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 conductiviti es in the normal state of a high temperature superconductor, Based on the v anishing of the Wiedemann-Franz ratio in the extrapolated zero temperature limit, they conclude that the charge carriers in this material are not ferm ionic. Here I make a simple observation that the prefactor in the temperatu re dependence of the measured thermal conductivity is unusually large, corr esponding to an extremely small energy scale T-0 approximate to 0.15 K. I a rgue that T-0 should be interpreted as a collective scale. Based on model-i ndependent considerations. I also argue that the experiment leads to two po ssibilities: (1) the charge-carrying excitations are non-fermionic. And muc h of the heat current is in fact carried by distinctive charge-neutral exci tations; (2) the charge-carrying excitations are fermionic, but a subtle or dering transition occurs at T-0. (C) 2001 Published by Elsevier Science B.V .