Gross violation of the Wiedemann-Franz law in a quasi-one-dimensional conductor

When charge carriers are spatially confi ned to one dimension, conventional Fermi-liquid theory breaks down. In such Tomonaga – Luttinger liquids, quasiparticles are replaced by distinct collective excitations of spin and charge that propagate independently with different velocities. Although evidence for spin – charge separation exists, no bulk low-energy probe has yet been able to distinguish successfully between Tomonaga – Luttinger and Fermiliquid physics. Here we show experimentally that the ratio of the thermal and electrical Hall conductivities in the metallic phase of quasi-one-dimensional Li 0.9 Mo 6 O 17 diverges with decreasing temperature, reaching a value fi ve orders of magnitude larger than that found in conventional metals. Both the temperature dependence and magnitude of this ratio are consistent with Tomonaga – Luttinger liquid theory. Such a dramatic manifestation of spin – charge separation in a bulk three-dimensional solid offers a unique opportunity to explore how the fermionic quasiparticle picture recovers, and over what time scale, when coupling to a second or third dimension is restored.