Princeton researchers recently conducted an experiment on a class of materials known as ‘frustrated magnets.’ The name comes because the material should be magnetic at low temperatures, yet they aren’t.
The experiment centers around a behavior known as the Hall Effect, and whether or not frustrated magnets exhibit the behavior.
The Hall Effect was named after American physicist Edwin Herbert Hall. He discovered the behavior in 1879. Here’s the Princeton press release explaining what the Hall Effect is.
When a magnetic field is applied to an electric current flowing in a conductor such as a copper ribbon, the current deflects to one side of the ribbon.
“To talk about the Hall Effect for neutral particles is an oxymoron, a crazy idea,” said N. Phuan Ong, Princeton’s Eugene Higgins Professor of Physics. Why? Because the Hall Effect occurs in charge-carrying particles. Frustrated magnets are non-charged (neutral) particles.
While most physicists thought the idea of frustrated magnets exhibiting the Hall Effect was ludicrous, some physicists speculated that the particles may exhibit the behavior if cooled to extremely cold temperatures. Materials cooled to near absolute zero behave according to the laws of quantum mechanics, not regular physical laws we see in our everyday lives.
Ong and his fellow Princeton researchers set out to see if the Hall Effect was present in frustrated magnets at the right temperature. They used a class of magnets called pyrochlores.
“These materials are very interesting because theorists think the tendency for spins to align is still there, but, due to a concept called geometric frustration, the spins are entangled but not ordered,” Ong said.
Graduate student Jason Krizan created the pyrochlore crystals that would be used in the experiment. Fellow graduate student Max Hirschberger handled the setup for the experiments. The press release explains how HIrschberger set up the tests.
To test each crystal, Hirschberger attached tiny gold electrodes to either end of the slab, using microheaters to drive a heat current through the crystal. At such low temperatures, this heat current is analogous to the electric current in the ordinary Hall Effect experiment.
While applying the heat current, Hirschberger also applied a magnetic field in the direction perpendicular to the heat. Guess what he observed? The heat current deflected to one side of the crystal. Meet the Hall Effect.
The researchers were surprised by the result. Ong suggested they repeat the experiment. But this time, reverse the heat current. If this was the Hall Effect, the heat current should deflect to the opposite side. And, it did.
“All of us were very surprised because we work and play in the classical, non-quantum world,” Ong said. “Quantum behavior can seem very strange, and this is one example where something that shouldn’t happen is really there. It really exists.”
So, what does this discovery mean? It could open the door to new research avenues for advanced electronics in the future. One area could be the hunt for a particle known as a spinon. Theorists speculate this particle “could be the carrier of a heat current in a quantum system such as the one explored in the present study,” according to the press release.
The results were published in the journal Science.
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