Evolution of the spectral lineshape at the magnetic transition in Sr $$_2$$ 2 IrO $$_4$$ 4 and Sr $$_3$$ 3 Ir $$_2$$ 2 O $$_7$$ 7

Sr $$_2$$ 2 IrO $$_4$$ 4 and Sr $$_3$$ 3 Ir $$_2$$ 2 O $$_7$$ 7 form two families of spin-orbit Mott insulators with quite different charge gaps and an antiferromagnetic (AF) ground state. This offers a unique opportunity to study the impact of long-range magnetic order in Mott insulators. It appears to play a different role in the two families, as there is almost no change of the resistivity at the magnetic transition $$T_N$$ T N in Sr $$_2$$ 2 IrO $$_4$$ 4 and a large one in Sr $$_3$$ 3 Ir $$_2$$ 2 O $$_7$$ 7 . We use angle-resolved photoemission to study the evolution of the spectral lineshape through the magnetic transition. We use Ru and La substitutions to tune $$T_N$$ T N and discriminate changes due to temperature from those due to magnetic order. We evidence a shift and a transfer of spectral weight in the gap at $$T_N$$ T N in Sr $$_3$$ 3 Ir $$_2$$ 2 O $$_7$$ 7 , which is absent in Sr $$_2$$ 2 IrO $$_4$$ 4 . We assign this behavior to a significantly larger coherent contribution to the spectral lineshape in Sr $$_3$$ 3 Ir $$_2$$ 2 O $$_7$$ 7 , which evolves strongly at $$T_N$$ T N . On the contrary, the Sr $$_2$$ 2 IrO $$_4$$ 4 lineshape is dominated by the incoherent part, which is insensitive to $$T_N$$ T N . We compare these findings to theoretical expectations of the Slater vs Mott antiferromagnetism within dynamical mean field theory. Graphic Abstract