Superfluids: Observation of 'second sound' in a quantum gas: ...fluids in this state conduct heat extremely efficiently, with energy transport occurring in a distinct temperature wave. Because of the similarities to a sound wave, this temperature wave is also called second sound...
In the laboratory, Grimm's team of quantum physicists prepared a quantum gas consisting of about 300,000 lithium atoms. They heated the cigar-shaped particle cloud locally with a power-modulated laser beam and then observed the propagating temperature wave. "While in superfluid helium only one entropy wave is generated, our Fermi gas also exhibited some thermal expansion and, thus, a measurable density wave," explains Grimm the crucial difference.
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