Low-temperature Heat Transport Of Spin Frustrated Materials R₂Ti₂O₇

Spin frustrated materials are one of the most important subjects in condensed matter physics. The geometrically frustrated systems fail to enter a conventional magnetic ordering state as a result of substantial degeneracy ground states. The spin fluctuation resists till extremely low temperature, and induces abundant exotic magnetic ground states, such as spin ice and spin liquid states. Moreover, the quantum spin ice has attracted extensive research interests these years, which is regarded as a strongly correlated system with spin frustration. The quantum magnetic monopole therein is a kind of novel elementary excitation.In this thesis we study the spin frustrated materials R₂Ti₂O₇?R = Dy, Yb, Tb?with pyrochlore struture. We take the low-temprature heat transport measurement to study the spin fluctuation and magnetic excitations in these materials. At low temperature, the scattering of phonon is contributed to the magnetic fluctuation and magnetic excitations. The heat conductivity can be a useful tool to detect ground state properties in these matirial and provide information to understand these exotic ground states.In chapter one, we introduce the geometrical frustration in pyrochlore structure,the abundant ground states of rare earth titanium oxides, and research progress of spin ice Dy₂Ti₂O₇, spin liquid Tb₂Ti₂O₇, and Yb₂Ti₂O₇ with quantum transitions at low temperature, including some unresolved issues of these materials.In chapter two, we study the irreversibility and strong relaxation effect of thermal conductivity in Dy₂Ti₂O₇. We find the irreversibility is highly related to the relaxation effect, and the machanism for H// [111] is different from for H// [100] and [110]. The decrease of ??H? at low field can not be explained by the depression of magnetic monopole transport by magnetic field. Moreover, thermal conductivity is strongly influenced by the magnetic field even at high field ?H> 7 T?, which was unrevealed in earlier experiments. These behaviors indicate that the ? of Dy₂Ti₂O₇ is mainly attributed to phonons, the magnetic monopoles play a minor role in carrying heat.In addition, the slow spin dynamics is responsible for the irreversibility and relaxation effect.In chapter three, we study the single-crystal growth and the ultra-low temperature heat transport of quantum spin ice Yb₂Ti₂O₇. We observe a kink in ??T? at 200 mK, which is associated to the quantum phase transition, Above the transtion temperature,the quantum magnetic monopoles contribute to the thermal transport; the field dependencies of ? are mainly determined by the suppression of spin fluctuations when the spins are polarized, and the field-induced transitions can cause a sharp diplike anomaly in ??H?. Inaddition, the ??H? show sharp dip corresponding to the magnetic field-induced transition. It is difficult to understand that the T3 behavior of k4T) cannot be observed in high field, which indicates that besides the impacts asociated with the low-field magnetic transitions and spin fluctuations, some other factors are also important.In chapter four, we study the single crystal growth of Zr-doped Tb₂Ti_Z-xZr_XO₇ ?x =0.02, 0.1, and 0.4? and the magnetic susceptibility, specific heat, thermal conductivity conductivity were measured. We found that the doped samples show obvious magnetic anistropy, the specific heat analysis indicates that the Zr doping induces the splitting of ground-state doublet, the thermal conductivity results with Zr doped samples reveal that the magnetic corelation of Tb³⁺ ions is weakened and spin fluctuation was supressed.