Probing ultrafast charge density wave diffusion in a trilayer nickelate*

Low valence nickelates are a close analog to cuprates, sharing structural and electronic motifs and hosting superconductivity and charge density wave (CDW) phases in close proximity. Whether these correlated phases have common origins in both systems is an open question. While static CDWs are thought to compete with superconductivity, CDW fluctuations could enhance or even contribute to superconducting pairing [1]. We use non-resonant ultrafast x-ray diffraction to observe the suppression and subsequent recovery of the CDW phase in the stripe-ordered trilayer nickelate La₄Ni₃O₈ [2] in response to an 800 nm pump excitation. The intensity of the (-1/3, 1/3, 9) CDW peak was monitored with a 2D detector, simultaneously measuring the response along a 2D momentum cut through the CDW peak. Conventional analysis required large time and momentum binning, due to weak CDW intensity and high background and noise levels. However, using the machine learning algorithm Xray-TEmperature series Clustering (X-TEC) [3] to track and analyze the time-evolution of individual pixels, we were able to determine momentum-dependent recovery rates and compare them to similar observations in La_1.75Ba_0.25CuO₄. We find that La₄Ni₃O₈ exhibits diffusive dynamics, similar to the cuprate La_1.75Ba_0.25CuO₄ [4], rather than inertial dynamics as in conventional Peierls CDW systems. This indicates that in both cuprates and nickelates charge order can fluctuate at any temperature and can contribute to the emergent electronic behavior in both classes of materials.

[1] E. Fradkin, et al. Rev. Mod. Phys. 87, 457 (2015).

[2] J. Zhang. et al. PNAS, 113, 8945-8950 (2016).

[3] J. Venderley, et al. PNAS 119(24) (2022).

[4] M. Mitrano, M. et al. Sci. Adv. 5, (2019).