Surface Lattice, Orbital and Spin Reconstructions and Couplings in Hexagonal Multiferroics

Delicate surfaces create an intriguing and challenging research subject because of embedded novel physical phenomena and exotic properties. Broken symmetry when it is introduced to original ferroic orders in multiferroics can pose exciting challenges for understanding the reconstructed couplings. Here, we report a peculiar surface state and reconfigurable functionalities within a representative hexagonal multiferroics, YMnO₃. The reconstructed spin, orbital and lattice couplings are achieved by changes in O 2p and Y 4d orbital (p-d) hybridization, which is atomically revealed utilizing state-of-the-art aberration-corrected (S)TEM. DFT calculations further verify the key roles of specific in-plane oxygens in modulating electronic structures and reconstructions, which should be regarded as an atomic multiferroic element. Meanwhile, the controversial origin of improper ferroelectricity can also be clearly understood based on its absence in defective unitcells. Our findings advance understandings of surface science in strongly correlated oxides, and provide new insights into design and implementation of surface devices by simply controlling the oxygen stoichiometry.