Electronic structure of pure and defective PbWO4, CaWO4, and CdWO
Optical constants, reflectivity, and exciton binding energy
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Optical constants, reflectivity, and exciton binding energyAlthough density functional theory is rigorously a ground-state formalism, there has recently been considerable progress in developing methods to calculate optical properties using density functional results as the starting point [19]. As a first step toward investigating the optical properties, we have calculated the imaginary part of the dielectric constant from the self-consistent LAPW wavefunctions  and one-electron eigenvalues Enk, using the code developed by Abt and Ambrosch-Draxl [20]. There are of course no excitonic effects included in these calculations. Taking the Kramers-Kronig transform of 2, we obtain the calculated spectrum of 1 after adjusting the calculated band gap and calculated visible refractive index to agree with experiment. [21 ] Our calculated reflectivity was compared to the experimental measurement by Shpinkov et al [22] in Refs. [2,3]. The agreement between the measured and calculated reflectivity for PbWO4 is surprisingly good. The sharp peak in the calculated spectrum at the band edge is due to a near singularity in the joint density of single-particle states. Since no lower-energy discrete features are found in the experimental spectrum, we conclude that whatever exciton discrete states are observable in the absorption spectrum should have a low binding energy compared to the 0.3 eV width of the experimental reflectivity peak. The suggestion of a small exciton binding energy in PbWO4 was supported by consideration in Ref. [3] of the measured optical and static dielectric constants for PbWO4 which are quite large -- 1(1.9 eV) εopt = 5.06 [21] and 1(0 eV) εstatic = 23.6, respectively, for a-axis polarization.
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