Different critical exponents on two sides of the magnetic transition in two dimensions

Abstract: The possibility of different critical exponents on two sides of a magnetic transition has been theoretically predicted [PRB 13, 2222 (1976), PRL 115, 200601 (2015)] and experimentally realized [PRL 128, 015703 (2022)] in three-dimensional systems. However, the emergence of such a scenario in two dimensions is not yet established. Here, we report distinct two-dimensional (2D) critical exponents below and above the magnetic transition temperature (TC) through critical scaling analysis in a quasi-2D non-centrosymmetric, Dzyaloshinskii-Moriya interaction (DMI) active hybrid perovskite (C7H9NBr)2CuBr4. The system exhibits critical exponents of 2D Ising-type below TC and 2D XY-type above TC . In contrast, isostructural Cl and centrosymmetric Br analogs, in which the DMI effect is expected to be weaker, exhibit symmetric 2D critical behavior. These results highlight the crucial role of DMI in (C7H9NBr)2CuBr4 as a symmetry-breaking perturbation that alters the magnetic critical behavior differently above and below TC . Our study is the experimental realization of hitherto unreported asymmetric 2D critical behavior in structurally flexible and dimensionally tunable hybrid perovskites.