LCN researchers observe many-body physics in an atomic layer of spin ice

A half-century old prediction about how a sheet of atoms will behave has been experimentally verified by researchers at the LCN. 

Any familiar lump of matter – whether a drop of water or a grain of salt – contains a lot of atoms, typically up to a trillion trillion. Even if the atoms are arranged one atom thick on a surface there will typically be a million billion atoms over the whole surface.  Although physics comprehends the properties of single atoms, to calculate the properties of so many atoms interacting together is an almost intractable problem – the so called “many-body problem” of physics. The technique of “statistical mechanics” aims to solve such problems, but even when models of atomic interaction are highly simplified, there are only a few historical examples where such models of many-body behaviour can be solved exactly. 

One such exact solution was provided in 1965 by the American theoretician Elliott Lieb. He was able to exactly calculate a familiar property -- the specific heat – of a simplified model of atomic interaction. The model (called the F-model) relates to a two-dimensional system held together by so-called “hydrogen bonds” between oxygen and hydrogen and the exact solution makes a definite prediction of how the specific heat of such a system should vary with temperature. 

Researchers at the LCN, in collaboration with researchers form Oak Ridge National Laboratory, USA, and Oxford University have made thin films of the magnetic material “spin ice” and shown that it is possible to make films that contain only a single atomically-thick layer. In spin ice the atoms are magnetic and these tiny atomic “compass needles” interact to create a many body problem of a million billion interacting parts. 

 

FModel
Figure: A component of the specific heat of a spin ice thin film compared with the exact solution of the “F-model” of many-body behaviour

LCN researchers carefully measured the specific heat of the spin ice thin films and discovered an extremely close match to the predicted exact specific heat of the F-model.  The reason is that interactions between the magnetic atoms in spin ice precisely mimics that between oxygen and hydrogen in the F-model. 

We believe this is the first time that the exact F-model specific heat has been observed in experiment” says Dr. Laura Bovo, lead author of the work. “It shows that we have a magnetic film that can be understood on all scales from the atomic to the macroscopic.”

The work, published in Nature Communications, represents a major step forward in understanding the properties of spin ice in thin form. Spin ice is famous for its currents of magnetic monopoles – “magnetricity” – and the team is working on methods of connecting such currents to electronic probes. Thin films are useful in this regard because their flat surfaces allow probes to be attached directly to the surface. 

Link to article: Phase Transitions in Few-Monolayer Spin Ice Films. Nature Communications.  L. Bovo, C. M. Rouleau, D. Prabhakaran, S. T. Bramwell.                                                                                    

Other contributors
Laura Bovo (former LCN); C. M. Rouleau (Oak Ridge National Laboratory); D. Prabhakaran (Clarendon Laboratory)
LCN Authors
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F-model
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