Polar, checkerboard charge order in bilayer nickelate La3Ni2O7

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.

Figures (2)

• Figure 1: Polar charge order in bilayer nickelate La$_3$Ni$_2$O$_7$ at ambient pressure.a, Centrosymmetric, orthorhombic $Amam$ structure model of La$_3$Ni$_2$O$_7$, previously reported in the literature. There is only a single independent nickel site, surrounded by gray oxygen octahedra. b, Polar orthorhombic $Am2m$ structure proposed in the present work. This model contains two inequivalent nickel sites with smaller (orange) and larger (blue) NiO$_6$ octahedral environments. The orange and blue lines indicate shorter and longer Ni-O bonds. c, Broken $a$-glide symmetry in the $Am2m$ structure. The glide operation maps a nickel site to the other independent site; the symmetry is broken if the sites are inequivalent. Broken glide-mirror plane indicated in gray. d, Polarity $P$ induced by the charge alternation along the $b$ axis in combination with the bond alternation due to octahedral tilting around the $a$ axis. The nickel sites with orange and blue octahedra carry a valence of $+2.785(1)$ and $+2.347(1)$, respectively, as estimated from the valence-bond sum (Methods). The tilting angles are $\theta_1 = 192.183(16)^\circ$ and $\theta_2=170.945(14)^\circ$. e, Another polar domain obtained by swapping charges in panel d. f, X-ray diffraction pattern on the $h0l$ plane at $50~$K. Here, $h0l$ ($h=-1$) reflections associated with the broken $a$-glide plane are highlighted by the black rectangle.
• Figure 2: Structure refinement based on synchrotron X-ray diffraction (XRD).a, Refinement of XRD data at $T = 50\,$K assuming the $Amam$ structure model. Observed and calculated intensities, expressed in terms of $|F_\mathrm{obs}|^2$ and $|F_\mathrm{calc}|^2$, show a poor agreement in the regime of weak intensity. Here, $F_\mathrm{obs}$ and $F_\mathrm{calc}$ denote the observed and calculated structure factors, respectively. The poor agreement of model and experiment is quantified by the reliability factor $R = 4.22\,\%$ and goodness-of-fit (GOF) of $7.13 \gg 1$. b, Refinement assuming the proposed $Am2m$ structure. $|F_\mathrm{obs}|^2$ and $|F_\mathrm{calc}|^2$ show a much better agreement with $R = 1.28\,\%$ and $\text{GOF}=1.07$. Inset focuses on $h0l$ ($h=\text{odd}$) reflections (orange circles) associated with the broken $a$-glide.