Polar, checkerboard charge order in bilayer nickelate La3Ni2O7

TL;DR

This study uses advanced synchrotron X-ray diffraction to uncover a polar charge order in La3Ni2O7, revealing broken symmetry and providing insights into its phase competition and superconductivity under pressure.

Contribution

It identifies a polar crystal structure with checkerboard charge order in La3Ni2O7, challenging previous centrosymmetric models and advancing understanding of its unconventional superconductivity.

Findings

Detected faint reflections indicating broken glide-mirror symmetry.

Confirmed polar structure induced by charge order and octahedral tilting.

Provided evidence linking charge order to phase competition and superconductivity.

Abstract

Competing charge and spin orders are central to uncovering the nature of unconventional superconductivity. Here we utilize synchrotron X-ray diffraction on a high-quality single crystal to reveal the charge order of La$_3$Ni$_2$O$_7$ at ambient pressure, which competes with the high-temperature superconducting phase under pressure. Enabled by the high synchrotron photon flux and a large dynamic range, we resolve faint reflections -- nearly four orders of magnitude weaker than the main Bragg reflections -- that were overlooked in prior diffraction studies. This observation evidences a broken glide-mirror symmetry, leading to a polar crystal structure, rather than the widely used centrosymmetric structure model. The polarity is induced by checkerboard charge order on nickel sites in combination with octahedral tilting, reminiscent of bilayer manganese oxides. Our results provide a foundation for understanding phase competition and the mechanism of pressure-induced superconductivity in bilayer nickelates.