Dynamics of ethylene groups and hyperfine interactions between donor and anion molecules in $\lambda$-type organic conductors studied by $^{69,71}$Ga-NMR spectroscopy

TL;DR

This study uses $^{69,71}$Ga-NMR spectroscopy to investigate the hyperfine interactions and ethylene group dynamics in $ ext{lambda}$-type organic conductors, revealing how molecular motions influence magnetic interactions and interlayer coupling.

Contribution

It demonstrates the utility of NMR in probing interlayer interactions and provides insights into the role of short contacts and molecular motions in hyperfine interactions in organic conductors.

Findings

Ethylene group dynamics induce quadrupolar relaxation at high temperatures.

Hyperfine interactions develop below 100 K as ethylene motion freezes.

HF interaction strength varies between different organic salts, influenced by Cl-chalcogen contacts.

Abstract

We present the results of $^{69,71}$Ga-NMR measurements on an organic antiferromagnet $\lambda$-(BEDSe-TTF)$_2$GaCl$_4$ [BEDSe-TTF=bis(ethylenediseleno)tetrathiafulvalene], with comparison to reports on $\lambda$-(BETS)$_2$GaCl$_4$ [BETS=bis(ethylenedithio)tetraselenafulvalene] [T. Kobayashi et al., Phys. Rev. B 102, 235131 (2020)]. We found that the dynamics of two crystallographically independent ethylene groups induce two types of quadrupolar relaxation in the high-temperature region. As the ethylene motion freezes, hyperfine (HF) interactions develop between $\pi$ spin and Ga nuclear spin below 100 K, and thereby magnetic fluctuations of the $\pi$-spin system are detected even from the Ga site. The HF interaction in $\lambda$-(BETS)$_2$GaCl$_4$ was more than twice as large as in $\lambda$-(BEDSe-TTF)$_2$GaCl$_4$, implying that the short contacts between Cl atoms and the chalcogens of fulvalene part are essential for the transferred HF interaction. We propose that NMR using nuclei in anion layers is useful for studying interlayer interactions in organic conductors, which have not been studied experimentally. In addition, because the mechanism of the transferred HF interaction is considered to be the same as $\pi$-$d$ interaction in isostructural Fe-containing $\lambda$-type salts, our findings aid in the understanding of their physical properties.