The Mystery of Two Gaps in a New Superconductor express
Electrons in a crystal sometimes behave like a stadium crowd doing the wave: areas of higher density alternate with nearly empty ones. This wave blocks the free flow of charges — under normal conditions, the material is not superconducting. But apply pressure, and the wave vanishes, letting electrons move without resistance. Ultrashort laser flashes (a method of optical spectroscopy with precise light measurements) allowed a peek into the wave's structure. It turned out that breaking the wave requires overcoming two energy steps—54 and 67 millielectronvolts. Like a lock needing two differently shaped keys. At helium temperatures, when thermal jitters almost fade, the crystal's own vibrations, phonons, become audible. Their dance adapts to the electron wave, altering the system's overall disorder. The ideas of John Bardeen on electron pairing and Lev Landau on electron liquids intertwine here into a single mechanism.
Original abstract
as written by the authors arXiv:2604.02843
Recently, high-temperature superconductivity has been established in bilayer La3Ni2O7, which exhibits a density-wave (DW) transition at ~ 150 K under ambient pressure. The DW order is believed to be linked to superconductivity, as it is suppressed upon the emergence of superconductivity at high pressures. Here, we explore the ultrafast dynamics of high-energy electronic excitations from 10 K to room temperature under ambient pressure using time-resolved optical spectroscopy. Two high-energy electronic excitations at ~1.8 and ~ 2.4 eV, arising from distinct interband transitions, are identified. They exhibit different DW gaps of approximately 54 and 67 meV, respectively, along with relaxation dynamics that can be well described by the Rothwarf-Taylor model. In addition, we observe four coherent Raman-active phonon modes that exhibit distinct coupling with different electronic excitations. The phonon softening with increasing temperature can be well described between ~100 K and room temperature by a semi-quantitative model, which includes thermal expansion and anharmonic phonon-phonon coupling. At cryogenic temperatures, deviations from the measured temperature-dependent phonon frequencies and the model fits suggest an additional contribution from electron-phonon coupling. Our study provides direct evidence of the complex gap structure and phonon dynamics in this material, offering critical insights into the DW mechanism and many-body effects.
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