Femtosecond Resolution of Soft Mode Dynamics in Structural Phase Transitions

• Published in: Science (American Association for the Advancement of Science) Vol. 258; no. 5083; pp. 770 - 774
• Main Authors: Dougherty, Thomas P., Wiederrecht, Gary P., Nelson, Keith A., Garrett, Mark H., Jensen, Hans P., Warde, Cardinal
• Format: Journal Article
• Language: English
• Published: Legacy CDMS American Society for the Advancement of Science 30.10.1992; American Association for the Advancement of Science
• Subjects: Analysis; Crystal lattices; Crystals; Cubic crystals; Exact sciences and technology; Lattice vibrations; Mathematical minima; Perovskite; Phase transformations (Statistical physics); Phase transitions (Physics); Phonons; Physics; Potential energy; Quantum ensemble theory; Quantum statistical mechanics; Raman scattering; Raman spectroscopy; Research Article; Solid-State Physics; Statistical physics, thermodynamics, and nonlinear dynamical systems; Vibration mode; Femtosecond; Time resolution; Phase transformation; Ferroelectric materials; Perovskite type compound; Raman scattering; Transition metal Compounds; Phonon mode; Barium Titanates; Potassium Niobates; Inorganic compound; Soft mode
• ISSN: 0036-8075; 1095-9203
• DOI: 10.1126/science.258.5083.770

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.