Evidence of Frozen Moments at Low T in κ-(BEDT-TTF)₂Hg(SCN)₂Br

Quantum Spin Liquids (QSLs) and the phenomena behind their manifestation continue to be a topic of debate in solid state physics. The charge transfer salt, κ-(BEDT-TTF)₂Hg(SCN)₂Br or (κ-HgBr), has been proposed as a “quantum dipole liquid” [1], possibly exhibiting the necessary properties for QSL emergence. We examine the magnetic properties of κ-HgBr using 13C NMR Spectroscopy. 1/T₁ measurements show a first order transition at T_MI = 90 K, a possible crossover at T = 20 K, and a pronounced peak at 5 K characteristic of freezing moments. Analysis of the NMR spectra reveals an onset of broadening at T_MI which becomes more pronounced at low T reaching a FWHM of approximately 1350 ppm at T = 2.5 K. The behavior and magnitude of the linewidth with respect to temperature is indicative of coupling to electron moments which are gradually freezing as temperature decreases and agrees with the 1/T₁ behavior.

[1] Hassan et al., Science 360, 1101–1104 (2018)