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Low-dimensional ferroelectricity and Dirac materials with protected bandcrossings are fascinating research subjects. Based on first-principlescalculations, we predict the coexistence of spontaneous in-plane polarizationand novel 2D emergent fermions in dynamically stable quadruple-layer (QL)XSbO$_2$ (X= Li, Na). Depending on the different polarization configurations,QL-XSbO$_2$ can exhibit unconventional inner-QL ferroelectricity andantiferroelectricity. Both ground states harbor robust ferroelectricity withenhanced spontaneous polarization of 0.56 nC/m and 0.39 nC/m for QL-LiSbO$_2$and QL-NaSbO$_2$, respectively. Interestingly, the QL-LiSbO$_2$ possesses twoother metastable ferroelectric (FE) phases, demonstrating the first 2D examplewith multiple FE orders. The ground FE phase can be flexibly driven into one ofthe two metastable FE phases and then into the antiferroelectric (AFE) phase.During this phase transition, several types of 2D fermions emerge, forinstance, hourglass hybrid and type-II Weyl loops in the ground FE phase,type-II Weyl fermions in the metastable FE phase, and type-II Dirac fermions inthe AFE phase. These 2D fermions are robust under spin-orbit coupling. Notably,two of these fermions, e.g., an hourglass hybrid or type-II Weyl loop, have notbeen observed before. Our findings identify QL-XSbO$_2$ as a unique platformfor studying 2D ferroelectricity relating to 2D emergent fermions.

Enhanced in-plane ferroelectricity, antiferroelectricity, and unconventional 2D emergent fermions in QL-XSbO$_2$ (X= Li, Na)