Crystalline Order At 40,000 K

Physicists at the Christian-Albrechts Universität in Kiel and Ernst-Moritz-Arndt Universität in Greifswald (Germany) have been able to rig a ball of dust particles holding to a crystalline structure even in the middle of a hot plasma.

Most crystals---that is, solid materials in which atoms are arrayed in a regular stacked-cannonball order---melt at temperatures of hundreds or thousands of degrees. The heartiest crystal, diamond, succumbs at 4000 K. The heat is just too much for the atomic bonds and the defining gridiron structure weakens and melts.

Another sort of “crystal,” at low temperatures, is the optical crystal consisting of an artificial and diffuse array of atoms held at the interstices of a 3-dimensional lattice by the electric fields of cross-cutting laser beams.

The plasma crystal, by great contrast, consists of a herd of charged 3.5-micron-sized polymer particles amidst a gas-discharge. Juggling two mighty forces---the mutual repulsion of the particles among themselves and the compressive force on them by the surrounding plasma---the particles manage to arrange themselves into neat concentric spheres, to a total ball diameter of several mm (see figure at Physics News Graphics).

It is ironic that J.J. Thomson, the discoverer of the electron, had suggested in 1904 that the layout of the periodic table of elements could be explained if atoms had exactly this sort of onionlike architecture, with negative charges held poised in a wider sea of positive charges. This idea was wrong for atoms but does describe the arrangement of the dust particles in this plasma.

To sum up: in a plasma where the electron temperature is 40,000 K (the positive-ion temperature is less than 1000 K), an orderly Coulomb ball consisting of aligned, concentric shells of dust particles can survive for long periods.

The two outstanding features of the ball (other than its survival at such high temperatures) are that it represents a true transparent crystal; with a microscope and video camera individual particles in the middle of the structure can be imaged by laser light. The other feature is the slowness of the dynamics. The particles move about with a characteristic timescale of milliseconds rather than the femtosecond scale of atoms in a conventional crystal.

The study of laboratory plasma crystals, the experimenters believe, gives fundamental insight into strongly coupled matter and applies directly to the study of intergalactic nebulae, comet tails, the rings of Saturn and, back here on Earth, in the improvement of various microchip processing steps. (Oliver Arp et al., Physical Review Letters, upcoming article; contact Dietmar Block block@physik.uni-kiel.de, 49-431-880-3862)