Monday, July 9, 2012

Freezing magnetic monopoles: How dipoles become monopoles and vice versa

Freezing magnetic monopoles: How dipoles become monopoles and vice versa: "Steady flows of magnetic monopoles are apparently impossible," Powell said, "but transient currents have been demonstrated, and one could imagine creating an alternating current, the magnetic equivalent of AC electricity..."

Normally all magnetic poles should be confined within two-pole couplets---the traditional magnetic dipole. However, at a low enough temperature, around 5 K, "frustration" among the magnetic atoms---they want to align with each other but can't because of the inherent geometry of the material---leads to a disordered state with strong, synchronized fluctuations. Unpaired magnetic poles can form amid this tumult. That is, particles (quasiparticle excitations, to be exact) in spin ice with a net magnetic "charge" can exist and move about. A gas of electric charges is called a "plasma," so some scientists refer to the analogous tenuous cloud of magnetic charges as a "monopole plasma."
Stephen Powell's paper, published presently in the journal Physical Review Letters, explores what happens when the fluctuations are frozen by, for example, still-colder temperatures or a high-strength magnetic field. He shows how the monopoles are confined into magnetically neutral dipoles again. He is the first to prescribe the phase transition from the monopole phase (also called the Coulomb phase since the monopoles feel the same inverse-square force effect as electric charges) into the pole-confined phase.

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