Daily Overview: Today’s highlighted work focuses on deepening the understanding of the electronic structure of hybrid Ruddlesden–Popper nickelates. In Ref. [1], the authors combine high-pressure, high-temperature Raman spectroscopy with synchrotron infrared spectroscopy to systematically investigate the structural transition from the tilted Amam phase to the untilted Fmmm/I4mmm phase in the bilayer nickelate La₃Ni₂O₇ and the associated evolution of its electronic properties. Raman measurements confirm the pressure-driven structural transition and reveal the emergence of a Fano line shape, indicating enhanced electron–phonon coupling. High-temperature data further show that the Amam phase has an upper temperature limit in the temperature–pressure phase diagram and identify a coexistence region extending from approximately 6 GPa/400 K to 15.25 GPa/544 K. Infrared reflectivity measurements demonstrate that the carrier density increases by nearly two orders of magnitude across the structural transition, with the system evolving from a weakly metallic state into a highly metallic state, indicating strong coupling between the structural transition and electronic properties. Notably, superconductivity emerges near the boundary of the coexistence region at about 6 GPa, suggesting that although an untilted high-symmetry structure and metallicity are necessary conditions for superconductivity, they alone are not sufficient to trigger it. This provides important constraints for understanding the superconducting phase diagram of nickelates. arXiv submission processing window: 2026-09-03 00:00 to 2026-09-03 00:00 UTC.

1. Metallic crossover through the tilt-free transition in La$_3$Ni$_2$O$_7$ at high pressure and temperature

Summary: Combining high-pressure and high-temperature Raman spectroscopy with synchrotron infrared spectroscopy, this study systematically investigates the structural transition from the tilted Amam phase to the tilt-free Fmmm/I4mmm phase in the bilayer nickelate La₃Ni₂O₇ and the associated evolution of electronic properties. Raman measurements confirm the pressure-driven structural transition and reveal the emergence of a Fano line shape, indicating enhanced electron–phonon coupling; high-temperature data show similar spectral features above 544 K, revealing that the Amam phase has an upper temperature limit in the temperature–pressure phase diagram and defining a coexistence region from 6 GPa/400 K to 15.25 GPa/544 K. Infrared reflectivity measurements show that the carrier density increases by nearly two orders of magnitude accompanying the structural transition, indicating that the system crosses from a weak metallic state to a highly metallic state. These results demonstrate that the structural transition is strongly coupled to the electronic properties, and that superconductivity emerges near the boundary of the coexistence region at about 6 GPa; the tilt-free high-symmetry structure and metallicity are necessary conditions for superconductivity, but are not sufficient by themselves to trigger it.