  We have performed inelastic neutron scattering experiments to investigate the
magnetic excitations in the quasi-face-centered-cubic (fcc) iridate double
perovskites La2ZnIrO6 and La2MgIrO6, which are characterized by A-type
antiferromagnetic ground states. The neutron scattering data, which reveal
gapped spin wave excitations with a very weak dispersion, are shown to be
well-described by theoretical calculations on a model Hamiltonian with dominant
Kitaev interactions. Our surprising finding shows that in contrast to honeycomb
materials, where the Kitaev interaction manifests itself via complex magnetic
order, even conventional magnetic orders in certain geometrically frustrated
systems might owe their existence to unconventional Kitaev exchange.
