We propose through first-principles investigation that the GaTeCl monolayer is an excellent two-dimensional (2D) multiferroic with giant mechanical anisotropy. The calculated phonon spectrum, molecular dynamic simulations, and elastic moduli confirm its dynamic and mechanical stability, and our cleavage energy analysis shows that exfoliating one GaTeCl monolayer from the existing GaTeCl bulk is feasible. The calculated in-plane ferroelectric polarization reaches 578 pC m⁻¹. The energy barriers per formula unit of the ferroelastic 90° rotational and ferroelectric reversal transitions are 476 meV and 754 meV, respectively, being the greatest in the 2D multiferroics family so far. Importantly, on the other hand, a tensile stress of 4.7 N m⁻¹ perpendicular to the polarization can drive the polarization to rotate by 90°. These can make the GaTeCl monolayer have not only robust ferroelasticity and ferroelectricity but also easy mechanical controllability. Furthermore, the GaTeCl monolayer has giant piezoelectricity and optical second harmonic generation, especially in the range of visible light, and a tensile stress of 0.3 N m⁻¹ along the polarization can make the indirect gap transit to the direct gap. These interesting mechanical, electronic, and optical properties of the GaTeCl monolayer show its great potential in high-performance multi-functional applications.

中文翻译：

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具有巨大二维铁电性和理想多功能性的可控鲁棒多铁性GaTeCl单层

我们通过第一性原理研究提出，GaTeCl单层是具有出色的机械各向异性的出色的二维（2D）多铁性体。计算得到的声子谱，分子动力学模拟和弹性模量证实了其动态和机械稳定性，并且我们的裂解能分析表明，从现有的GaTeCl本体中剥离一个GaTeCl单层是可行的。计算出的面内铁电极化达到578 pC m ^-1。铁弹性90°旋转和铁电反向跃迁的每个公式单位的能垒分别为476 meV和754 meV，是迄今为止二维多铁化合物家族中最大的。另一方面，重要的是，拉伸应力为4.7 N m ^-1垂直于偏振的光可以驱动偏振旋转90°。这些可以使GaTeCl单层不仅具有坚固的铁弹性和铁电性，而且具有容易的机械控制性。此外，GaTeCl单层具有巨大的压电性和光学二次谐波的产生，特别是在可见光范围内，并且沿着极化的0.3N m ^-1的拉伸应力可使间接间隙转变成直接间隙。GaTeCl单层的这些有趣的机械，电子和光学特性显示了其在高性能多功能应用中的巨大潜力。