Coexistence of polar distortion and conduction in doped 2D group-IV ferroelectrics:SiGe, SiSn,andGeSn.

Since the concept of ferroelectric metal predicted in the 1960s has been experimentally realized in the bulk Weyl semimetal WTe2 [Sharma et al., Sci. Adv. 5, eaax5080 (2019)], it is significant to find the ultrathin polar metal or ferroelectric metal due to the demand of miniature of electronic nanodevices. Here, 2D buckled monolayers composed of group-IV elements such as SiGe, SiSn, and GeSn are selected as prototype. Then, the stability of 2D ferroelectricity in the above monolayers are confirmed based on the results of first-principles calculations. Most interesting, a robustly metallic polar state has been found in the above 2D ferrolectrics under both the electron doping and hole doping, and the polar distortion becomes even more remarkable when the electrons are doped as compared with the undoped system. Thus, the coexistence of polar state and conduction is theoretically verified in the doped group-IV monolayers. We hope the 2D ferroelectric materials can be used as a starting point to look for the polar metals with atomic thickness, and further broaden their applications in 2D electronics or spintronics in the future.