Microscopic evidence of charge- and spin-density waves in La₃Ni₂O_7-δ revealed by ¹³⁹La-NQR

The recent discovery of superconductivity in La₃Ni₂O_7-δ with a transition temperature Tc close to 80 K at high pressures has attracted significant attention, due particularly to a possible density wave (DW) transition occurring near the superconducting dome. Identifying the type of DW order is crucial for understanding the origin of superconductivity in this system. However, owing to the presence of La₄Ni₃O₁₀ and other intergrowth phases in La₃Ni₂O_7-δ samples, extracting the intrinsic information from the La₃Ni₂O₇ phase is challenging. In this study, we employed ¹³⁹La nuclear quadrupole resonance (NQR) measurements to eliminate the influence of other structural phases in the sample and obtain microscopic insights into the DW transition in La₃Ni₂O_7-δ. Below the DW transition temperature T^DW ~ 153K, we observe a distinct splitting in the ± 5/2 ↔ ± 7/2 transition of the NQR resonance peak at the La(2) site, while only a line broadening is seen in the ± 3/2 ↔ ± 5/2 transition peak. Through further analysis of the spectra, we show that the line splitting is due to a unidirectional charge modulation. A magnetic line broadening is also observed below TDW, accompanied by a large enhancement of the spin-lattice relaxation rate, indicating the formation of magnetically ordered moments in the DW state. Our results suggest a simultaneous formation of chargeand spin-density wave order in La₃Ni₂O_7-δ, thereby offering critical insights into the electronic correlations in Ni-based superconductors.