  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.
