Excitons condense and form macroscopic patterns

Excitons condense and form droplets, which can be explained by strong phase fluctuations in the condensate. The phenomenon is similar to superconductor-insulator quantum transition.
Negatively charged electron and positively charged holes, existing in a semiconductor, attract each other and can form bound particles or “electron-hole” atoms, know as excitons. Recent experiments by "Leonid Butov (UCSD)" and his collaborators suggest that these particles can condense into a single quantum state and form a superfluid. This is similar to superfluidity of He atoms which can also form a condensate.
The sample in which excitons can be created and studied is a double quantum well consisting of several thin layers of semiconducting and insulating materials (see review of excitons in coupled quantum wells [1]). Free electrons and holes are created by shining a laser on to the surface of the sample. Free electrons and holes then form indirect excitons. The location of the excitons can be observed when they recombine and emit light of a characteristic wavelength. The central bright spot(see the image) corresponds to the indirect excitons created at the spot where the laser beam hits the sample (the light is filtered to show only indirect exciton emission). These excitons have high temperatures, because they are heated by the laser beam. The aura around the central spot marks the location of the indirect excitons that are formed away from the laser excitation spot, and have temperature close to that of the lattice. The observed bead pattern is formed spontaneously at low temperatures [2] (T<2K) and is though to be related to formation of a degenerate Bose gas of excitons[2].

[1] L.V. Butov,;J. Phys.: Condens. Matter 16, R1577 (2004). [2]L.V. Butov, A.C. Gossard, and D.S. Chemla; Nature 418, 751 (2002).

Related Links:
  • Introduction to quantum exciton gases and exciton condensation
  • Nanowerk Nanotechnology Portal



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