Microscopic Evidence for Spin Density Wave Order in La<sub>3</sub>Ni<sub>2</sub>O<sub>7</sub> Probed by La-NQR

In this article we review 139La-nuclear magnetic/quadrupole resonance (NMR/NQR) studies on the bilayer nickelate La3Ni2O7 at ambient pressure, which distinguish two inequivalent La sites, La(1) between the NiO2 planes and La(2) on the out-of-plane blocking layers. The 139La(2)-NQR in La3Ni2O7 further distinguishes the La(2)a site of the ideal La3Ni2O7 from the La(2)b site close to local defects. The 139La(2)-NQR spectrum analyses revealed the appearance of two intrinsic La(2)a sites with zero and finite internal fields below T*~150 K. It is consistently explained by the spin density wave order with the single spin-spinless stripe and the moderately reduced Ni moment that is antiferromagnetically aligned. It will give us a clue to understand the relationship with the novel high-Tc superconducting states under high pressure.