Low-Temperature Heat Transport Of Spin-Liquid Materials

Geometrically frustrated spin systems are one of the most interested subjects in condensed matter physics, due to their exotic ground state properties and quantum phase transition behaviors. Hence, it is expected to be excellent systems to seek exotic quantum-mechanical ground states, such as the quautum spin liquids, which has received considerable attention recently. Physics of quantum spin liquids is very helpful in understanding the mechanism of high-temperature superconductivity, and quantum-spin-liquid materials are possible to realize the application in quantum information. Although substantial theoretical researches have been carried out for years, until recently only few frustrated magnets have been found to be promising candidates as quantum spin liquids.We carefully studied the low-T heat transport properties of rare-earth pyrochlore structure Tb2Ti2O7and rare-earth langasites Nd3Ga5SiO14, which were reported as spin-liquid candidates. In these two types of spin frustrated materials, geometrically frustrated effect induced by competing spin interactions gives rise to macroscopic degeneracy of ground state. The low-T heat transport has been found to be an effective way to probe the properties of quantum ground state, magnetic-field induced quantum phase transitions and also the spin-phonon coupling, which will provide usefull information to understand the nature of spin-liquid ground state.In chapter one, we introduce the main progress on geometricallty frustrated spin systems and spin-liquid systems. First, the spin frustrated magnets with spin liquid state are summarized in detail. Second, the ground state properties and research progress of Tb2Ti2O7are presented. Third, the ground state properties and research progress of Nd3Ga5SiO14are introduced. Finally, the basic knowledges of heat transport are briefly offered.In chapter two, high-quality rare-earth titanates R2Ti2O7(R=Gd, Tb, Dy, Ho, Y, Er, Yb and Lu) and Y-doped Tb2-xYxTi2O7(x=0.2,1.0and1.6) single crystals were successfully obtained using an optical floating-zone method, and growth conditions were carefully studied. It is found that the growth rate, atmosphere and oxygen pressure should be carefully adjusted to get high-quality single crystals and they differ from each other for different rare-earth ions. The structure and crystallinity of single crystals were characterized by X-ray diffraction and Laue photographs. The basic physical properties of R2Ti2O7were characterized by the magnetic susceptibility and specific heat measurements. Moreover, to study the magnetic dilution effect of the spin liquid Tb2Ti2O7, we carefully measured the magnetic susceptibility, specific heat and thermal conductivity of Tb2-xYxTi2O7single crystals.Chapter Three reports a study on the thermal conductivity of Tb2Ti2O7single crystals at low temperatures. It is found that the thermal conductivity shows a phonon-glass-like behavior in zero field. The mean free path of phonons is even smaller than that of amorphous materials and is3-4orders of magnitude smaller than the sample size at0.3K. The phonon-glass-like thermal conductivity behavior is due to the phonon scattering by the strong spin fluctuations in the spin-liquid state of Tb2Ti2O7. Moreover, the magnetic-field dependencies of k(H) show complicated behaviors and the striking finding is that at low temperatures the k(H) can be enhanced by30-40times with increasing field, which is mainly because that magnetic field suppresses the spin fluctuations and therefore weakens the phonon scattering. And the comparison with TbYTi2O7thermal conductivity further indicates the phonons are strongly scattered by the spin fluctuations in Tb2Ti2O7.In chapter four, we study the low-temperature heat transport of Nd3Ga5SiO14, which is a spin-liquid candidate, to probe the nature of the ground state and the effect of the magnetic field on the magnetic properties. The thermal conductivity shows a purely phononic transport in zero field. The external magnetic field along the c axis induces a diplike behavior of k(H), which can be attributed to a simple paramagnetic scattering on phonons. However, the magnetic field along the ab plane induces another steplike decrease of k. This kind of k(H) behavior is discussed to be related to a field-induced partial order, which yields low-energy magnetic excitations that significantly scatter phonons. These results point to a paramagnetic ground state that partial magnetic order can be induced by magnetic field along the ab plane, which is also signified by the low-T specific heat data.