Mott transition in the A15 phase of Cs$_{3} $C$_{60}$: absence of pseudogap and charge order

TL;DR

This study uses NMR to investigate the Mott transition in Cs₃C₆₀'s A15 phase, revealing the absence of a pseudogap and charge order, and clarifying the nature of the transition without symmetry change.

Contribution

It provides detailed NMR evidence that the pseudogap feature is linked to the Mott transition itself, not to symmetry changes or charge order, in a multiorbital 3D system.

Findings

No cell symmetry change at the Mott transition.

Pseudogap-like features are signatures of the Mott transition.

Insulator and superconductor phases coexist only in a narrow pressure range.

Abstract

We present a detailed NMR study of the insulator to metal transition induced by an applied pressure $p$ in the A15 phase of Cs$_{3}$C$_{60}$. We evidence that the insulating antiferromagnetic (AF) and superconducting (SC) phases only coexist in a narrow $p$ range. At fixed $p$, in the metallic state above the SC transition $T_c$, the $^{133}$Cs and $^{13}$C NMR spin lattice relaxation data are seemingly governed by a pseudogap like feature. We prove that this feature, also seen in the $^{133}$Cs NMR shift data is rather a signature of the Mott transition, which broadens and smears out progressively for increasing $(p,T)$. The analysis of the variation of the quadrupole splitting $\nu _{Q}$ of the $^{133}$Cs NMR spectrum precludes any cell symmetry change at the Mott transition and only monitors a weak variation of lattice parameter. These results open an opportunity to consider theoretically the Mott transition in a multiorbital three dimensional system well beyond its critical point.