Resonant Midinfrared Modulation of the Charge Transfer Band in an Organic Mott Insulator

The coupling between molecular vibrational modes and electrons is key to many light-induced phenomena in κ-type organic compounds, e.g., photomolecular high-temperature superconductivity [1]. κ-(ET)₂Cu₂(CN)_3, or (κ-CN), is a prototypical organic Mott insulator hosting genuine Mott-Hubbard physics on a triangular lattice [2]. Having a molecular (ET)₂ dimer, instead of an atom, behaving as one electron on each lattice site, κ-CN provides a flexible platform for tuning its Hubbard parameters. In this study, we interrogate how light-induced vibrations of the C=C bonds (ν₂₇) on the (ET)₂ dimers affect the charge degrees of freedom in κ-CN. Using ultrafast midinfrared pump pulses, we observed multi-component dynamics of the charge transfer band, including a fast response, a coherent phonon oscillation and a slow response that is resonantly amplified around the ν₂₇ mode. This indicates modulations of electronic properties in κ-CN due to local distortions of the (ET)₂ dimers. The stretching of the C=C bonds in a κ-type material was calculated to modulate the Hubbard U [1]. This result suggests an ability to modulate on-dimer electronic interactions, U, in κ-CN with light, perturbing its Hubbard Hamiltonian, leading to emergent states in the material.



[1] M. Buzzi et al., Photomolecular High-Temperature Superconductivity, Phys. Rev. X 10, 031028 (2020).

[2] Pustogow A., Thirty-Year Anniversary of κ-(BEDT-TTF)2Cu2(CN)3: Reconciling the Spin Gap in a Spin-Liquid Candidate, Solids 3(1):93-110 (2022).