Atomic Structure of Domain Walls in a Polar Metal

Image Caption: 
Atomic and Electronic Structure of Domains Walls in a Polar Metal in Ca3Ru2O7: (a) [110] view of Ca3Ru2O7 with a magnified perovskite bilayer showing the Ca displacements (black arrows) which are used to define polar displacement vector vP and angle θP. The Ca, Ru, and O atoms are represented by blue, gray, and red circles, respectively. ADF-STEM images along the [110] zone of (b) H-H, (c) T-T 90° domain walls with orientation of vP indicated by the yellow arrows and the overlaid red and blue features. Zoomed in regions of the wall show below panels (b)-(c) depict the characteristic polar displacements of the Ca atoms across the domain walls. (e)-(f) Maps of θP calculated from the ADF-STEM images of H-H, and T-T, 90° domain walls with DFT overlaid in the black boxes. Scale bars 2 nm.

Greg Stone,1 Danilo Puggioni,Shiming Lei,1 Mingqiang Gu,2 Ke Wang,1 Yu Wang,3 Jianjian Ge,3 Xue-Zeng Lu,Zhiqiang Mao,1 James M. Rondinelli,2 and Venkatraman Gopalan,1 1Pennsylvania State University,  2Northwestern University, 3Tulane University

DMR-1420620 (primary), ARO W911NF-15-1-0017, DOE DE-SC0012375, DE-SC0012432


Polar metals counterintuitively bring two well-known phenomena into coexistence: bulk polar displacements and an electronic Fermi surface giving rise to metallic conduction. Little is known about the polar domains or domain walls in such materials. Using electron microscopy imaging and first-principles density functional theory, the MRSEC team showed that uncharged head-to-tail walls, and “charged” head-to-head and tail-to-tail walls can exist in crystals of polar metals Ca3Ru2O7. A significant change occurs in the local density of states occurs at these walls; the H-H and T-T 90 degree domain walls iexhibit distinct electrostatic profiles with a screening length of ~1 nm and a potential variation of 30 meV to 170 meV. Note that a typical silicon-based transistor requires 60 meV/decade to switch between the on and the off states. We surmise that such significant built-in potential could potentially be of use as a variable work function gate material and in interface-space charge mediated devices, such as memristors. 

Physical Review B, 99, 014105 (2019)

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