Spectroscopic Studies on Mechanism of Metal-Insulator Transition of Titanium and Vanadium Oxides

最近，Tanaka提出了一个新的金属-绝缘体转变(MIT)机制的统一模型[1]。对于Ti_2O_3，MIT发生在450K左右，从低温的绝缘相到高温的金属相。在他的模型中，钛离子对的Ti3d电子构型从(a_&lt；1g&gt；↑；a_&lt；1g&gt；↓)变化到(a_&lt；1g&gt；↑；e_g(π)↑))，这种变化导致了Ti2O_3的MIT。他还指出，在Ti2p-3DX射线吸收光谱及其线性二色性(XAS/LD)中可以清楚地观察到这种电子组态的变化。在本研究中，我们在广岛同步辐射中心Hisor的光束线BL14上生长了Ti2p-3DXAS/LD单晶，并在300K(绝缘相)和520K(金属相)之间进行了Ti2p-3DXAS/LD实验。XAS谱显示了一个显著的LD，反映了Ti3D态的各向异性。520K时的激光强度比300K时的强度降低了60%，表明Ti3D态的各向异性减小。XAS/LD谱的多重分析表明，在520K和300K时，(a&lt；1g&gt；↑；a&lt；1g&gt；↓)：(a&lt；1g&gt；↑；e_g(π)↑)=0.55：0.45，0.15：0.85。激光强度只在MIT温度附近变化，说明Ti3d电子组态的变化与MIT有关。这些结果支持Tanaka提出的MIT模型[1]。我们还从Ols-XAS谱中发现了Ti3d态的变化。此外，Ti2p软X射线发射谱仅在金属相处有一个弹性峰。SoC。日本。73,152(2004年)。

Recently, Tanaka proposed a new unified model for a mechanism of Metal-Insulator Transition (MIT) for Ti_2O_3 and VO_2 [1]. In case of Ti_2O_3, MIT takes place around 450 K from an insulating phase at low temperatures to a metallic phase at high temperature phase. In his model, in accordance with MIT, a Ti 3d electron configuration of the Ti ion pair changes from (a_<1g>↑ ; a_<1g>↓) to (a_<1g>↑; e_g(π)↑) and this change leads to MIT of Ti_2O_3. The mechanism of MIT for VO_2 is also interpreted based on the same scenario. He also pointed out that such a change of the electronic configuration is clearly observed in the Ti 2p-3d x-ray absorption spectra and its linear dichroism (XAS/LD).In the present study, we have grown Ti_2O_3 single crystals and carried out the Ti 2p-3d XAS/LD experiments between 300 K (insulating phase) and 520 K (metallic phase) on the beamline BL14 of HiSOR, Hiroshima Synchrotron Radiation Center. The XAS spectra show a remarkable LD, reflecting an anisotropy of the Ti 3d states. The LD intensity at 520 K is reduced to 60 % relative to that at 300 K, which indicates a decrease of the anisotropy of the Ti 3d states. The multiplet analyses for the XAS/LD spectra give that (a_<1g>↑; a_<1g>↓) : (a_<1g>↑ ; e_g(π)↑) = 0.55:0.45, 0.15:0.85 at 520 and 300 K, respectively. The LD intensity changes only around MIT temperature, indicating that the change of the Ti 3d electronic configuration is related to MIT. These results support the MIT model proposed by Tanaka [1]. We have also found the change of the Ti 3d states from the O ls XAS spectra. In addition, the Ti 2p soft x-ray emission spectra exhibit an elastic peak only at metallic phase.[1] A. Tanaka, J. Phys. Soc. Jpn. 73, 152 (2004).