π-electron S = 1/2 quantum spin-liquid state in an ionic polyaromatic hydrocarbon

Molecular solids with cooperative electronic properties based purely on μ electrons from carbon atoms offer a fertile ground in the search for exotic states of matter, including unconventional superconductivity and quantum magnetism. The field was ignited by reports of high-temperature superconductivity in materials obtained by the reaction of alkali metals with polyaromatic hydrocarbons, such as phenanthrene and picene, but the composition and structure of any compound in this family remained unknown. Here we isolate the binary caesium salts of phenanthrene, Cs(C₁₄H₁₀) and Cs₂(C₁₄H₁₀), to show that they are multiorbital strongly correlated Mott insulators. Whereas Cs₂(C₁₄H₁₀) is diamagnetic because of orbital polarization, Cs(C₁₄H₁₀) is a Heisenberg antiferromagnet with a gapped spin-liquid state that emerges from the coupled highly frustrated Δ-chain magnetic topology of the alternating-exchange spiral tubes of S = 1/2 (C₁₄H₁₀)^.- radical anions. The absence of long-range magnetic order down to 1.8 K (T/J ≈ 0.02; J is the dominant exchange constant) renders the compound an excellent candidate for a spin-1/2 quantum-spin liquid (QSL) that arises purely from carbon μ electrons.