Evidence for spinon localization in the heat transport of the spin-12ladder compound(C5H12N)2CuBr4

Abstract

We present experiments on the magnetic-field-dependent thermal transport in the spin-$\frac{1}{2}$ ladder system ${({\text{C}}_{5}{\text{H}}_{12}\text{N})}_{2}{\text{CuBr}}_{4}$. The thermal conductivity $\ensuremath{\kappa}(B)$ is only weakly affected by the field-induced transitions between the gapless Luttinger-liquid state realized for ${B}_{c1}<B<{B}_{c2}$ and the gapped states, suggesting the absence of a direct contribution of the spin excitations to the heat transport. We observe, however, that the thermal conductivity is strongly suppressed by the magnetic field deeply within the Luttinger-liquid state. These surprising observations are discussed in terms of localization of spinons within finite ladder segments and spinon-phonon umklapp scattering of the predominantly phononic heat transport.