Abstract:
This talk focuses on bilayer Ruddlesden–Popper nickelates, particularly La₃Ni₂O₇ (LNO) and La₂SmNi₂O₇ (LSNO). Single-crystalline and polycrystalline samples of these materials exhibit superconductivity with transition temperatures of up to 96 K under high pressure. More recently, superconductivity has also been realized at ambient pressure in LNO and Pr- or Sm-substituted LNO thin films. In this talk, we address two key topics.
(1) Electronic structure of La₃Ni₂O₇ thin films and its implications for superconductivity
Polarization-dependent x-ray absorption spectroscopy is employed to investigate the electronic states and effective dimensionality of LNO thin films with different thicknesses. The thin films exhibit an electronic structure distinct from that of bulk LNO, together with the emergence of the γ band. We compare these electronic features with those predicted in bulk LNO under pressure, highlighting both their similarities and differences. Our results suggest that electronic-structure characteristics associated with the γ band and the out-of-plane d_(3z^2-r^2 ) orbital may play important roles in the emergence of nickelate superconductivity, but may not be by themselves sufficient conditions.
(2) Competing ordered phases in La₂SmNi₂O₇
Density-wave orders have been reported in bulk LNO. Although both charge-density-wave and spin-density-wave states have been proposed, their relationship and respective ordering temperatures remain controversial. To clarify these issues and identify other possible ordered states in LSNO, we perform temperature-dependent synchrotron x-ray diffraction measurements. Ultrafast optical spectroscopy and electrical-transport measurements are further incorporated to provide information on the evolution and interplay of the competing phases.
Together, the above studies provide new insights into the electronic structure, collective ordering phenomena, and possible ingredients required for superconductivity in bilayer nickelates.
Keywords – Ruddlesden–Popper nickelates, electronic structure, density wave