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Physics

Quasiparticle formation

A team of physicists at the University of Innsbruck, in Austria, simulated the formation of a type of quasiparticle – a collective excitation in an environment that behaves as if it were a particle – and observed it in real time (Science, October 7, 2016). The researchers recorded the birth of Fermi polarons, a type of quasiparticle, in an ultracold quantum gas composed of potassium atoms embedded in a cloud of lithium atoms. In condensed-matter systems, the process takes about 100 attoseconds (1 attosecond is equal to 1×10−18 of 1 second). In the gas simulation, where the system was less dense, the polarons formed in a few microseconds. Rudolf Grimm, study coordinator, uses a metaphor to explain the concept of a quasiparticle. “You could picture it as a skier on a powder day,” says Grimm. “The skier is surrounded by a cloud of snow crystals. Together they form a system that has different properties than the skier without the cloud.” This system is a quasiparticle.

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