KIAS- APCTP Workshop on "Quantum Materials"
at Korea Institute for Advanced Study (KIAS), Seoul, Korea
July 19(Wed)-22(Sat), 2006
Title | Orbital dependent spectral weight changes in layered ruthenates |
Abstract |
Recently, the role of orbitals in the physical properties of transition-metal oxides has attracted lots of attention in the condensed matter physics community. It has been found that intriguing physical properties of numerous transition-metal oxides could be explained by taking account of the orbital degree of freedom. Especially in the layered ruthenates, the orbital degrees of freedom are known to play key roles in their intriguing physical properties.
Quite recently, we demonstrated that optical spectroscopy could be a very useful and powerful method to investigate the orbital degree of freedom. In this talk, we will address some intriguing physical phenomena occurring in layered ruthenates, Ca2-xSrxRuO4 and Ca3Ru2O7, by investigating their optical spectra.
After the discovery of p-wave superconductivity in Sr2RuO4, the layered ruthenates Ca2-xSrxRuO4 have attracted lots of attention. While the metal-insulator transition occurs for x<0.2, anomalous enhancements of magnetic susceptibility and specific heat coefficient were observed near x = 0.5. These surprising experimental behaviors have been explained in terms of the orbital-selective Mott transition, where most theoretical works assume opening of Mott gap in the narrow dyz/dzs bands at x=0.5. However, the sum rule arguments on our optical spectra of Ca2-xSrxRuO4 clearly demonstrated that the heavy mass state should not come from the strong renormalization of the dyz/dzs bands. Based on the newly derived extended Drude model for a multiband system, we could show that the mass anomaly at x = 0.5 could be compatible with our optical conductivity data only if the strong renormalization occur in the dxy band. Another intriguing orbital dependent spectral change occurs in Ca3Ru2O7. Optical spectra of this double-layered perovskite ruthenate show a pseudo-gap opening around 200 cm-1 below 50 K, which should be attributed to the partial k-space gap opening due to the density wave instability. However, its spectral weight change is quite different from the prediction of the BCS theory for the density wave: the reduced spectral weight below the pseudogap energy is redistributed to a strong incoherent excitation around 800 cm-1. These intriguing spectral changes might be understood in terms of an orbital-related excitation in the spin/charge density wave ground state. |