Heat transport and spin-charge separation in the normal state of high temperature superconductors

Hill et al. have recently measured both the thermal and charge conductivities in the normal state of a high temperature superconductor, Based on the vanishing of the Wiedemann-Franz ratio in the extrapolated zero temperature limit, they conclude that the charge carriers in this material are not fermionic. Here I make a simple observation that the prefactor in the temperature dependence of the measured thermal conductivity is unusually large, corresponding to an extremely small energy scale T-0 approximate to 0.15 K. I argue that T-0 should be interpreted as a collective scale. Based on model-independent considerations. I also argue that the experiment leads to two possibilities: (1) the charge-carrying excitations are non-fermionic. And much of the heat current is in fact carried by distinctive charge-neutral excitations; (2) the charge-carrying excitations are fermionic, but a subtle ordering transition occurs at T-0. (C) 2001 Published by Elsevier Science B.V.