Laser Raman study of quasi-spin wave hydrogen tunneling modes in KH 2PO 4 and KD 2PO 4

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The low frequency laser excited Raman spectra of KH 2PO 4 and KD 2PO 4 single crystals have been measured at room temperature for various orientations and a complete symmetry assignment of the observed modes has been made. In addition to the strongly damped “ferroelectric” quasi-spin wave-like proton mode of symmetry B 2 observed by Kaminov, a non-degenerate proton mode at 397 cm −1 and a double degenerate proton mode around 80 cm −1 of predicted symmetry properties have been found. The frequencies of all these proton modes are reduced on deuteration by a factor of ten or more, thus demonstrating their collective proton tunneling nature.