Femtosecond electron diffraction

“Snapshots of cooperative atomic motions in the optical suppression of charge density waves” might not seem like an exciting title. But the paper , which appeared in last Thursday’s Nature was my favorite of last week.

Charge density waves are periodic distortions in crystals that consist of stacked chains or planes of atoms. Above a critical temperature, the valence electrons are free to move about the crystal. Below that temperature, the atoms shift positions slightly, moving closer to some neighbors than to others. The shift has the effect of marooning the electrons in puddles around the atoms, like fish trapped in rockpools at low tide. The crystal becomes an insulator and acquires an additional periodicity.

Rudolf Peierls worked out the basic physics of charge density waves in a 1934 paper. One-dimensional crystals, Peierls pointed out, are intrinsically unstable and inevitably buckle. Charge density waves are manifestations of the Peierls distortion in bulk materials. Neutron scattering experiments confirmed the waves’ existence in the 1950s.

Charge density waves, then are well established. What got me excited about the new paper are the words “snapshot” and “cooperative atomic motions” in its title. The nine-author team from universities in Canada, Germany, and Slovenia has succeeded in observing a Peierls distortion on the few-hundred-femtosecond timescale over which it occurs.

To pull off that coup de recherche, the team took a thin crystal of one of the best-studied charge-density-wave materials, 1T-TaS₂, and chilled it below the temperature at which charge density waves appear. Next, the team zapped the crystal—first with a brief pulse of light and then, after a precisely controlled and adjustable delay, with a brief pulse of electrons.

As an electron pulse passed through the crystal, it diffracted off the regular array of atoms to form a pattern that embodied the crystal’s instantaneous structure. From those patterns collected at different delays, the team could track how charge density waves form.

The light pulse gave the valence electrons enough energy to flow freely. Responding to the electrons’ liberation, the more sluggish atoms started to shift back to their undistorted positions. However, even as the atoms were shifting, the electrons began to cool and sank back into confinement around the atoms. The atoms, too, regained their original, Peierls-distorted configuration.

In the schematic, which depicts the process, the atoms are red and the electron distribution is purple: light for low density; dark for high density. The whole process took less than 4 picoseconds to play out.

The paper’s implications go beyond charge density waves. As the authors point out at the very beginning of the abstract:

Femtosecond electron diffraction, as the technique is called, should yield a host of interesting results in the future. It could even prove decisive in solving one of the hardest problems in physics: explaining high-T_c superconductivity.