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Description
The fission barrier height serves as a crucial quantity in understanding the stability of superheavy nuclei and estimating the survival probabilities of a compound nucleus in a heavy ion fusion reaction, while its variation across the nuclear chart characterizes the landscape of the island of stability'' of superheavy nuclei. In this work, we perform the first systematic microscopic calculations of fission barrier heights for even-even superheavy nuclei with triaxiality included. In our calculations, the multidimentionally-constrained relativistic mean-field model is used. Our results reveal that the fission barrier heights remain $\sim$ 5 MeV in the vicinity of the doubly magic nucleus ${}^{304}120$, indicating a surprising landscape of flatness around theisland of stability''. We demonstrate that this flatness originates from the mutually suppressed magicity, whose microscopic origin lies in the distinct density distributions of protons and neutrons, arising from the different orbital angular momenta occupied by nucleons near the Fermi surface. This anomalous behavior is crucial for a quantitative understanding of superheavy nuclear stability and survival. Our findings provide a fresh view of shell evolution and its impact in this region.