On August 10, the CERN community of scientists at the particle accelerator facility in Geneva came up with another elementary subject of quantum physics which dates back to the 1930ies when Paul Dirac predicted the possible existence of magnetic monopoles in accordance with quantum theory.
Magnetic Monopoles and Dipoles (CERN)
As we already learned at school, the north- and south-pole of an elementary magnet cannot be separated by breaking the magnet apart in its middle. Instead, two different elementary magnets would appear that have, both, their own sets of north- and south-pole.
Quantum theory, however, should allow the existence of such isolated magnetic monopoles in a vacuum. Modern physics being unable to detect let alone generate such monopoles, therefore has to resort to an appropriate model which is far more easier to handle.
Here now scientists refer to the fact that some kind of magnetic anomaly can be generated in crystallized matter. It's the measurable magnetism of certain atoms that becomes the source of such anomaly and which might be related to a slight distortion of a regular crystal lattice, for instance due to a slight change in composition. As this arrangement resembles the "freezing of an anomaly" while the arrangement of atomic spins is similar to that of hydrogen atoms in frozen water, scientists have come to name such material a "spin ice".
Arrangement of hydrogen atoms (black circles) about oxygen atoms (open circles)
in frozen water. Two hydrogen atoms (bottom ones) are close to the central oxygen
atom while two of them (top ones) are far and closer to the two other (top left and
top right) oxygen atoms.
Two teams including Tom Fennell at the Institute Laue-Langevin (ILL) in France and Jonathan Morris at the Helmholtz Centre in Berlin, Germany, are already working on that subject. The Morris group studied crystalline Dy2Ti2O7, which has a tetrahedral unit cell with two Dy spins pointing into the centre of the tetrahedron and two pointing out, very similar to the above mentioned arrangement of H spins in frozen water.
" The spins in a spin ice do not line up like those in a ferromagnet. Instead physicists believe that they join up to create magnetic flux lines within the material that resemble a knotted mess of strings. These are known as Dirac strings because they resemble the tubes of flux that should connect magnetic monopoles according to Dirac's calculations. " (physicsworld.com)
Here now comes the CERN facility with its technical abilities. The so-called MoEDAL experiment at the Large Hadron Collider LHC is designed specifically to look at the effects related with the appearance of monopoles. MoEDAL is composed of a largely passive detector, installed next to the LHCb experiment.
If monopoles exist, they are believed to be very massive. As the LHC produces collisions at unprecedented energy, physicists may be able to observe such particles if they are light enough to be in the LHC’s reach. For instance, high-energy photon–photon interactions could produce pairs of North and South monopoles. Monopoles could manifest their presence via their magnetic charge and through their very high ionizing power, estimated to be about 4700 times higher than that of the protons.
As to the marials to be used in spin ice research, here are some further facts:
The most prominent compounds with spin ice properties are dysprosium titanate (Dy2Ti2O7) and holmium titanate (Ho2Ti2O7). Both belong to the group of pyrochlores with its cubic crystal structure (Fd-3m) describing materials of the type A2B2O6 and A2B2O7 where the A and B species are generally rare-earth or transition metal species; e.g. Y2Ti2O7.The pyrochlore structure is a super structure derivative of the simple fluorite structure (AO2 = A4O8), where the A and B cations are ordered along the ⟨110⟩ direction (*). The additional anion vacancy resides in the tetrahedral interstice between adjacent B-site cations. These systems are known to be particularly susceptible to geometrical frustration and novel magnetic effects.
*= The ⟨110⟩ direction of one single cubic "building brick" (elementary cell) that continually adds to a cubic crystal lattice during its growth is the diagonal direction on one of its six square sides.
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