth is significantly stronger than theoretically predicted and is even further enhanced upon entering the transition region where the metallic and insulating phases coexist. On the other hand, even at its very edge of stability the metallic ground state preserves a large coherent Fermi surface with no significant enhancement of scattering.