丁香园AV

Synopsis

The past decade has seen enormous advances in the synthesis and ultrafast optical spectroscopy of quantum materials that exhibit fascinating phenomena ranging from superconductivity to metal-insulator transitions. Many of these materials exhibit colossal changes to external perturbation, which includes electromagnetic excitation. This opens up exciting possibilities such as photoinduced phase transitions with the goal to create and control novel states with unique properties. From equilibrium and non-equilibrium perspectives, layered transition metal oxides offer enormous possibilities for light-based discovery and control, given the delicate interplay between interactions and dimensionality, and the ever-present prospect for hosting superconductivity.

In this talk, I will present our recent results on La_2-xBa_xCuO₄ and Sr₂IrO₄. In  La_2-xBa_xCuO₄ (LBCO, x = 0.115), we have been using the c-axis Josephson plasmon response (JPR) to track the evolution of superconductivity following photoexcitation with low fluence near-infrared pulses. We have observed a long-lived metastable state where the JPR blueshifts and persists for hundreds of picoseconds. The observed blueshift arises from photoinduced collapse of the high energy spectral weight associated with stripe order in LBCO. The electrodynamic response of this metastable state is consistent with enhanced superconducting fluctuations.

The spin-orbit coupled Mott insulator Sr₂IrO₄ has attracted considerable interest because of its exotic J_eff = 1/2 Mott state arising from the interplay of on-site Coulomb repulsion and strong spin-orbit coupling. We have investigated magnetization dynamics of this enigmatic compound. Specifically, we measured coherent magnons of the J_eff = 1/2 Mott state using Kerr rotation following excitation with infrared circularly polarized pulses. Notably, for photons below the Mott gap, coherent magnon generation is nearly an order of magnitude more efficient in comparison above-gap excitation. We discuss the origin of this in terms of two-magnon absorption.