Charge and Spin Coupling in 3d-Transition-Metal Oxides with Perovskite-Related Structures

本研究旨在阐明一系列具有钙钛矿结构的3D过渡金属氧化物中掺杂空穴的电荷动力学。这些氧化物基本上是反铁磁性绝缘体。我们通过光谱学证明，当自旋系统被稀释或长程自旋有序被掺杂空穴阻挡时，光谱重量从高能区转移到低能区。低能激发形成所谓的中红外吸收带，这是所有空穴掺杂的3D过渡金属氧化物的共性。红外中带的强度与局域自旋有序有关。由于二维强烈的自旋涨落，自旋系统与电荷动力学之间的密切联系在高T_c的铜氧化物中表现得最为生动。研究已经扩展到一维镍和铜氧化物，其中由于一维更强的自旋涨落，即使在母体绝缘体中也形成了一些量子自旋流体。Y_2BaNiO_5的MuSR实验证实了量子自旋流体的存在，并成功地进行了空穴掺杂，但化合物仍具有绝缘性。本研究的结果和策略将为进一步研究掺杂量子自旋流体中的电荷动力学提供基础。

The present research aimed to elucidate the charge dynamics of doped holes in a series of 3d-transition-metal oxides with perovskite-related structures. These oxides are basically antiferromagnetic insulators. We demonstrate by the potical spectroscopy that as the spin system is diluted or the long-range spin order is frustrated by the doped holes, the spectral weight is transferred from high-energy to low-energy region. The low-energy excitations form a so-called mid-IR absorption band which is generic to all 3d-transition-metal oxides doped with holes. The intensity of the mid-IR band is connected with the local spin order. The close connection between the spin system and the charge dynamics is most vividly displayd in the high-T_c Cu oxides owing to strong spin fluctuations in two dimensions. The research has been extended to one-dimensional Ni and Cu oxides in which some quantum spin fluid is formed even in a parent insulator due to much stronger spin fluctuations in one dimension. The presence of quantum spin fluid has been confirmed by muSR experiment for Y_2BaNiO_5, and hole doping into this compound has been successfully done, though the compound remains to be insulating. The result and strategy developed in the present research will provide a basis for future study of the charge dynamics in doped quantum spin fluid.