Low-Temperature Heat Transport And Magnetic Transitions Of Transition-Metal Compounds

In the transition-metal compounds, the interactions among lattice, charge, spin, and orbital degrees of freedom could result in many kinds of quantum state. External parameters, such as magnetic field, pressure, and chemical composition, could induce the competition and transition among different quantum states and the occurrences of quantum critical phenomena. These compounds consequently exhibit many exotic physical properties, such as high-Tc superconductivity, colossal magnetoresistance, multiferroicity and giant thermoelectricity, etc. The various magnetisms (like quantum magnetism) and magnetic phase transitions caused by the spin correlations are also very important physical properties and hot topic of the transition-metal compounds. Recently, it is found that the heat transport exhibits a lot of peculiar phenomena, which is due to the dual roles of elementary excitations in magnetic states. The magnetic excitations can either transport heat as carriers or strongly scatter phonons. Therefore, it is helpful for us to deeply understand the magnetisms and magnetic phase transitions of the transition-metal compounds by studying the heat transport properties of the magnetic excitations.In this dissertation, the low-T heat transport properties are studied for several representative transition-metal compounds, such as low-dimensional quantum magnets, high-Tc cuprates, and multiferroic materials. The dissertation is composed of five chapters; the major contents of each chapter are as follows.In chapter One, the exotic physical properties and their developments of low-T heat transport study of transition-metal materials are summarized. First, the quantum ground states and phase transitions in low-dimensional quantum magnets are introduced. The heat transport properties of magnetic excitations are also included. Second, the important role of thermal conductivity in determining the symmetry of the order parameter and the metal-insulator crossover of the ground state of normal state in the high-Tc cuprates is reviewed. In order to judge whether the phonon conductivity of the superconductor depends on the magnetic field, the heat transport properties of the parent insulating cuprates are discussed. Finally, the spin-induced multiferroicity is briefly introduced. Detailed introduction of the exchange striction mechanism in the rare-earth ortheferrites is given.Chapter Two reports the heat transport study of quasi-one-dimensional spin chain material BaCo2V2O8, with the magnetic field applied along and perpendicular to the spin chain direction. In the longitudinal field, the nearly isotropic temperature dependencies of thermal conductivity demonstrate the scattering of magnetic excitations on phonons. With increasing the field, the thermal conductivity shows a sharp decrease when the system undergoes a transition from the Neel order to the incommensurate state of Co2+spins. The case for the transverse field is different, which presents a field-independent thermal conductivity at low temperatures except for a deep minimum around10T. In the transverse field, the phase boundary given by our heat transport results is different from that determined by specific heat, suggesting that some unknown field-induced magnetic transition is observed inside the Neel state.In chapter Three, a kind of quasi-one-dimensional organic spin chain material (CH3)2CHNH3CuCl3(IPA-CuCl3) is studied at ultra-low temperatures. The ground state of IPA-CUCl3is gapped and the system enters the long-range antiferromagnetic order when the gap is closed by the applied field. This was discussed as a magnon Bose-Einstein Condensation (BEC). Due to the presence of the gap, the zero-field thermal conductivity is completely ascribed to the phonon transport, which reaches the boundary scattering limit at about1K. The drastic increase of thermal conductivity across the BEC transition is obviously originated from the contribution of magnon heat conduction, and the magnons exhibit a ballistic transport behavior below500mK. In addition, the field dependencies of thermal conductivity at ultra-low temperatures indicate that the BEC state is gapped with the gap value about0.013meV, which is contrary to the gapless state observed through neutron scattering technique.Chapter Four reports the thermal conductivity of electron-doped cuprates Nd2-xCexCu04. In the insulating Nd2CuO4, the strong magnetic-field dependence of thermal conductivity results from the phonon scattering by the paramagnetic spins. The magnetic excitations of Nd3+could also take part in transporting heat. The magnetic structure of the ground state of Nd2CuO4and its evolution in different field directions are suggested from the field dependencies of thermal conductivity. On the other hand, the paramagnetic scattering on phonons is still dominant in the heat transport of Nd2-xCexCuO4(x=0.02,0.10,0.18) samples. Due to the strong magnetic-field dependencies of phonon conductivity, it is unable to separate the electron term from the phonon conductivity and obtain the quasiparticle heat transport properties for this kind of electron-doped cuprate superconductors.Chapter Five presents the low-T electric polarization, magnetization, and thermal conductivity of multiferroic orthoferrites RFeO3(R=Gd, Dy). GdFeO3shows an irreversible field-dependent thermal conductivity while the magnetization is reversible. The phonon scattering by the ferroelectric domain walls could be responsible for the irreversible thermal conductivity. In spite of similar crystal structures, the case for DyFeO3is more complicated. When the crystal is cooled in zero magnetic field, the low-T thermal conductivity, electric polarization and magnetization all show irreversible field dependencies, and these irreversible behaviors are present only in the initial sweeping-up-and-down run and are absent in the following sweeping field process. The irreversible behaviors suggest a metastable state formed in the zero-field cooling, which disappears once the system undergoes a transition after applying field, leading to the irreversible field dependencies of physical properties.