MgB_2各向异性光学性质的第一性原理研究

Ab-initio investigation of anisotropic optical properties of MgB_2

使用密度泛函第一性原理研究了超导体MgB2单晶各向异性的光学性质.在描述光学性质的基本理论和计算方法的基础上,计算了MgB2的光电导谱、反射谱以及电子能量损失谱,并通过MgB2的各个原子分解态密度图对所得到的反射谱和损失谱的各个谱峰做了详尽地分析.从光电导谱上来看,x方向与z方向有着很大差别,而在反射谱与电子能量损失谱中,x方向与z方向的特征峰位置都是相互符合的.从光导谱来看,沿x方向的第一个带间吸收峰出现在20000 cm-1处,而沿z方向出现在40000cm^-1处.考虑到温度效应对其光学性质的影响,在计算光学矩阵元时加入Lorentz展宽δ=0.10 eV.计算结果和最近实验结果有比较好的一致,只是带间吸收谱峰位置和实验之间存在约1000 cm^-1（-0.124 eV）的差别.总体上该研究的计算结果从定性上和定量上都与最新各向异性光电导实验结果在误差范围内符合很好.

The density functional theory and full-potential linearized augmented plane wave （FLAPW） method are used to study the anisotropic optical properties of superconductor MgB2 . The optical conductivity spectra, the reflectivity spectra and electron energy loss spectra have been calculated. The calculated optical conductivity spectra along xx-axis display quite different features in comparison with that along the zz-axis,while the reflectivity spectra coincide well with the corresponding electron energy loss spectra, especially with respect to the characteristic peaks. In the optical conductivity spectra, the first intra-band absorption peak along xx-axis appears at 20000 cm^-1 while along zz-axis it appears at a high frequency of about 40000 cm^-1 In order to simulate the temperature effect in these spectra, the Lorentz expansion （-δ = 0.10 eV） is added to the optical matrix elements in the calculation for 0 K. In comparison with existing experimental data,a minor discrepancy of 1000 cm^- 1 （ - 0. 124 eV） has been found in the calculated optical spectra.