The pyrochlore material $\rm Ho_{ 2}Ti_{ 2}O_{ 7}$ has been suggested to show ``spin ice'' behaviour. We present neutron scattering and specific heat results that establish unambiguously that Ho$_2$Ti$_2$O$_7$ exhibits spin ice correlations at low temperature. Diffuse magnetic neutron scattering from Ho$_2$Ti$_2$O$_7$ is found to be quite well described by a near neighbour spin ice model and very accurately described by a dipolar spin ice model. The heat capacity is well accounted for by the sum of a dipolar spin ice contribution and an expected nuclear spin contribution, known to exist in other Ho$^{ 3+}$ salts. These results settle the question of the nature of the low temperature spin correlations in $\rm Ho_{ 2}Ti_{ 2}O_{ 7}$ for which contradictory claims have been made.
Categories: cond-mat.dis-nn