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Journal Article

Femtosecond Resolution of Soft Mode Dynamics in Structural Phase Transitions

Thomas P. Dougherty, Gary P. Wiederrecht, Keith A. Nelson, Mark H. Garrett, Hans P. Jensen and Cardinal Warde
Science
New Series, Vol. 258, No. 5083 (Oct. 30, 1992), pp. 770-774
Stable URL: http://www.jstor.org/stable/2880323
Page Count: 5
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Femtosecond Resolution of Soft Mode Dynamics in Structural Phase Transitions
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Abstract

The microscopic pathway along which ions or molecules in a crystal move during a structural phase transition can often be described in terms of a collective vibrational mode of the lattice. In many cases, this mode, called a "soft" phonon mode because of its characteristically low frequency near the phase transition temperature, is difficult to characterize through conventional frequency-domain spectroscopies such as light or neutron scattering. A femtosecond time-domain analog of light-scattering spectroscopy called impulsive stimulated Raman scattering (ISRS) has been used to examine the soft modes of two perovskite ferroelectric crystals. The low-frequency lattice dynamics of KNbO$_3$ and BaTiO$_3$ are clarified in a manner that permits critical evaluation of microscopic models for their ferroelectric transitions. The results illustrate the advantages of ISRS over conventional Raman spectroscopy of low-frequency, heavily damped soft modes.

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