Absence of Pauling's residual entropy in thermally equilibrated Dy2Ti2O7

The discovery of the spin-ice phase in Dy2Ti2O7 numbers among the most significant findings in magnetic materials in over a decade(1-3). Spin ice has since been associated with the manifestation of magnetic monopoles(4,5) and may even inform our understanding of emergent quantum electrodynamics(6). The spin-ice model is based on an elegant analogy to Pauling's model of geometrical frustration in water ice, and predicts the same residual entropy, as confirmed by numerous measurements(1,2,7-11). Here we present results for the specific heat of Dy2Ti2O7, demonstrating why previous measurements were unable to correctly capture its low-temperature behaviour. By carefully tracking the flow of heat into and out of the material, we observe a non-vanishing specific heat that has not previously been detected. This behaviour is confirmed in two samples of Dy2Ti2O7, in which cooling below 0.6 K reveals a deviation from Pauling's residual entropy, calling into question the true magnetic ground state of spin ice.