【作者】 刘旭光；

【导师】 孙学峰；

【作者基本信息】 中国科学技术大学 ， 凝聚态物理， 2013， 博士

【摘要】 准一维量子磁性体系具有丰富的相图和奇特的物理性质。理论研究表明,对于可积的一维量子磁性体系来说,该体系的热输运是弹道的；对于不可积的体系则是扩散的。实验上在一些不可积的体系(如S=1/2自旋梯子,甚至S=1自旋链)观察到了非常大的磁激发热导率和平均自由程,这表明该体系有可能存在弹道的热输运。因此,准一维磁性材料的热输运仍存在较多需要深入研究的问题。另外,研究准一维量子磁性材料的磁热输运还有助于了解磁激发的散射机制及其色散,增加对这些体系的磁性质的认识。针对上述问题,本论文研究了两种S=1/2的阻挫自旋链材料和一种S=1的Haldane链材料的低温热输运性质。探讨了阻挫材料处于反铁磁有序态时磁激发参与传热的可能性；并讨论了Haldane链材料中巨大的磁热导率行为。论文正文共分为四章,每章的主要内容概括如下：第一章综述准一维量子磁性体系的热传导理论和实验的研究进展。本章首先简单地介绍了低维量子磁性体系的研究历史和现状,之后扼要介绍了热传导测量方法和分析手段,然后重点介绍了S=1/2自旋链体系、S=1Haldane链体系和S=1/2的自旋梯子体系等三种准一维量子磁性体系的热传导理论和实验结果,最后指出了在这些体系的热传导研究中依然存在些问题。第二章主要介绍Zn掺杂LiCu2O2单晶的热输运研究结果。LiCu2O2属于S=1/2阻挫自旋链体系,并且具有内禀的多铁性。发现LiCu2O2单晶的热导率随温度的变化表现出奇特的双峰现象,并研究了非磁性Zn掺杂和外磁场对LiCu2O2单晶的热导率的影响,结果表明48K左右的热导率峰主要来自声子热导率的贡献,14K附近的热导率峰是长程磁有序态的磁激发作为载热准粒子参与热输运的结果。结果还表明其热输运行为类似于三维反铁磁体。第三章主要介绍S=1的Haldane链材料Ni(C3H10N2)2NO2ClO4(简称NINO)单晶的磁热输运研究结果。发现平行和垂直于自旋链方向的热导率表现出明显的各向异性,分析表明这种各向异性特征是由磁激发参与热输运所导致的,并且从总的热导率中分离出磁激发热导率的贡献,发现其在低温下具有近似的e指数的温度依赖关系,暗示着这种材料中弹道的自旋输运。第四章主要介绍S=1/2的类钻石链阻挫材料蓝铜矿(即Cu3(CO3)2(OH)2)单晶的热传导研究结果。热导率随温度和磁场的变化关系表明低温下该材料的声子在部分温区受到显著的共振散射。当体系处于1/3磁化平台时,自旋能隙的打开和增大使磁激发的数量减少,因此声子受到的磁散射减弱,这使被压制的声子热导率得以明显恢复。该结果揭示了该体系较强的自旋声子—耦合。更多还原

【Abstract】 Quasi-one-dimensional (Quasi-ID) quantum spin systems exhibit abundant phase diagrams and exotic physical properties. Theoretical studies revealed that the heat transport is ballistic in the integrabel ID quantum spin systems; it manifests diffusive character for the unintegrabel systems. However, huge magnetic thermal conductivity and large mean free path of magnetic excitations were observed experimentally in S-1/2spin ladders and S=1spin chains, which belong to the unintegrabel systems, indicating the possible ballistic heat transport in these systems. Therefore, there are still needs for further studies on heat transport in the quasi-ID spin systems. In addition, the study of magnetic thermal conductivity in the quasi-ID spin systems is helpful for understanding the scattering mechanism and the dispersion of the magnetic excitations, and the magnetism of these low-dimensional systems.In this dissertation, the low-temperature heat transport properties are studied for two kinds of S=1/2frustrated spin chains and one S=1Haldane chain systems. The heat conduction of magnetic excitations in the antiferromagnetic state for the frustrated systems and the huge magnetic thermal conductivity for the Haldane chain are discussed. The dissertation consists of four chapters; the main contents of each chapter are as following.In chapter One, the theoretical and experimental investigations of heat conduction in quasi-ID quantum spin systems are reviewed. First, the research history and current situation of low-dimensional quantum spin systems are simply introduced, and then the measurement technology and analysis methods of thermal conductivity are also introduced. In particular, the heat transport properties for S=1/2spin chains, S=1Haldane chains and S=1/2spin ladders are focused on. Finally, some open questions in the heat transport study of these systems are mentioned.Chapter Two reports the heat transport study of Zn-doped LiCu2O2single crystals. LiCu2O2contains S=1/2frustrated spin chains and exhibits intrinsic multiferroicity. The main finding is that the temperature dependence of thermal conductivity of LiCu2O2single crystal shows a peculiar double-peak feature. The effect of Zn doping and applying magnetic field on the thermal conductivity is carefully investigated. It is found that the high-temperature peak at about48K is resulted from the phonon conductivity, while the low-temperature peak at about14K is originated from the heat conduction of magnetic excitations in the long-range ordered state. The results indicate that the heat transport of LiCu2O2is similar to that of three-dimensional antiferromagnets.In chapter Three, the heat transport study of an S=1Haldane chain compound Ni(C3H10N2)2NO2ClO4is present. The thermal conductivities show strong anisotropy for the heat flow along and perpendicular to the chain direction, demonstrating that the magnetic excitations take part in conducting heat in this material. The conductivity of magnetic excitations separated from the total conductivity shows an approximately exponential temperature dependence at low temperatures, indicating the ballistic-like spin transport.Chapter Four shows the heat transport study of the S=1/2diamond-like chain frustrated material Cu3(CO3)2(OH)2. From the temperature and magnetic field dependencies of thermal conductivities, it is found that the resonant phonon scattering is strong at low temperatures. The phonon thermal conductivity is significantly recovered at the1/3magnetization plateau. It is due to the open and increase of the spin gap, which reduces the population of the magnetic excitations and leads to the weakened scattering on phonons. These results indicate a strong spin-phonon coupling in this material.更多还原