Janus ZrFX (X = Cl, Br, I) monolayers with giant vertical piezoelectricity exhibiting electronegativity difference ratio effectShou-Xin Cui, Biao Ma, Bao Zhao and Jun LiEur. Phys. J. B, 99 1 (2026) 9DOI: https://doi.org/10.1140/epjb/s10051-026-01123-4