Fluctuation, insulation and superconductivity: the pressure-dependent phase-diagram of Rb$_2$Mo$_6$Se$_6$

TL;DR

This study investigates how pressure influences the phase diagram of Rb$_2$Mo$_6$Se$_6$, revealing a complex interplay between insulating fluctuations and superconductivity as the material transitions from one-dimensional to three-dimensional behavior.

Contribution

It provides the first detailed pressure-dependent transport measurements on Rb$_2$Mo$_6$Se$_6$, uncovering the coexistence and competition of insulating and superconducting phases across a wide pressure range.

Findings

Insulating behavior reinforced by fluctuations at low pressure.

Superconductivity emerges above 12 GPa and coexists with insulating phases.

Critical temperature peaks despite reduced density of states.

Abstract

The quasi-one-dimensional (q1D) material Rb$_2$Mo$_6$Se$_6$ has been proposed to display a nontrivial combination of low-dimensional fluctuations and a dynamical charge density wave (CDW) at ambient pressure. This may lead to a progressive metal to insulator cross over at low temperature. To explore the link between the crystal dimensionality and this insulating instability, we have performed hydrostatic pressure-dependent electrical transport measurements on single crystals of Rb$_2$Mo$_6$Se$_6$. At low pressure, we observe thermally-activated behaviour consistent with a temperature-dependent gap $E_g(T)$ opening below a characteristic temperature $T_{Rmin}$. Upon increasing the pressure $T_{Rmin}$ initially rises, indicating a reinforcement of the low temperature insulating state despite a continuous reduction in $E_g(P)$. We interpret this as a signature of suppressed fluctuations as the dimensionality of the electronic structure rises. However, $T_{Rmin}$ drops above 8.8 GPa and superconductivity emerges at 12 GPa. Between 12-24.2 GPa the superconducting and insulating instabilities coexist, with superconductivity surviving up to the maximum attained pressure (52.8 GPa). Analysis of the magneto-transport reveals two distinct regions: at high pressures the anisotropy gradually falls and the superconducting state appears unremarkable. In contrast, coexistence with the gapped insulating phase creates a superconducting dome. The emergence of a peak in the critical temperature Tc despite the depleted density of states is indicative of enhanced coupling. Our journey from the extreme 1D to 3D limits in this prototypical q1D metal reveals an intriguing relationship between superconducting and insulating ground states which is simultaneously competitive and symbiotic.