NMR evidence for very slow carrier density fluctuations in the organic metal (TMTSF)$_2$ClO$_4$

TL;DR

This study uses NMR to show that slow carrier density fluctuations, linked to anion ordering, cause large spin-echo decay rates in (TMTSF)$_2$ClO$_4$, revealing insights into charge dynamics in organic conductors.

Contribution

It provides direct NMR evidence that carrier density fluctuations, rather than molecular orientation, dominate spin-echo decay in (TMTSF)$_2$ClO$_4$, highlighting the role of anion ordering.

Findings

Carrier density fluctuations cause large spin-echo decay rates.

Anion ordering at 25K influences fluctuation dynamics.

Spectral broadening relates to finite domain sizes.

Abstract

We have investigated the origin of the large increase in spin-echo decay rates for the $^{77}$Se nuclear spins at temperatures near to $T=30K$ in the organic superconductor (TMTSF)$_2$ClO$_4$. The measured angular dependence of $T_2^{-1}$ demonstrates that the source of the spin-echo decays lies with carrier density fluctuations rather than fluctuations in TMTSF molecular orientation. The very long time scales are directly associated with the dynamics of the anion ordering occurring at $T=25K$, and the inhomogeneously broadened spectra at lower temperatures result from finite domain sizes. Our results are similar to observations of line-broadening effects associated with charge-ordering transitions in quasi-two dimensional organic conductors.