【作者】 王敏；

【导师】 万云；

【作者基本信息】 西北大学 ， 凝聚态物理， 2023， 硕士

【摘要】 随着科学界对二维材料研究的深入展开,以及设计和材料合成手段的飞速发展,新型的二维材料层出不穷,如何理解这些二维材料的新奇物理和化学性质是当前凝聚态物理的一个重要方向。近期Nature报道了一种合成钽自插层TaS₂材料的新方法（Nature,581（2020）,171-177）,引起了研究者的广泛关注。作为过渡金属二硫族化合物材料中的一种,TaS₂表现出了丰富的相结构,如多晶型、超导相、磁性、电荷密度波等,这暗示了该类材料包含多种可调的自由度,并且它们之间还具有复杂的相互作用。自插层方法的引入,不仅成功合成了新的材料,而且这种新的人工材料为进一步理解和操纵上述多种自由度及其相互作用提供有效的平台。本文采用第一性原理的方法,系统地研究了上述自插层方法得到的TaS₂的电子结构和磁性,为进一步研究其复杂多体相互作用提供了坚实的理论基础。具体的研究内容如下:（1）利用第一性原理方法系统地模拟了Nature工作中观察到的Ta插层的Ta₇S₁₂的几何结构、电子性质、磁性和磁各向异性,并与未插层的2H-TaS₂进行了对比研究。结果表明:未插层的2H-TaS₂是一种非磁金属,而Ta插层的Ta₇S₁₂是一种铁磁金属,插层Ta原子的5d轨道在E_F附近提供了更多的态密度,这使得Ta₇S₁₂很容易满足Stoner的铁磁基态判据,根据不同磁基态之间的能量差,估计Ta₇S₁₂的居里温度约为865 K,远大于室温。进一步的计算还表明Ta₇S₁₂的每个单胞具有97 me V的面外磁各向异性能（MAE）。通过态密度和二阶微扰理论分析发现,Ta₇S₁₂的磁各向异性能主要是由插层Ta原子的d_xy和d_x-y²²轨道之间的自旋轨道耦合相互作用引起的。我们的研究证明了金属原子插层策略对增强2H-TaS₂磁各向异性并提高其居里温度的有效性。（2）1T-TaS₂随着温度的降低会经历多次复杂的电荷密度波（CDW）转变,并且在足够低的温度下表现出金属-绝缘体转变、超导和CDW相竞争等现象。本文用第一性原理方法系统研究了Ta插层对1T-TaS₂及其CDW相的电子结构和磁性的影响。研究表明:1T相TaS₂是一种非磁性金属,CDW相TaS₂是一种层间反铁磁而层内呈现铁磁态的绝缘体。当Ta原子插入双层1T-TaS₂之间时,更容易诱导1T相向CDW相转变,并且插层原子位置能够决定David-star（大卫星团）的分布,同时电子结构由非磁性金属态转变为半金属态,这不同于普通电荷密度波的绝缘态。当在插层系统中掺入适量的空穴时,大卫星团逐渐被破坏,并最终恢复到最初的1T相。我们的研究丰富了TaS₂的相图,验证了通过自插层方法对CDW状态操纵的有效性,也为拓展CDW物理性质调控方法提供了新思路。更多还原

【Abstract】 The rapid development of design methods and synthesis techniques has contributed to the boom of 2D material science and led to the continuous expansion of the two-dimensional material family.It is an urgent and important task in condensed matter physics to understand the novel physical and chemical properties of these new materials.Recently,a new 2D materials have been synthesized（Nature,581（2020）,171-177）,through self-intercalation tantalum into TaS₂,which has attracted extensive attention from researchers.As one of the transition metal disulfide materials,TaS₂ exhibits rich phase structure,such as polycrystalline,superconducting sequence,magnetism,charge density wave,etc,which implies that it contains a variety of degrees of freedom and complex interactions among them.Through the self-intercalation method,we not only obtained a new type of artificial synthesis,but also changed the properties of the original materials,providing a new platform for us to further understanding and manipulation of the above various degrees of freedom and their interactions.In this paper,the electronic structure and magnetic properties of TaS₂ obtained by the self-intercalation method are studied systematically by using the first principles method,which provides a helpful information for further study of the complex many-body interaction.The specific research contents are as follows:（1）We systematically studied the geometric structure,electronic properties,magnetism and magnetic anisotropy of Ta-intercalated Ta₇S₁₂ observed in the Nature using a first principles methods,and compared with the pristine 2H-TaS₂.The results show that the pristine 2H-TaS₂ is a non-magnetic metal,while the Ta-intercalated Ta₇S₁₂ is a ferromagnetic metal.The 5d orbitals of the intercalated Ta atoms provide more density of states around E_F,which makes Ta₇S₁₂ prefer ferromagnetic ground state according to Stoner’s criterion.Based on the energy difference between different magnetic ground states,we estimated that the Curie temperature of Ta₇S₁₂ is about 865 K,much higher than the room temperature.In particular,we also observed the large out-of-plane magnetic anisotropy energy（MAE）97me V per cell of Ta₇S₁₂.The magnetic anisotropy energy of Ta₇S₁₂ is mainly caused by spin-orbit coupling interaction between the d_xy and d_x-y²² orbitals of the intercalated Ta atom,which is analyzed by density of states and second-order perturbation theory.Our study demonstrated the effectiveness of the metal atom intercalation strategy in enhancing the magnetic anisotropy and increasing the Curie temperature of 2H-TaS₂.（2）1T-TaS₂ not only undergoes multiple complex charge-density wave（CDW）transitions with decreasing temperature,but also exhibits metal-insulator transitions,superconductivity and CDW competition along with this cooling process.In this paper,the effects of Ta intercalation on the electronic structure and magnetic properties of 1T-TaS₂ and its CDW phase have been systematically studied by first principles method.The results show that the 1T phase TaS₂is a non-magnetic metal,and the CDW phase TaS₂ is an insulator with interlayer antiferromagnetic and intralayer ferromagnetic order.When the Ta atom is intercalated into the space within the bilayer 1T-TaS₂,the 1T phase is prefer transition to CDW phase,and the position of intercalated atom determine the distribution of David-star clusters,meanwhile the electronic structure has changed from non-magnetic metal to half-metallic state which is different from ordinary charge density wave’insulating state.Furthermore,when appropriate holes are doped to the intercalation system,the CDW David-star cluster is gradually destroyed and the material eventually restore to its original 1T phase.Our study enriched the phase diagram of TaS₂,emphasized the effective manipulation of CDW states through self-intercalation,and also provided a new idea for expanding the regulation methods of CDW physical properties.更多还原