拉曼光谱对准一维量子磁体 TiOCl 变温和加 压下的相变研究

STUDY ON PHASE TRANSITIONS OF QUASI-ONE-DIMENSIONAL QUANTUM MAGNET TiOCl UNDER VARIABLE TEMPERATURE AND PRESSURE BY RAMAN SPECTRUM

郝占阳

11849277

硕士

材料工程

陈伟强

2020-05-29

2020-02-02

哈尔滨工业大学

深圳

低维量子磁体由于其基态的新颖性及其与高温超导的关系而受到了人们的 广泛关注。对于准一维反铁磁体，人们把很多的注意力集中在准一维 S=1/2 海 森堡链材料上，这些材料经历了自旋-派尔斯相变到二聚化单重态基态的转变。 对于这些存在自旋-派尔斯相变的材料的高质量单晶研究能够极大地丰富我们 关于量子基态性质的研究，甚至在对这些材料进行非磁性掺杂时，它们可能会 发生金属态到绝缘体的转变，并且可能显示出超导性。而氯氧化钛（TiOCl）作 为一种有间隙基态的准一维自旋 1/2 量子磁体，非常具有进一步研究的价值。 拉曼光谱是一种能够准确、便捷地对材料的结构进行表征的测试手段, 而 且从拉曼光谱中我们可以得到材料的某些磁性信息。我们采用拉曼光谱来对 TiOCl 单晶进行测试，从而了解其在温度变化和在压力变化下出现的相变。 我们通过化学气相传输法（CVT）成功制备了 TiOCl 单晶，通过扫描电镜 （SEM）表征了材料的形貌，通过 X 射线衍射仪（XRD）表征了材料的结构， 通过能谱仪（EDS）表征了材料的成分，从而表明单晶质量较好。然后，通过 磁学性质测量系统（MPMS）对 TiOCl 单晶的磁性进行了相关测试，得到了不 同磁场下的磁化率曲线，发现在温度为 67 K 和 92 K 附近存在两个相变，其中 前者为一级相变，后者为二级相变。接下来，我们在室温下对 TiOCl 单晶进行 了拉曼光谱测试，发现其沿 b 轴方向比 a 轴方向多出一个宽峰，证明其存在各 向异性，其他三个峰的峰位基本一致，且都为 Ag 峰。除此之外，我们对 TiOCl 单晶进行了变温拉曼光谱测试，在温度依赖的拉曼光谱中发现其随着温度降低 会有峰的出现和劈裂，在 67 K 附近和 92 K 附近都存在相变，且 bb 方向存在的 宽峰随温度降低逐渐红移并变得非常尖锐，这种不寻常的变化可能跟低温下的 自旋声子相互作用有关。同时我们也对 TiOCl 单晶进行了高压拉曼光谱测试， 发现随着压力的不断提高，会有很多新峰的出现以及合并，同时也会有旧峰的 劈裂或消失，通过拉曼峰的一系列变化确定在压力~7 Gpa、~15 Gpa 以及~22 Gpa 附近存在相变，在低压~1.62 Gpa 和~5 GPa 附近的拉曼峰也出现了比较异常的 变化。本文通过以上一系列的实验研究，来得到 TiOCl 单晶的结构、磁性和拉曼 光谱信息，对材料的磁化率、声子和相变进行了分析。最终发现，从常温到 92 K 的温度范围和常压到 7 GPa 的压力范围内，TiOCl 的相变行为有一定的相关 性，而当压力增加到 15 GPa 时，TiOCl 会从准一维体系转变为二维体系，并且 沿着 a 轴和 b 轴都会形成二聚态。将我们的测试结果和已有的研究内容相结合后，我们对 TiOCl 的温度-压力相图进行了绘制和补充。

Low-dimensional quantum magnets have received widespread attention because of their novelty of the ground state and its relationship with high-temperature superconductivity. For quasi-one-dimensional antiferromagnets, much attention has been paid to the quasi-one-dimensional S=1/2 Heisenberg chain materials, which have undergone the spin-Peierls phase transition to dimerization singlet ground state. The studies on the high quality single crystals of these materials with spin-Peierls phase transition can greatly enrich our research on the properties of quantum ground states, and when these materials are non-magnetic doped, they may undergo metal-to-insulator transitions and may exhibit superconductivity. As a quasi-one-dimensional spin 1/2 quantum magnet with energy gap ground state, titanium oxychloride (TiOCl) is of great value for further research. Raman spectrum is a kind of characterization method that can accurately and conveniently characterize the structure of materials, and even from Raman spectrum, we can get the magnetic information of materials. In this dissertation, TiOCl single crystal is characterized by Raman spectrum, so as to understand its phase transitions under temperature changes and pressure changes. We synthesized the TiOCl single crystal successfully by chemical vapor transport (CVT). The morphology was characterized by scanning electron microscope (SEM), the structure was characterized by X-ray diffraction (XRD), the composition was characterized by energy dispersive spectrometer (EDS), and the results indicated that the quality of the single crystal is better. And then, the magnetic properties of TiOCl single crystals were characterized by the Magnetic Property Measurement System (MPMS) to obtain the magnetic susceptibility curves under different magnetic fields. It was found that there are two phase transitions near the temperature of 67 K and 92 K respectively, of which the former was a first order phase transition and the latter was a second order phase transition. Next, Raman spectrum of TiOCl single crystal was measured at room temperature. And it was found that there was one continuous broad peak along the b-axis direction than the a-axis direction, proving that there was anisotropy, in addition, the peak positions of the other three peaks were basically the same, and their vibration modes are all Ag modes. In addition, we measured the temperature-dependent Raman spectrum of TiOCl single crystal. And it was found in the temperature-dependent Raman spectrum that there will be new peaks appeared or peaks spliting as the temperature decreased. There were phase transitions at 67 K and 92 K respectively,and the continuous broad peak along the bb direction gradually redshifts and becomes very sharp as the temperature decreases. This unusual change may be related to the spin-phonon interaction at low temperature. Meantime, the pressure-dependent Raman spectrum of TiOCl single crystal was measured. And the results show that with the continuous increase of pressure, there will be many new peaks appearing and merging, and at the same time, there will also be splitting or disappearance of the original peaks. The series of changes confirms that there are phase transitions around the pressures of ~7 Gpa, ~15 Gpa and ~22 Gpa respectively, meantime, the Raman peaks near the low pressure ~1.62 Gpa and ~5 GPa also exhibit relatively abnormal changes. In this dissertation, through the above series of experimental studies, the structure, magnetism and Raman spectrum information of TiOCl single crystals are obtained. The magnetic susceptibility, phonon and phase transitions of the material are analyzed. Finally, it is found that the phase transition behavior of TiOCl is correlated from room temperature to 92 K and from ambient pressure to 7 GPa, and when the pressure increases to 15 GPa, TiOCl will change from a quasi-one-dimensional system to two-dimensional system, and dimerization will be formed along the a and b axes. After combining our test results with the existing research results, we have drawn and supplemented the temperature-pressure phase diagram of TiOCl.

准一维磁体 相变 自旋-派尔斯相变 二聚态 拉曼光谱

quasi-one-dimensional magnet phase transition spin-Peierls phase transition dimerization Raman spectrum

中文

联合培养

南方科技大学

郝占阳. 拉曼光谱对准一维量子磁体 TiOCl 变温和加 压下的相变研究[D]. 深圳. 哈尔滨工业大学,2020.