Heat transport of the spin-ice materials Ho2Ti2O7 and Dy2Ti2O7

The elementary excitations of the spin-ice materials Ho2Ti2O7 and Dy2Ti2O7 in zero field can be described as independent magnetic monopoles. We investigate the influence of these exotic excitations on the heat transport by measuring the magnetic-field dependent thermal conductivity κ. Additional measurements on the highly dilute reference compounds HoYTi2O7 and DyYTi2O7 enable us to separate κ into a sum of phononic (κph) and magnetic (κmag) contributions. For both spin-ice materials, we derive significant zero-field contributions κmag, which are rapidly suppressed in finite magnetic fields. Moreover, κmag sensitively depends on the scattering of phonons by magnetic excitations, which is rather different for the Ho- and the Dy-based materials and, as a further consequence, the respective magnetic-field dependent changes κph(B) are even of opposite signs.