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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