Independent Freezing of Charge and Spin Dynamics inLa1.5Sr0.5CoO4

We present elastic and quasielastic neutron scattering measurements characterizing peculiar short-range charge-orbital and spin order in the layered perovskite material ${\mathrm{La}}_{1.5}{\mathrm{Sr}}_{0.5}{\mathrm{CoO}}_4$. We find that below $T_c\approx 750\mathrm{K}$ holes introduced by Sr doping lose mobility and enter a statically ordered charge glass phase with loosely correlated checkerboard arrangement of empty and occupied $d_{{3z}^2{-r}^2}$ orbitals ( ${\mathrm{Co}}^{3+}$ and ${\mathrm{Co}}^{2+}$). The dynamics of the resultant mixed spin system is governed by the anisotropic nature of the crystal-field Hamiltonian and the peculiar exchange pattern produced by the orbital order. It undergoes a spin freezing transition at a much lower temperature, $T_s\lesssim 30\mathrm{K}$.