Andrea Cavalleri (now at LBL) and his colleagues at UC-San Diego and the University of Quebec worked with a sample consisting of a 200-nm thick film of vanadium oxide (VO2).

A 50-fsec laser pulse entered the sample, causing what is believed to be not one, but two phase transitions: one structural (the unit cell size increases a bit), monitored with short x-ray pulses; and one electrical (insulator-to-metal), monitored by short pulses of visible light.

All of this was done on an unprecedentedly short timescale. This allowed the researchers to observe that the manifestation of the solid in its new crystalline form did not happen piecemeal, but practically all at once; this had never been seen before.

For all the speed, though, this experiment still did not settle an old question in condensed matter physics: Which comes first, the structural change in the sample or the electrical change?

Because the crystalline reordering is so fast -- only hundreds of fsec -- and is reversible, and because x rays scatter differently from the two contrasting crystalline forms, it might be possible to use this whole process as an ultrafast "Bragg switch" for sub-picosecond portions of a longer x-ray wavetrain.

The transformation from insulator to metal is an important example of the large catalog of solid-to-solid phase transitions in physics which usually occur because of a change in pressure or temperature; the ice-induced failure of the sealing ring on the Challenger mission is one example.

(Reference: Cavalleri et al., Physical Review Letters, 3 December 2001.)

(Editor's Note: This story, with minor editing, is based on PHYSICS NEWS UPDATE, the American Institute of Physics Bulletin of Physics News Number 567, November 29, 2001, by Phillip F. Schewe, Ben Stein, and James Riordon.)


[Contact: Andrea Cavalleri]

05-Dec-2001