  Auxetic behavior quantified by the negative Poisson's ratio (NPR) is commonly
attributed to geometry evolution with re-entrant mechanism or other mechanical
factors, which is thought to be independent of electronic structures. Thus,
searching for electronic effect dominated auxetic behavior is challenging.
Herein, from state-of-the-art first-principles calculations, by studying a
class of two-dimensional (2D) transition metal chalcogenides (TMCs), namely
X2Y2-type (X=Cu, Ag, Au; Y=O, S, Se) rectangular TMCs (R-TMCs), we identify
that the monolayer R-Cu2Se2 unconventionally demonstrates a
structure-independent anisotropic NPR. In contrast, the NPR is absent in other
R-TMCs. The emerging NPR is attributed to the strong strain response of
intralayer interaction in R-Cu2Se2, which can be traced to the lone pair
electrons and weak electronegativity of Se atoms under multi-orbital
hybridization. The emerging NPR would make R-Cu2Se2 a promising candidate in
electronics protection, and our study would provide valuable clues and useful
guidance for designing advanced auxetic materials.
