Researchers with the Lawrence Berkeley National Laboratory (Berkeley Lab), in collaboration with a scientist at the University of California's Santa Barbara campus, have reported the observation of excitons that display a macroscopically ordered electronic state which indicates they have formed a new exciton condensate. The observation also holds potential for ultrafast digital logic elements and quantum computing devices.
"The excitons were expected to form a quantum liquid or even a Bose-Einstein condensate, this state had been predicted in theory since the 1960s, but the macroscopically ordered exciton state that we found is a new state that was not predicted," says Leonid Butov, a solid state physicist who holds a joint appointment with Berkeley Lab's Materials Sciences Division (MSD) and with the Institute of Solid State Physics at the Russian Academy of Sciences.
Just as the Nobel prize-winning creation of Bose-Einstein condensate atoms offered scientists a new look into the hidden world of quantum mechanics, so, too, would the creation of Bose-Einstein condensate excitons provide scientists with new possibilities for observing and manipulating quantum properties.
The creation of a new exciton condensate was reported in the August 15, 2002 issue of the journal Nature, in a paper co-authored by Butov, Arthur Gossard of UC Santa Barbara's Department of Electrical and Computer Engineering, and Daniel Chemla, director of Berkeley Lab's Advanced Light Source.
The new exciton condensate was observed at Berkeley Lab using photoluminescence on samples composed of the semiconductors gallium arsenide and aluminum gallium arsenide. The semiconductor samples were of extremely high quality and were prepared by Gossard in Santa Barbara.
The observations were made by shining laser light on specially designed nano-sized structures called quantum wells which were grown at the interface between the two semiconductors. These quantum wells allow electrons and electron holes (vacant energy spaces that are positively-charged) to move freely through the two dimensions parallel to the quantum well plane, but not through the perpendicular dimension. Under the right energy conditions, application of an electrical field in this perpendicular direction will bind an electron in one quantum well to a hole in another across a potential barrier to create a relatively stable exciton.
"An exciton functions as a quasi-particle, akin to a hydrogen atom," says Butov, "which means that by reducing temperature or increasing density, it is a candidate to form a Bose-Einstein condensate."
Trapped in the quantum wells, their movement restricted to two-dimensions, the excitons created by Butov and his colleagues condensed at the bottom of the wells as their temperature dropped. Because the mass of these excitons was so much smaller than that of the atoms used to form atomic Bose-Einstein condensates, the critical temperature at which condensation occurred, about one degree Kelvin (-272 degrees Celsius or -459 degrees Fahrenheit) was much higher. By comparison, to create the first atomic Bose-Einstein condensates back in 1995, researchers at the University of Colorado had the daunting task of chilling a ball of rubidium atoms to as close to absolute zero as the laws of physics allow.
Under photoluminescence, the macroscopically ordered exciton state that Butov and his colleagues observed appeared against a black background as a bright ring that had been fragmented into a chain of circular spots extending out to one millimeter in circumference.
"The existence of this periodic ordering shows that the exciton state formed in the ring has a coherence on a macroscopic length of scale," says Butov. "This coherence is a signature of a condensate. The next step is to do a coherence spectroscopy study, particularly at lower temperatures, that will verify the properties of this new state."
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