钛基自旋-派尔斯晶体的制备及其掺杂晶体磁序的研究

The preparation and physics properties of low dimensional quantum materials have been a subject of the crossing field of materials science and condensed matter physics. One of such family of quantum magnets are spin-Peierls compounds, the complex interplay between charge, spin, orbital, and lattice degrees of freedom has made spin-Peierls compounds prime candidates for studying unusual magnetic ground states. The project will focus on the improvement of the crystal quality of TiOCl and TiOBr, which are grown by chemical vapor transport method firstly. The distortion of crystal structure in intercalated TiOCl and TiOBr, which is the result of strong magnetoelastic coupling, will be studied by temperature-dependent x-ray diffraction (XRD) experiments. The magnetic phase transition will also be studied by magnetic susceptibility, heat capacity, and dielectric measurements. The dimerization of spin magnetic moment is suppressed due to the introduction of alkalis. A further study is calculations of the band structures of intercalated TiOCl and TiOBr, which are based on the XRD data, by the ab initio quantum mechanical software package CASTEP. The discussion between theoretical and experimental results will be performed to uncover the roles of dopant. This project will improve the understanding of insulator-metal transitions and the superconductivity in doped spin-Peierls crystals, which were caused by the introduction of dopant.

低维量子磁性材料的制备与物性研究成为当前材料科学与凝聚态物理交叉领域的前沿课题之一。自旋-派尔斯化合物是此种材料的一类，电荷、自旋、轨道以及晶格自由度之间复杂的相互作用使其成为目前研究晶体自旋磁矩耦合以及高温超导机理的一个热点。本课题采用化学气相传输法制备碱金属Na、K掺杂的自旋-派尔斯晶体TiOCl和TiOBr，通过优化反应条件获得高质量的晶体。采用同步辐射X射线衍射技术观察由于晶体内部强烈的磁耦合而产生的结构形变并得到其调制结构，进而研究晶体在低温下的磁相变。研究碱金属的的引入对低温下自旋磁矩的二聚化的影响以及对自旋-派尔斯相变的抑制。观察掺杂晶体低温下的绝缘体-金属相变并采用从头算的方法计算掺杂晶体的电子能带结构，建立碱金属掺杂TiOCl以及TiOBr的理论模型并进行讨论。本课题对理解掺杂引起的含钛自旋-派尔斯晶体的绝缘体-金属相变以至超导相变有着重要的科学意义。

过渡族金属卤氧化物MOX（M=Ti，Fe，Cr，V；X=Cl，Br等）是典型的类FeOCl结构。金属原子与氧原子形成M-O双层与卤素原子X单层沿结晶学c方向堆叠，M-O双层内由于3d电子的轨道相互作用，原子间有着强烈的相互作用，层间以范德华力相连，作用力微弱。因此MOX是一种理想的研究原子间电荷、自旋、轨道以及晶格自由度之间复杂相互作用的材料，同时由于其层状结构还可能具备可饱和吸收等非线性光学性质。本项目制备了几种MOX单晶，并根据不同晶体的内在性质研究了他们的低温相变以及非线性光学性质，并从理论上进行了第一性原理计算来支持实验的结果。研究表明，少量的Na掺杂并不会从根本上改变TiOCl在低温下的物理学基态，但实验证据表明额外电子的加入会抑制TiOCl的自旋派尔斯相变发生；理论计算表明CrOCl在低温下会发生正交到单斜的相变，单斜角α为90.1度，从而印证了实验的准确性；以CrOCl作为可饱和吸收体研究了其非线性光学效应，分别在1.06μm 以及1.34 μm实现了被动调Q激光输出，首次报道了Cr3+离子的可饱和吸收效应。