Magnets reveal their exotic quantum side

The unit cell of URu2Si2

The unit cell of URu2Si2
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A heavy-metal magnet shows intriguing behaviour in which unusual quantum effects come into play.

Metals and metallic compounds are characterised by the fact that their outer atomic electrons are free to wander through the material – making them electrically conducting. Physicists have developed a successful quantum description called Fermi-liquid theory which explains their behaviour. It usefully predicts how their electrical, magnetic and thermal properties change with temperature - describing, for example, a phase transition from a disorderly non-magnetic state to a magnetic state in which the electron spins are ordered.
In recent years, this theory has suffered a spectacular assault, through the continuing discovery of metallic materials whose electronic properties deviate from the expected norm – non- Fermi liquid behaviour. These include the remarkable superconducting copper oxides which conduct electricity without resistance at relatively high temperatures, and some heavy-metal alloys in which the itinerant electrons behave as though they are many times heavier than normal. 

Quantum criticality 

Theorists predict that the key to this renegade behaviour is a new kind of transformation triggered by quantum fluctuations (as predicted by Heisenberg’s uncertainty principle). This so-called quantum critical transition is based on the notion of what would happen if the temperature, pressure or the composition of material were changed such that a particular phase transition occurred only at absolute zero. Quantum fluctuations would then dominate, producing a rather weird state thought to extend well above absolute zero – the quantum critical phase.
Quantum critical behaviour is now a hugely important area of study. Just recently, an American collaboration working with ISIS scientists uncovered a novel quantum phase transition in a heavy metal alloy, uranium ruthenium silicide, which appeared when successive amounts of ruthenium were substituted by rhenium.
The pure material is superconducting close to absolute zero, becoming antiferromagnetic (magnetic spins oppositely aligned) at higher temperatures. However as more rhenium is added, it becomes a ferromagnet (spins aligned parallel) retaining non- Fermi liquid behaviour at low temperatures.
Using the ISIS High Energy Transfer spectrometer, the researchers followed the changes in magnetic alignments at four different compositions. Neutrons interact with the electrons spins in the material, gaining or losing energy in ways that reveals what happens at the ferromagnetic quantum critical transition. They found that the quantum critical behaviour follows expected rules – but depends on the alloy composition and breaks down in the presence of short-range magnetic forces. Their study shows that the antiferromagnetic and ferromagnetic interactions compete in a subtle way that characterises behaviour at an antiferromagnetic-ferromagnetic quantum critical point.
Such ground-breaking studies give us a better understanding of materials like the high-temperature superconductors that may underpin technologies of the future.

Dr V Krishnamurthy (Oak Ridge National Laboratory, USA), vemurukv@ornl.gov

DT Adroja, NP Butch, R Osborn, SK Sinha, JL Robertson, MC Aronson, SE Nagler, MB Maple.

Research date: December 2007

Further Information

Magnetic short range correlations and quantum critical scattering in the non-Fermi liquid regime of URu2-x RexSi2 (x = 0.2-0.6), VV Krishnamurthy et al., Physical Review B 78, 024413 (2008).

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