An international team of physicists has developed a method for taking ultrafast "sonograms（声波图） " that can track the structural changes that take place within solid materials in trillionth-of-a-second intervals as they go through an important physical process called a phase transition. Common phase transitions include the melting of candle wax before it burns and dissolving sugar in water. They are purely structural changes that produce dramatic changes in a material's physical properties and they play a critical role both in nature and in industrial processes ranging from steel making to chip fabrication（制造，装配） .

The researchers have applied this method to shed new light on the manner in which vanadium（钒） dioxide, the material that undergoes the fastest phase transition known, shifts between its transparent and reflective phases.

Many of these transitions, like that in vanadium dioxide, take place so rapidly that scientists have had difficulty catching them in the act. "This means that there is a lot that we still don't know about the dynamics of these critical processes," said Professor of Physics Richard Haglund, who directed the team of Vanderbilt researchers who were involved.

To build a more complete picture of this phenomenon in vanadium dioxide (VO2), one of the most unusual phase-change materials known, Vanderbilt researchers collaborated with physicists at the Fritz Haber Institute of the Max Planck Society in Berlin, who have developed the powerful new technique for obtaining a more complete picture of ultrafast phase changes. Details of the method, which can track the structural changes that take place within materials at intervals of less than a trillionth of a second, are reported in the Mar. 6 issue of the journal Nature Communications.