Microscopic evidence for spin-spinless stripe order with reduced Ni moments within $ab$ plane for bilayer nickelate La$_3$Ni$_2$O$_7$ probed by $^{139}$La-NQR

The intrinsic electronic properties of La$_3$Ni$_2$O$_{7}$ have been selectively investigated by nuclear quadrupole resonance (NQR) at the La(2) site outside the NiO$_2$ bilayers. The La(2)$_{\rm a}$ site of the ideal La$_3$Ni$_2$O$_{7}$ is clearly distinguished from the La(2)$_{\rm b}$ site close to the local defects. Below 150K, almost half of the intrinsic La(2)$_{\rm a}$ sites are dominated by a finite internal field within the $ab$ plane, while the other half are dominated by zero internal field. The result is fully consistent with the single spin-spinless stripe order of ($\cdots\uparrow\circ\downarrow\circ\uparrow\circ\cdots$), where the reduced Ni magnetic moments are parallel to the $ab$-plane. Even for the La(2)$_{\rm b}$ site, the result is also explained within the same model by considering the inhomogeneous internal magnetic fields enhanced around the nearby defects such as oxygen vacancies. These findings provide unambiguous microscopic evidence for the single spin-spinless stripe order below 150 K at ambient pressure.