Large-Tiny Spin-Stripe SDW Order in La 3 Ni 2 O 7 via 139 La-NQR Spectroscopy: Reconsideration Based on the Am 2 m Structure

The lower-symmetry Am2m structure was recently proposed as the intrinsic ambient-pressure phase of La 3 Ni 2 O 7 based on synchrotron x-ray diffraction, whereas earlier studies investigated its properties assuming the commonly accepted Amam structure. Here, we reexamine the crystal structure in La 3 Ni 2 O 7 and the spin configuration of spin-density-wave (SDW) state via 139 La nuclear quadrupole resonance (NQR). The observation of two distinct La(2) sites in the NQR spectra is naturally accounted for by the Am2m structure without any special assumptions, whereas it cannot be reconciled within the Amam framework. Within the Am2m framework, we reanalyze the spin configuration of the SDW phase and identify it as a large-tiny spin-stripe structure, arising naturally from two inequivalent Ni sites with large and tiny magnetic moments. These results establish the itinerant nature of the SDW order at ambient pressure in La 3 Ni 2 O 7 , providing a basis for understanding its relationship to the emergence of high-T c superconductivity under high pressure.