News Item

Experiment yields evidence of Dirac Strings and monopolar magnetic excitations

A team of Scientists form Berlin and St Andrews has experimentally confirmed for the first time the existence of Dirac Strings and monopolar magnetic excitations in a material. The study, to be published in the Journal Science, brings fresh evidence to a long debate about one of the fundamental puzzles of modern physics: the asymmetry between electric and magnetic "charges". While the theoretical principles behind the asymmetry in question are far from simple, the evidence is often part of our everyday experience, as the team state:

"Anyone who played with a couple of fridge magnets knows they have two poles, north and south, and that no matter into how many pieces one breaks them, each of them will also have a north and south pole. All the magnets we know – all the forms of magnetism we know- are based on magnetic dipoles, that is, on elementary components with two poles. Yet, we know that in the case of electricity, there are independent positive and negative charges (protons and electrons for example), that we can find in isolation."

The British physicist Paul Dirac proposed the existence of elementary magnetic monopoles in 1931. This early work linked the magnetic monopoles with solenoidal lines that carry magnetic flux, Dirac’s strings. All attempts to yield reproducible experimental evidence of the existence of these monopoles, elementary or emergent, had been unsuccessful until the group from Berlin and St Andrews began to study one specific magnetic system in detail. The key to the team's success was theoretical work lead by Roderich Moessner, from Dresden, Germany. Moessner proposed the appearance of monopoles and Dirac strings as emergent collective excitations in a class of frustrated magnetic systems, the so-called spin-ice.

neutron scattering exptBy cooling the material Dysprosium Titanate to temperatures below 1K (approx. -272C), the team were able to determine that the magnetic dipolar moments of this material reorganize themselves into a sort of “magnetic spaghetti” (see image above). Evidence for this "spaghetti” state (the Dirac strings) came from neutron diffraction studies of the material (shown in progress opposite). By analysing small deviations in the path of different neutron beams passing through the cryogenically cooled Dysprosium Titanate, the scientists were able to reconstruct the magnetic field distribution inside the material from the beam geometry and the known magnetic moment of the neutrons. Furthermore, by applying an external magnetic field to the material the term were able to "stretch" the strings and lower the density of the string networks to promote monopole dissociation. At temperatures from 0.6 to 2 Kelvin, the strings are visible and have magnetic monopoles at their ends.

The work represents a significant advance in the field of Condensed Matter science. It is the culmination of another sucessful scientific collaboration for the St Andrews group. Researchers from the School grew the Dysprosium Titanate crystal using facilities at St Andrews and contributed to the neutron experiments in Berlin. The experiemtntal project was concieved by Dr Santiago Grigeria (St Andrews) and Prof Alan Tennant (Berlin).

These exciting new results promise to influence broader aspects of related science.

Links to more information:


tje 04/09/09