Quantum effects writ large

HOUSTON, Feb. 15, 2007 — A team of physicists from Rice University, Rutgers University, and the Max Planck Institute for Chemical Physics of Solids in Dresden, Germany, this week reports in the journal Science the discovery of surprising quantum effects in a member of a broad class of materials that include high-temperature superconductors and quantum magnets. The effects were observed in a compound that was cooled nearly to absolute zero, a temperature low enough to bring about a "quantum critical point," a tipping point at which the quantum properties of the material undergo a radical change.

"Physicists have long held that the macroscopic properties of a material at a quantum critical point are completely described in terms of fluctuations of a classical variable called the order parameter," said Rice University theoretical physicist Qimiao Si. "Our results show instead that inherently quantum effects play an important role, and that these can be seen in thermodynamic measurements."

In this week's paper, researchers reported finding telltale signs of a link between quantum effects and thermodynamic properties in the "heavy fermion" compound YbRh2Si2 (YRS) containing the elements ytterbium, rhodium and silicon. This material contains a quantum critical point that separates a magnetic phase from a non-magnetic one.

"Quantum criticality epitomizes the collective organization of a large number of microscopic particles in matter," said Rutgers theorist Elihu Abrahams. "The new research sheds light on such a collective state of strongly interacting electron systems."

The frontier of research in condensed matter physics that is involved centers on the way that quantum effects influence the physical properties of chunks of material containing many billions of billions of atoms. The 1986 discovery of high-temperature superconductivity in copper-oxide ceramics led condensed matter theorists to realize that quantum effects of strongly correlated electron systems are much more complex than have been anticipated in textbook descriptions. One effect whose importance has been increasingly recognized over the past few years is quantum criticality.

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