Two-dimensional (2D) ferroelectrics, ferromagnets, and multiferroics have attracted numerous attention due to their applications in novel miniaturized devices such as nonvolatile memories, field effect transistors, magneto-optic devices, and sensors. Our recent studies on these systems lead to the following findings: We discover that there exist two novel kinds of ferroelectric (FE) states in perovskite oxides thin films, namely, a surface-effect induced FE state, and a hybrid improper FE state. Surprisingly, the FE polarization in these two FE states increases with the decrease of the film thickness [1]; We find that the anisotropic exchange coupling in CrI3 and CrGeTe3 is of Kitaev-type, and the interplay between this Kitaev interaction and single ion anisotropy (SIA) naturally explains the Ising behavior of CrI3 versus the Heisenberg behavior of CrGeTe3[2]; We propose new avenues towards realizing 2D multiferroicity [3,4].

References
[1] Jinlian Lu, Wei Luo, Junsheng Feng, and Hongjun Xiang*, Nano Lett. 18, 595 (2018).
[2] Changsong Xu, Junsheng Feng, Hongjun Xiang*, and L. Bellaiche*, submitted.
[3] Chengxi Huang, Yongping Du, Haiping Wu*, Hongjun Xiang*, Kaiming Deng, Erjun Kan*, submitted.
[4] Wei Luo, Ke Xu, and Hongjun Xiang*, Phys. Rev. B 96, 235415 (2017).

Hongjun Xiang received his Bachelor degree and PhD in chemical physics from the University of Science and Technology of China in 2001 and 2006, respectively. He worked on magnetic systems at North Carolina State University from September 2006 to August 2007. After that, he moved to the National Renewable Energy Laboratory where he performed research on energy related materials for about two years. Since October 2009, Hongjun Xiang has been a Professor of Physics at Fudan University. His current research focus is on theoretical studies of ferroelectrics, magnets, multiferroics, and two-dimensional materials.