Interacting electron spins in $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]I investigated by ESR spectroscopy

TL;DR

This study uses ESR spectroscopy to explore electron spin interactions in a quasi-two-dimensional organic conductor, revealing a spin gap, weak interlayer diffusion, and local magnetic moments at low temperatures.

Contribution

First detailed ESR analysis of $$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]I, uncovering spin gap formation and magnetic behavior distinct from related compounds.

Findings

Weak interlayer spin diffusion compared to analogues

Spin gap of approximately 18 K indicating spin-singlet state

Presence of local magnetic moments below 4 K

Abstract

We performed angular and temperature-dependent electron-spin-resonance measurements in the quasi-two-dimensional organic conductor $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]I. The interlayer spin-diffusion is much weaker compared to the Cl- and Br-analogues, which are antiferromagnetic insulator and paramagnetic metal, respectively; $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]I behaves insulating when cooled below $T$ = 200 K. A spin gap ($\Delta\approx$ 18 K) opens at low temperatures leading to a spin-singlet state. Due to intrinsic disorder a substantial number of spins ($\sim$ 1 $\%$) remains unpaired. We observe additional signals below $T$ = 4 K with a pronounced anisotropy indicating the presence of local magnetic moments coupled to some fraction of those unpaired spins.