Studies Of Spin-orbit Coupling And Spin Frustraction Effect In Iridium Compounds

The investigation of the interplay between the electronic, orbit, spin, lattice and orbital degrees of freedom attracts a lot of attentions, in the strongly correlated electron systems field of condensed matter physics. The competition and collaboration between these energies result in multiple ground states, such as modulation-induced superconductivity, quantum criticality state, topological phase transition, metallic-insulating transition, etc. The spin-orbital coupling plays an important role in these extraordinary physical properties, due to the theoretical predictions of exotic phases including Axion insulator, strong topological insulator, and Mott insulator. As spin-orbit coupling (SOC) is essential to realize some topological states, and given the propensity of electron correlations to induce a variety of novel phenomena, there is now intense interest in 5d transition metal compounds with simultaneously strong SOC and intermediate electron correlation.It is generally believed that physical properties of iridium compounds is mainly determined by the formation of Ir4+(5d5, s=1/2).5d orbital electron wave function has a broader space of ductility. In compounds, under the action of the Jahn Teller effect and the crystal field splitting, orbital degeneracy were lifted. Therefore, an intensive study of these systems can enrich our understanding on the condensed matter physics, especially on the behavior of correlated electron and spin-orbitally coupling. Based on these ideas, in this dissertation, the author devoted her effort to the study of the mechanism and spin dynamics in the iridium-based compounds:the spinel CuIr2S4 and the pyrochlore iridate Gd2Ir2O7. This works are achieved the following key findings:1. The orbitally-induced Peierls phase transition in the Na-doped Cu1-xNaxIr2S4 has been investigated. The phase transition temperature on warming TMW increases while that on cooling TMC decreases with x increasing, which means that the phase transition is broadened by the doping of Na ions. However, the average phase transition temperature TM4 is not changed. The Raman spectroscopy and X-ray diffraction studies demonstrate that no structural distortion is induced by the doping of Na in Cu1-xNaxIr2S4, which excludes the lattice distortion effect. The tendency to metallic behavior with the doping of Na indicates hybridization between Na+ and surrounding S2- ions via the outer shell p electrons, which enhances the itinerant characteristic of electrons. Therefore, above TM, there are two kinds of itinerant electrons. The first kind of itinerant electrons are from metallic Ir ions, which will participate into the orbitally-induced phase transition and disappear below TM·The second kind of itinerant electrons are from the hybridization between Na and S, which will persist through the phase transition but not participate into the phase transition. The phase transition is hindered but not prohibited, due to the scattering from the second itinerant electrons. When the temperature decreasing, the dimerization is delayed. When the temperature increasing, the de-dimerization is also postponed. It is suggested that the scattering from itinerant electrons and lattice defects may be responsible for the broadening of the orbitally-induced Peierls phase transition.2. When A3+ ions are magnetic, the magnetic moments from A site would modulate the magnetic ordering on B site in A2Ir2O7 system. As we know, the Gd3+ ions exhibit strong magnetic moments in the compounds. In this work, the pyrochlore iridate Gd2Ir2O7 has been investigated by means of the electron paramagnetic resonance (EPR) spectroscopy. A magnetic transition at T*～20 K is found in addition to the transition at Tc～130 K, which has not been reported previously. The phase transition at Tc, which corresponds to a resistivity transition into variable-range-hopping (VRH) phase, is resulted from the long-range spin ordering of Ir4+ ions in all-in/all out state. However, the magnetic transition at T*, which is regardless of resistivity transition, is not caused by Ir4+ but Gd3+ ions. Both of the magnetic transitions are irrelevant to the lattice distortion, and can be suppressed by the strong external magnetic field. We suggest that the competition between the enhanced paramagnetism of Gd3+ ions and the f-d interaction should be responsible for the magnetic transition at T*.