Density wave (DW) order is believed to be correlated with superconductivity in the recently discovered high-temperature superconductor La₃Ni₂O₇. However, experimental investigations of its evolution under high pressure are still lacking. Here, we explore the quasiparticle dynamics in bilayer nickelate La₃Ni₂O₇ single crystals using ultrafast optical pump-probe spectroscopy under high pressures up to 34.2 GPa. At ambient pressure, the temperature-dependent relaxation dynamics demonstrate a phonon bottleneck effect due to the opening of an energy gap around 151 K. The energy scale of the DW-like gap is determined to be 66 meV by the Rothwarf-Taylor model. Combined with recent experiential results, we propose that this DW-like transition at ambient pressure and low temperature is spin density wave (SDW). With increasing pressure, this SDW order is significantly suppressed up to 13.3 GPa before it completely disappears around 26 GPa. Remarkably, at pressures above 29.4 GPa, we observe the emergence of another DW-like order with a transition temperature of approximately 135 K, which is probably related to the predicted charge density wave (CDW) order. Our study provides the experimental evidences of the evolution of the DW-like gap under high pressure, offering critical insights into the correlation between DW order and superconductivity in La₃Ni₂O₇.

Carbon monoxide clathrate hydrate is a potentially important constituent in the solar system. In contrast to the well-established relation between the size of gaseous molecule and hydrate structure, previous work showed that carbon monoxide molecules preferentially form structure-I rather than structure-II gas hydrate. Resolving this discrepancy is fundamentally important to understanding clathrate formation, structure stabilization and the role the dipole moment/molecular polarizability plays in these processes. Here we report the synthesis of structure-II carbon monoxide hydrate under moderate high-pressure/low-temperature conditions. We demonstrate that the relative stability between structure-I and structure-II hydrates is primarily determined by kinetically controlled cage filling and associated binding energies. Within hexakaidecahedral cage, molecular dynamic simulations of density distributions reveal eight low-energy wells forming a cubic geometry in favour of the occupancy of carbon monoxide molecules, suggesting that the carbon monoxide-water and carbon monoxide-carbon monoxide interactions with adjacent cages provide a significant source of stability for the structure-II clathrate framework.

The development of organic solid-state luminescent materials, especially those sensitive to aggregation microenvironment, is critical for their applications in devices such as pressure-sensitive elements, sensors, and photoelectric devices. However, it still faces certain challenges and a deep understanding of the corresponding internal mechanisms is required. Here, we put forward an unconventional strategy to explore the pressure-induced evolution of the aggregation microenvironment, involving changes in molecular conformation, stacking mode, and intermolecular interaction, by monitoring the emission under multiple excitation channels based on a luminogen with aggregation-induced emission characteristics of di(p-methoxylphenyl)dibenzofulvene. Under three excitation wavelengths, the distinct emission behaviors have been interestingly observed to reveal the pressure-induced structural evolution, well consistent with the results from ultraviolet-visible absorption, high-pressure angle-dispersive X-ray diffraction, and infrared studies, which have rarely been reported before. This finding provides important insights into the design of organic solid luminescent materials and greatly promotes the development of stimulus-responsive luminescent materials.