The following article appeared as Appl. Phys. Lett 108, 262901 (2016) .
Judy G. Cherian, Turan Birol, Nathan C. Harms, Bin Gao, Sang-Wook Cheong, David Vanderbilt, and Janice L. Musfeldt, Optical spectroscopy and band gap analysis of hybrid improper ferroelectric Ca3Ti2O7
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