arXiv 0709.37112007-09cond-mat.str-el

光电导法研究LaSrMnO4中Mott-Hubbard与电荷转移行为

Mott-Hubbard versus charge-transfer behavior in LaSrMnO4 studied via optical conductivity

A. Gössling, M. W. Haverkort, M. Benomar, H. Wu, D. Senff, T. Möller, M. Braden, J. A. Mydosh, M. Grüninger

备注：: 13 pages, 10 figures
发表信息：: Phys. Rev. B 77, 035109 (2008)
DOI：: 10.1103/PhysRevB.77.035109

摘要：

利用椭圆偏振光谱法研究了绝缘LaSrMnO_4在0.75-5.8 eV能量范围内的光学电导率σ（Ω），温度范围为15 - 330 K。层状结构引起明显的各向异性。在sigma_1^a（omega）中观察到多峰结构（2、3.5、4.5、4.9和5.5 eV），而在sigma_1^c（omega）中仅在5.6 eV处存在一个峰。我们采用局部多重态计算，并获得（i）光学数据的良好描述，（ii）Mott-Hubbard多重态分裂和电荷转移吸收带的详细峰归属，以及（iii）电子结构的有效参数，例如，原位库仑排斥U_eff=2.2 eV，面内电荷转移能量Δ_a=4.5 eV，以及d^4构型的晶场参数（10 Dq=1.2 eV，Δ_eg=1.4 eV，Δ_t2g =0.2 eV）。最低吸收特征（在1-2 eV处）的光谱权重作为温度的函数而改变2倍，这可以归因于跨Neel温度的最近邻自旋-自旋关联函数的改变。将LaSrMnO 4有效地解释为Mott-Hubbard绝缘体自然解释了这种强烈的温度依赖性，不同吸收峰的相对权重以及明显的各向异性。通过透射率测量，我们确定了光学带隙Δ a_opt的起始值在15 K时为0.4-0.45 eV，在300 K时为0.1-0.2 eV。我们的数据表明，晶场分裂太大，不能解释的c轴晶格参数的温度依赖性的激发晶场能级的热占领。另外，我们建议，热人口的上哈伯德带引起的c轴晶格参数的收缩。

英文摘要：

Using spectroscopic ellipsometry, we study the optical conductivity sigma(omega) of insulating LaSrMnO4 in the energy range of 0.75-5.8 eV from 15 to 330 K. The layered structure gives rise to a pronounced anisotropy. A multipeak structure is observed in sigma_1^a(omega) (2, 3.5, 4.5, 4.9, and 5.5 eV), while only one peak is present at 5.6 eV in sigma_1^c(omega). We employ a local multiplet calculation and obtain (i) an excellent description of the optical data, (ii) a detailed peak assignment in terms of the multiplet splitting of Mott-Hubbard and charge-transfer absorption bands, and (iii) effective parameters of the electronic structure, e.g., the on-site Coulomb repulsion U_eff=2.2 eV, the in-plane charge-transfer energy Delta_a=4.5 eV, and the crystal-field parameters for the d^4 configuration (10 Dq=1.2 eV, Delta_eg=1.4 eV, and Delta_t2g=0.2 eV). The spectral weight of the lowest absorption feature (at 1-2 eV) changes by a factor of 2 as a function of temperature, which can be attributed to the change of the nearest-neighbor spin-spin correlation function across the Neel temperature. Interpreting LaSrMnO4 effectively as a Mott-Hubbard insulator naturally explains this strong temperature dependence, the relative weight of the different absorption peaks, and the pronounced anisotropy. By means of transmittance measurements, we determine the onset of the optical gap Delta^a_opt=0.4-0.45 eV at 15 K and 0.1-0.2 eV at 300 K. Our data show that the crystal-field splitting is too large to explain the anomalous temperature dependence of the c-axis lattice parameter by thermal occupation of excited crystal-field levels. Alternatively, we propose that a thermal population of the upper Hubbard band gives rise to the shrinkage of the c-axis lattice parameter.

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