Insulator to Metal in Only 100 Femtoseconds

A new experiment has, for the first time, studied in detail how a crystal undergoes a superfast phase change from the insulating state into a metallic state on a femtosecond time scale. Andrea Cavalleri (now at LBL, 510-495-2536, acavallieri@lbl.gov) and his colleagues at UC-San Diego and the University of Quebec work with a sample consisting of a 200-nm thick film of vanadium oxide (VO₂). A 50-fsec laser pulse enters 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 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 as to 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 a 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. (Cavalleri et al., Physical Review Letters, 3 December 2001.)