Elastic and inelastic neutron scattering study on (CuCl)LaTa$_2$O$_7$

A quasi-two-dimensional frustrated spin system, (CuCl)La(Nb$_{1- x}$Ta$_x$)$_2$O$_7$, shows a quantum phase transition upon doping of Ta ions from a singlet state to an ordered state at $x\sim0.4$. (CuCl)LaNb$_2$O$_7$ has been reported as the first ferromagnetically coupled Shastry-Sutherland singlets with the triplet excitations centered at 2 meV. We report elastic and inelastic neutron scattering measurements on a powder sample of (CuCl)LaTa$_2$O$_7$ with and without an magnetic field. Our results show that upon cooling this system undergoes a magnetic ordering below 7 K with a characteristic wave vector of $Q=(1/2 \ 0\ 1/2)$. The magnetic excitations in the ordered phase are dominated by a nearly dispersionless mode centered at 2 meV similar to the triplet excitations observed in (CuCl)LaNb$_2 $O$_7$. Under field, however, the 2 meV mode in (CuCl)LaTa$_2 $O$_7$ splits into two modes, clearly indicating that it is a spin wave expected for an ordered state.