Unveiling a Large Supermodulation Underlying Electronic Anisotropy in Uranium Chalcogenide

Uranium compounds often exhibit complex, strongly correlated electron behaviors (e.g., spin-triplet superconductor UTe₂[1], hidden order compound URu₂Si₂ [2], ferromagnetic superconductor UCoGe [3] and many more). Here we show a new order parameter in a layered uranium chalcogenide: supermodulation. The supermodulation reported herein is a large periodic structural distortion that forms corrugated layers of crystals which resembles that of high temperature superconductor, Bi₂Sr₂CaCu₂O₈ [4, 5]. Electronic anisotropy accompanies this structural distortion; the crystal is semiconducting along the supermodulation and metallic in orthogonal direction. Determining the precise atomic displacement pattern of this supermodulation is key to understanding the electronic structure.

Selected area electron diffraction (SAED) shows sharp and bright superlattice peaks with stronger intensities at higher frequencies indicating emergence of a long-range ordered structural superlattice. Atomic resolution annular dark-field scanning transmission electron microscopy (ADF-STEM) further reveals the superlattice order has strong out-of-plane modulation and forms corrugated structure. The strong interlayer coupling is apparent from antiphase out-of-plane order where sinusoidal modulation shows 180° phase shifts every layer. Cross-sectional SAED further confirms strong long-range interlayer antiphase order from absence of in-plane fundamental superlattice peak at (q,0,0) and strong and sharp presence of out-of-plane superlattice peak at (q,0,½). In addition, in situ heating reveals the superlattice is robust at high temperatures up to 700°C. In summary, we revealed a strong supermodulation with extreme thermal resilience in layered uranium compound. The precise structure of the supermodulation was mapped using high resolution electron microscopy with advanced image analysis techniques [6].

References:

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[6] This work was supported by the Rowland Institute at Harvard.