Very-low-temperature Thermal Conductivity Of (Pr, La)_2-xCexCuO₄ Single Crystals

Superconducting material is characterized by zero resistance and Meissner effect. The transition temperatures of high-temperature superconducting cuprates are much higher than that of conventional superconductors. In order to find new superconductors with higher transition temperatures, the study of the mechanism of high-temperature superconductivity is very important. To elucidate the quasiparticle transport in high- temperature cuprates, the magnetic-field (H) dependence of thermal conductivity (κ) is widely studied at low temperature. However, phonons and magnetic excitations make thermal conductivity behaviors very complex at low temperature. So it is useful to study parent insulating cuprate to gain insight into the possible magnetic-field (H) dependence of phonon conductivity at low temperature. In this thesis, the growth of the electron-doped cuprates and the thermal transport properties at low temperature are studied. The results are useful for understanding the magnetic-field (H) dependence of quasiparticle transport properties of cuprate superconductors.In chapter 1, we introduce the progresses in the study of high-temperature superconductivity and the crystal growth methods, such as self-flux method and optical floating zone method. At the same time we briefly review the research results in the thermal and electronic transport, particularly in detecting the normal state by these measurements under high magnetic fields.In chapter 2, high quality Pr2CuO4 single crystals are successfully grown by using CuO as a self-flux and with a proper thermal cycle. The obtained single crystals are characterized by various X-ray diffraction methods, including 00l diffractions and rocking curves, and Laue photographs determining the a, b and c axes. It is found that the crystals are of good quality. Additionally we measure magnetization and resistivity of Pr2CuO4 single crystals.In chapter 3, the magnetic-field (H) dependence of the thermal conductivity (κ) of Pr2CuO4 crystals are carefully studied, theκ(H) behavior shows a"dip-like"feature at low field and a"plateau"feature at high field. These phenomena are discussed to be due to the scattering of phonons by paramagnetic ions. The specific heat data of Pr2CuO4 crystals indicate that the paramagnetic ions are likely Pr4+ ions with zero-field splitting ground state. The present results demonstrate a strong spin-phonon coupling in high-Tc cuprates.