Tracking the order parameter dynamics during the melting of orbital order in manganites with optical rotation anisotropy

The loss of in-plane symmetry in layered quantum materials is ubiquitous, with many materials show a change from C₄ to C₂ symmetry when lowering the temperature. The formation of these C₂ symmetric phases are often thought to compete with other phases, such as high temperature superconductivity. Thus understanding how we can control the C₄ to C₂ symmetry breaking transition may allow us understand how we can “tip the balance” into which competing phase dominates the physics of the material.

Traditionally, dynamics of this phase transition have been tracked with time-resolved resonant diffraction, making use of ultra-bright X-ray FEL sources. However, limited access and time-resolution and penetration depth mismatch prevents detailed studies of such phase transitions. While it has been known for some time that optical spectroscopy can track the C₄ to C₂ symmetry breaking phase transition, previous experiments have lacked the ability to extract quantitative information.

In this work, I will show how we use time-resolve reflection anisotropy to extract the order parameter dynamics directly from optical experiments. We find that the order parameter is generally overdamped and weakly couples to coherent lattice vibrations. At the threshold required to restore C4 symmetry, we find a complete change in the entire lattice potential indicative of an order-disorder transition which involves a broad range phonon modes.