Observation of exciton-vibron coupling in cold dense nitrogen

Cold dense matter (CDM) exhibits unique quantum states or effects, and the quantum correlation effects in dense nitrogen become significant at low temperatures, which provides an opportunity to explore the matter under cold dense conditions. The optical band gap (Eg) of dense nitrogen gradually closes with increasing pressure, eventually leading to the metallization. At the same time, the exciton energy (Ex) also gradually decreases as the Eg decreases, this process may trigger a new quantum correlation transition correlated with the exciton coupling. Here we designed a state-of-the-art isothermal-compression Raman spectroscopy technology to probe vibrational properties in cold dense nitrogen, and clearly observed vibron fluctuations and phonon stiffening when we isothermally compressed the solid nitrogen above 130 GPa at below 150 K. This phenomenon is believed to be related to the pressure-driven exciton-vibron coupling in cold dense nitrogen due to the excitonic interaction can enhanced at low temperature, and the exciton binding energies and their lifetimes can be modified. We defined the exciton-vibron coupling state as the new μ-N2 phase, and also identified its pressure-temperature (P-T) phase boundaries. Our finding not only provides experimental basis for the existence of exciton-vibron coupling in cold dense nitrogen, but also provides a clear and manageable context for exploring complex interactions under extreme conditions.