Controllable two-dimensional robust multiferroic GaTeCl monolayer with giant ferroelectricity and superior multifunctions

We propose through first-principles investigation that GaTeCl monolayer is an excellent two-dimensional multiferroics with giant mechanical anisotropy and ferroelasticity. The calculated phonon spectrum, molecular dynamic simulations, and elastic modules confirm its stability, and our cleavage energy analysis shows that exfoliating one GaTeCl monolayer from the GaTeCl bulk is feasible. The calculated in-plane ferroelectric polarization reaches to 578 pC/m. The energy barriers per formula unit of the ferroelastic rotational and ferroelectric reversal transitions are 476.2 meV and 754.1 meV, respectively, which are the greatest in the two-dimensional multiferroics family so far, and makes the GaTeCl monolayer have robust ferroelasticity and ferroelectricity under room temperature. Uniaxial stress along the polarization direction can make the indirect gap transit to direct gap. Furthermore, the GaTeCl monolayer has giant piezoelectricity and optical second harmonic generation, especially for the range of visible light. These interesting mechanical, electronic, and optical properties of the GaTeCl monolayer show its great potential for multi-functional devices and nonlinear optoelectronic applications.