  Two-dimensional materials with Janus structure host novel physical properties
due to their inversional symmetry breaking. However, it remains elusive to
synthesize Janus monolayer crystals with tailored long-range magnetic orders.
Here, we have developed a general method to fabricate uniform Janus CrTeSe
monolayers by selective selenization of preformed CrTe2 monolayers with
molecular beam epitaxy. The uniform Janus structure of CrTeSe with high crystal
quality is confirmed by high-resolution scanning transmission electron
microscopy. Spin-polarized scanning tunneling microscopy/spectroscopy
measurements unveil that the Janus CrTeSe undergoes a charge density wave (CDW)
transition and a robust antiferromagnetic order. The magnetic anisotropy of
CrTeSe is drastically altered compared to monolayer CrTe2 by the breaking
symmetries induced from the Janus structure and the CDW transition, as is
substantiated with first principles calculations. Our research achieves the
construction of large-area Janus structures, and artificially tailors the
electronic and magnetic properties of Janus systems at the two-dimensional
limit.
