Doping and Raman scattering of strong spin-orbit-coupling compound Sr2-xLaxIrO4

Abstract

Novel unconventional physical phenomena, such as metal-insulator transition, high temperature superconductivity, colossal magneto-resistance and quantum criticality, are usually found in transition metal oxides (TMOs) with layered perovskite structures. Great success has been achieved in 3d TMOs, in which the localized 3d states yield strongly correlated narrow bands with a large on-site Coulomb repulsion U and a small band width W. Anomalous insulating behaviors are reported in the 5d TMOs, such as Sr2IrO4 system, which is surprising since the 5d TMOs are usually considered as weakly correlated wide band systems with largely reduced on-site Coulomb repulsion U due to delocalized 5d states. The crystal structure of Sr2IrO4 consists of two-dimensional (2D) IrO2 layers, similar to the parent compound La2CuO4 of the cuprates. Theoretically, a variational Monte Carlo study of Sr2IrO4 suggests that d-wave like superconductivity may appear but only within a narrow region of electron doping. In contrast, an s±*-wave phase is established for hole doping deduced from functional renormalization group, and triggered by spin fluctuations within and across the two conduction bands. Moreover, triplet p-wave pairing state with relatively high transition temperature emerges on the hole-doped side when the Hund's coupling is comparable to spin-orbit coupling. Several experiments are tried to search for the predicted unconventional superconductivity due to both electron-and hole-doping. However, to the best of our knowledge, it has not been found yet in the carrier-doped Sr2IrO4 system. Hence, more detailed studies are needed to explore the potential superconductivity.#br#A series of La doped Sr2-xLaxIrO4 samples is synthesized based on solid state reaction method. The evolution of the crystal structure is studied by the X-ray diffraction, scanning electron microscopy, together with the Raman spectrum. It is found that the crystal constant of the c-axis decreases with increasing doping level as well as the apical Ir—O1 bond length, indicating the lattice construction. Moreover, the distortion of the IrO6 octahedron reduces with increasing doping level. Therefore, blue shift occurs of the Raman scattering peaks. The temperature dependence of the Raman spectrum is also studied. It is found that the frequencies of the A1g and B1g vibration modes increase with temperature decreasing and an abnormal jump occurs around 110 K, which is believed to be correlated with the structural change and the magnetic transition around this temperature.