  We derive the coherent state representation of the integrable spin chain
Hamiltonian with supersymmetry group $SU(1,1|2)$. By the use of a projected
Hamiltonian onto bosonic states, we give explicitly the action of the
Hamiltonian on $SU(2)\times SL(2)$ coherent states. Passing to the continuous
limit, we find that the corresponding bosonic sigma model is the sum of the
known SU(2) and SL(2) ones, and thus it gives a string spinning fast on
$S^1_{\phi_1}\times S^1_{\phi_1}\times S^1_{\phi_2}$ in $\rm{AdS}_5 \times
S^5$. The full sigma model on the supercoset $SU(1,1|2)/SU(1|1)^2$ is given.
