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Universal Monopole Scaling near Transitions from the Coulomb Phase

The unit cell for “spin ice” materials consists of two tetrahedrons. The arrows show the orientation of the magnetic atoms within the material. (Courtesy Stephen Powell.)

Magnetic monopoles weren’t supposed to exist. If you try to saw a bar magnet in half, all you succeed in getting are two magnets, each with a south and north pole. In recent years, however, the existence of monopole quasiparticles consisting of collective excitations among many atoms has been demonstrated in the lab. Now Stephen Powell, a scientist at the PFC-supported Joint Quantum Institute, has sharpened the theoretical framework under which monopoles can operate.

Experiments with dysprosium titanate and other "spin ice" materials show that at a low-enough temperature, around 5 K, magnetic frustration leads to a state without order but with strong correlations between even distant atoms. While an isolated dipole cannot split, unpaired magnetic poles can emerge from this ensemble, with measurable effects.

Powell’s new paper explores what happens when the temperature is made colder still. He shows how the monopoles become confined into magnetically neutral dipoles again. He is the first to prescribe the phase transition from the monopole phase (also called the Coulomb phase) into the pole-confined phase.

Going to those lower temperatures and observing how monopoles freeze into dipoles will be difficult to achieve in the lab since it is hard to coax the magnetic atoms into interacting strongly enough. Powell believes it can be done. If, furthermore, this transition were like other phase transitions, then it should be subject to a body of laws called “universality,” which typify many such phenomena---water turning into steam is a favorite example.

Powell is the first to address how universality pertains to the freezing process. His framework for monopoles includes testable predictions about how to observe the transition from monopoles into confined poles.

Stephen Powell
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