Andrea Taroni
Available online 23 April 2011.
Scientists have successfully mapped out the phase diagram of an exotic state of matter known as a quantum spin liquid for the first time [Pratt et al., Nature (2011) 471, 612].
The international team of researchers, which includes scientists from the ISIS Neutron source at STFC Rutherford Appleton Laboratory and the University of Oxford, in the United Kingdom, as well as colleagues from Japan and Switzerland, achieved this feat by using the highly sensitive technique of muon spin rotation (μSR). This uses subatomic muon particles as a probe that can sense how a material's magnetic properties vary with changing temperature and magnetic field.
Originally proposed in the 1970s, quantum spin liquids are exotic states of matter that, unlike ordinary magnets, do not order magnetically down to the lowest attainable temperatures. They are characterized by strong quantum fluctuations promoting magnetic disorder, and a “geometrically frustrated” structure that gives rise to many possible magnetic ground states.
“The physics of a frustrated magnet is always a battle between chaos and apathy,” explains Peter Baker, an instrument scientist who participated in the μSR measurements performed at ISIS. “Chaos in that the magnet wants to be as disordered as possible, apathy in that it wants to find the lowest energy state accessible to it.” In most magnets, apathy wins out so that they eventually order. In a quantum spin liquid, however, frustration and quantum fluctuations conspire on the side of chaos to prevent the system finding a magnetically ordered ground state.
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Spins in a quantum liquid.
Francis Pratt, ISIS/Science and Technologies Facilities Council
In order to achieve this unusual state, Baker and his collaborators chemically engineered a layered molecular system known as κ-(BEDT-TTF)2Cu2(CN)3. Using μSR, they then examined its behavior under an applied magnetic field, down to milliKelvin temperatures. In this way, the scientists could chart the phase diagram of the material, and gain information on the nature of the quantum fluctuations present.
“By applying a small magnetic field we found that a weakly magnetically ordered state can be recovered – the forces of chaos are overwhelmed!” says Baker. “By forcing the system out of its spin liquid state you uncover some of the fog and start getting new information about the quantum fluctuations and how they behave.” This is significant, because it allows for a direct comparison with theoretical models of quantum spin liquids, and significantly narrows the range of directions likely to be fruitful in explaining this exotic state of matter completely.
Steven Bramwell, from the London Centre for Nanotechnology and University College London agrees these results add significantly to the understanding of unusual magnetic states. “This is an excellent example of how ISIS muons may be used to investigate magnetic properties that are too weak to be measured by other techniques,” he commented.
