Truncated mass divergence in a Mott metal

TL;DR

This study uses quantum oscillation measurements under high pressure to investigate the evolution of carrier velocity and concentration in NiS₂ as it approaches a Mott insulator, revealing that electronic slowing down dominates but the critical point remains hidden.

Contribution

It provides experimental evidence for electronic slowing down in a Mott transition and highlights the inaccessibility of the critical point due to phase diagram complexities.

Findings

Electronic slowing down governs the approach to the Mott insulating state.

The critical point with diverging effective mass is concealed by the insulating phase.

Mott criticality is interrupted by other phase transitions or emergent phases.

Abstract

Metal-insulator transitions in clean, crystalline solids can be driven by two distinct mechanisms. In a conventional insulator, the charge carrier concentration vanishes, when an energy gap separates filled and unfilled electronic states. In the established picture of a Mott insulator, by contrast, electronic interactions cause coherent charge carriers to slow down and eventually stop in electronic grid-lock without materially affecting the carrier concentration itself. This description has so far escaped experimental verification by quantum oscillation measurements, which directly probe the velocity distribution of the coherent charge carriers. By extending this technique to high pressure we were able to examine the evolution of carrier concentration and velocity in the strongly correlated metallic state of the clean, crystalline material NiS$_2$, while tuning the system towards the Mott insulating phase. Our results confirm that pronounced electronic slowing down indeed governs the approach to the insulating state. However, the critical point itself, at which the carrier velocity would reach zero and the effective carrier mass diverge, is concealed by the insulating sector of the phase diagram. In the resulting, more nuanced view of Mott localisation, the inaccessibility of the low temperature Mott critical point resembles that of the threshold of magnetic order in clean metallic systems, where criticality is almost universally interrupted by first order transitions, tricritical behaviour or novel emergent phases such as unconventional superconductivity.