Signatures of the quantum spin liquid state in triangular-based zig-zag polyaromatic hydrocarbon radicals.

Alkali-metal doped polyaromatic hydrocarbons (PAHs) have shown great potential in realizing exotic states of matter such as quantum spin liquids (QSLs). However, it is challenging to obtain new pure-phase candidates and perform experimental identifications accordingly. Here, we report the discovery and characterization of Cs(chrysene˙)(THF)·(THF) (1, THF = tetrahydrofuran), a pure-phase spin-½ organic magnet composed of triangular-based zig-zag magnetic layers, which give rise to strong spin frustration. Electron paramagnetic resonance and optical analyses show 1 is a Mott insulator. Despite the strong antiferromagnetic coupling, low-temperature specific heat and ac susceptibility demonstrate the absence of both long-range magnetic order and spin-glass phases down to 55 mK. Magnetic specific heat can be fitted to the power law, implying gapless spin excitation. Muon spin relaxation reveals constant spin fluctuation rates, suggesting persistent slow dynamics down to 0.3 K. Our results highlight PAHs as a promising avenue for exploring new QSLs.