Half-auxetic effect and ferroelasticity in a two-dimensional monolayer TiSe.

Two-dimensional (2D) materials with both auxetic effect and ferroelasticity are rare, however, have great potential applications in next generation microelectromechanical and nanoelectronic devices. Here, we report the findings of an extraordinary combination half-auxetic effect and ferroelasticity in a single p2mm-type TiSe monolayer by performing first-principles calculations. The unique half-auxetic effect, namely the material expand laterally under both uniaxial tensile strain, and compressive strain, is reported and explained by considering both the nearest and the next-nearest interactions. The ferroelasticity is stemming from the degeneracy breaking of the 3d-orbitals of Ti atoms in a distorted tetrahedron crystal field, or the so-called Jahn-Teller effect. The results provide a guideline for the future design of novel two-dimensional multiple functional materials at the nanoscale.