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

Abstract

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≈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.