Condensed-Matter Physics & Materials Science Seminar

The nature of the highly spin-orbit coupled Mott state of Sr2IrO4 suggests the ground state as well as the collective entangled spin and orbital excitations to be strongly dependent on the lattice degree of freedom. For this reason, Sr2IrO4 provides an ideal platform for controlling the physical properties of a correlated material by inducing local lattice distortions. We use epitaxial strain to modify the Ir-O bond geometry and perform momentum-dependent Resonant Inelastic X-ray Scattering (RIXS) both at the metal and ligand sites to unveil the response of the low energy elementary excitations. By applying tensile strain, we observe a large softening of the spin(-orbital) wave dispersion along the [h,0] direction and a simultaneous hardening along the [h,h] direction. This evolution entails a strain-driven crossover from anisotropic to isotropic interactions between the magnetic moments. We also show how the charge excitations are coupled to the lattice in Sr2IrO4. To this end, using O K-edge RIXS, we unveil the evolution of a dispersive electron-hole pair excitonic mode which shifts to lower (higher) energies upon compressive (tensile) strain, manifesting a reduction (increase) in the size of the charge gap. We show that this behavior originates in the modified hopping elements between the t2g orbitals induced by strain. Our work highlights the central role played by the lattice in determining both the spin(-orbital) as well as the charge excitations of Sr2IrO4 and confirms epitaxial strain as a promising route towards the control of the ground state of complex oxides in the presence of high spin-orbit coupling.