Charge Order without Spin Order in the Layered Frustrated Magnet κ-(BEDT-TTF)₂Hg(SCN)₂Cl

There are several quasi two-dimensional materials which do not order magnetically, including a number of geometrically frustrated organic charge transfer salts. To investigate the interplay between charge order (which may affect the frustration) and magnetic order, we use ¹H NMR spectroscopy to examine the magnetic properties of κ-(BEDT-TTF)₂Hg(SCN)₂Cl, which exhibits a first-order metal-insulator transition to a charge ordered state at T_CO = 30K. The high-temperature metallic phase exhibits Fermi-liquid behavior with constant (T₁T)⁻¹, while the spin-lattice relaxation rate is subject to an abrupt enhancement at T_CO and shows a standard paramagnetic response down to 2 K, below which the relaxation rates decrease as 1/T₁ ~ T². The smooth temperature dependence of T₁ and the absence of changes in the NMR spectra rule out magnetic order down to 25 mK suggesting decoupling of the spin and charge degrees of freedom, while the CO is expecetd to reduce the frustration. These observations are discussed in the context of the ground state without magnetic order. Ongoing field-dependent measurements may elucidate whether the magnetic interactions are subject to 1D physics.