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

Abstract

Sr $$_2$$ IrO $$_4$$ and Sr $$_3$$ Ir $$_2$$ O $$_7$$ 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 $$T_N$$ in Sr $$_2$$ IrO $$_4$$ and a large one in Sr $$_3$$ Ir $$_2$$ O $$_7$$ . 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 $$T_N$$ 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 $$T_N$$ in Sr $$_3$$ Ir $$_2$$ O $$_7$$ , which is absent in Sr $$_2$$ IrO $$_4$$ . We assign this behavior to a significantly larger coherent contribution to the spectral lineshape in Sr $$_3$$ Ir $$_2$$ O $$_7$$ , which evolves strongly at $$T_N$$ . On the contrary, the Sr $$_2$$ IrO $$_4$$ lineshape is dominated by the incoherent part, which is insensitive to $$T_N$$ . We compare these findings to theoretical expectations of the Slater vs Mott antiferromagnetism within dynamical mean field theory.