June 10, 2011

Ultrathin Copper-Oxide Layers Behave Like Quantum Spin Liquid

Magnetic studies of ultrathin slabs of copper-oxide materials reveal that at very low temperatures, the thinnest, isolated layers lose their long-range magnetic order and instead behave like a “quantum spin liquid” — a state of matter where the orientations of electron spins fluctuate wildly. This unexpected discovery by scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory and collaborators at the Paul Scherrer Institute in Switzerland may offer support for the idea that this novel condensed state of matter is a precursor to the emergence of high-temperature superconductivity — the ability to carry current with no resistance



A diagnostic technique called low-energy muon spin spectroscopy was used to detect and investigate magnetism in ultrathin layers.

The magnetic measurements revealed that when the slabs contained four or more copper-oxide layers, they showed anti-ferromagnetic ordering — just like thick, bulk crystals of the same materials, and even up to the same temperature. However, thinner slabs that contained just one or two copper-oxide layers showed an unexpected result: “While the magnetic moments, or spins, were still present and had about the same magnitude, there was no long-range static anti-ferromagnetic order, not even on the scale of a few nanometers. Rather, the spins were fluctuating wildly, changing their direction very fast,” Bozovic said.

Even more telling, this effect was stronger the lower the temperature of the sample. “That means these fluctuations could not be of thermal origin and must be of quantum origin — quantum objects fluctuate even at zero temperature,” Bozovic explained.

Physics Review Letters - Two-Dimensional Magnetic and Superconducting Phases in Metal-Insulator La2-xSrxCuO4 Superlattices Measured by Muon-Spin Rotation


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