Thermal Conduction Study In Quasi-One-Dimensional Quantum Magnets

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.