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L. Li, X. Cheng, J.R. Jokisaari, P. Gao, J. Britson, C. Adamo, C. Heikes, D.G. Schlom, L.Q. Chen, and X.Q. Pan, “Defect-Induced Hedgehog Polarization States in Multiferroics,” Physical Review Letters 120 (2018) 137602-1 – 137602-6.