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首页> 外文期刊>Physical review >Near-band gap electronic structure of the tetragonal rare-earth cuprates R_2CuO_4and the bismuth cuprate Bi_2CuO_4
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Near-band gap electronic structure of the tetragonal rare-earth cuprates R_2CuO_4and the bismuth cuprate Bi_2CuO_4

机译：四方稀土铜酸盐R_2CuO_4和铋铜酸盐Bi_2CuO_4的近带隙电子结构

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摘要

Complex optical dielectric function in the tetragonal rare-earth cuprates R_2CuO_4 (R=La, Pr, Nd, and Sm) and in the tetragonal bismuth cuprate Bi_2CuO_4 is studied in the spectral range of 0.6-5.4 eV using a method of optical ellipsometry. The dielectric spectra are studied for the two main polarizations and analyzed in terms of a cluster model for CuO_4~(6-) complexes taking into account intracenter p-d and intercenter d-d charge-transfer (CT) transitions. The band gap in the rare-earth cuprates is defined by an electric-dipole-allowed CT transitions centered at 1.54-1.59 eV in Pr, Nd, and Sm cuprates, and 2.1 eV in La cuprate. Optical response of Bi_2CuO_4 strongly differs from the rare-earth cuprates which we relate with strong covalency of Bi-O bonding and strong ionicity of Cu(3d)-O(2p) bonding. These features are manifested in suppression of low-energy intense intracenter p-d and intercenter d-d CT transitions, and by appearance of strong intense absorption bands near 5 eV. Regardless the strong distinctions of optical response, on one hand, of La, Pr, Nd, and Sm cuprates, and on the other hand, of the Bi cuprate, the dielectric gap in these compounds shows comparable values defined by a superposition of intracenter p-d CT transitions and two-center d-d CT transitions. Thus these cuprates should be classified as compounds intermediate between CT and Mott-Hubbard insulators.
机译：使用光学椭偏法研究了在稀土稀土四方酸盐R_2CuO_4（R = La，Pr，Nd和Sm）和四方铜铋酸盐Bi_2CuO_4中的复光学介电函数，其光谱范围为0.6-5.4 eV。研究了两种主要极化的介电谱，并根据考虑中心内p-d和中心间d-d电荷转移（CT）跃迁的CuO_4〜（6-）配合物的簇模型进行了分析。稀土铜酸盐中的带隙由允许电偶极子的CT跃迁定义，Pr，Nd和Sm铜酸盐的中心为1.54-1.59 eV，La铜酸盐的中心为2.1 eV。 Bi_2CuO_4的光学响应与稀土铜酸盐有很大的不同，我们与Bi-O键的强共价性和Cu（3d）-O（2p）键的强离子性有关。这些特征表现为抑制低能量的强烈中心内p-d和中心间d-d CT转换，以及在5 eV附近出现强烈的强吸收带。一方面，La，Pr，Nd和Sm铜酸盐的光学响应有很强的区别，另一方面，Bi铜酸盐的光学响应的​​显着区别，这些化合物中的介电间隙显示出可比的值，这些值由中心内pd的叠加确定CT过渡和两中心dd CT过渡。因此，这些铜酸盐应归类为CT和Mott-Hubbard绝缘体之间的中间化合物。

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