Thursday, October 22, 2009

Magnetic Monopoles and Magnetricity

Magnetic monopoles have never been seen in nature which makes Maxwell's equations fundamentally asymmetric between electric and magnetic fields, since there is no magnetic equivalent of a single isolated electric charge.

Recently a Nature article by Bramwell et al. show that in a certain kind of magnetically frustrated material called 'spin ice' one can see evidence of 'magnetricity' - a flow of magnetic charges just like electrical charge flow, and understood in terms of a magnetic analogue of the theory of electrolytes. (The material in which this has been observed is dysprosium titanate pyrochlore). The experimentalists actually observe real magnetic currents and are hence able to measure the magnetic charge ('monopole'). This, I believe, is the first example of a system where there is perfect symmetry between electric and magnetic charges. (The popular press has occasionally reported the existence of flux tubes -- dipoles which move independently in certain magnetic materials -- as equivalent to magnetic monopole quasiparticles but the present effect I believe is different -- I would appreciate some comments on these from experts).

One should realise though that this does not change Maxwell's equations in free space. Magnetic monopoles in free space have not been observed yet, except in one un-replicated experiment by Blas Cabrera in 1982. Thus, electromagnetism text books don't need to be revised any time soon.


Anant said...

Surely you are aware that Glashow is supposed to have sent a telegram saying
"Roses are red, violets are blue,
It is time for monopole two."

Strangely by 2009, the story has morphed into Weinberg sending him a letter to this effect. I cannot believe Weinberg (c)would have done something like that.

Another folk explanation I found via google is that the Cabrera result was because of some condensed matter flux-pinning effect in his system. Maybe he had discovered spin-ice?

Rahul Siddharthan said...

Slightly edited cut-and-paste of my email, as you requested:

There are two other relevant papers: one is by Morris et al., just published, and both are motivated by Castelnovo, Moessner and Sondhi.

This is a quick reaction, mainly to the Castelnovo et al paper, which I haven't yet read carefully but I think I get the idea.

is a very neat result, but of course the media (and the authors themselves) overstate the significance. These are not fundamental monopoles, but "things that look like monopoles" -- as will be obvious to many of your readers but perhaps not to the general public. Similarly, there is nothing fundamental about the "magnetic charge" that they measure: it is a property of this system only.

More significantly, they are not "quasiparticles" in the quantum-mechanical sense, or "manifestations of the correlations present in a strongly interacting many-body system" (the phrase used by Castelnovo et al). They are a purely classical phenomenon. This material can be described as a system of classical Ising spins that interact primarily via a long-ranged "dipole-dipole" interaction, but with a short-ranged correction due to superexchange. The local structure is tetrahedral, with the tetrahedra linked at corners and the spins sitting at the corners aligned along the centres of the adjoining tetrahedra. In the ground state two spins point in and two point out of each tetrahedron. An excitation would consist of a spin flip, where one tetrahedron has three in and one out, and the neighbouring one has three out and one in. It will superficially look like a "north pole" in one tetrahedron and a "south pole" in another. What Castelnovo showed is that this can propagate via further spin flips, so that the poles get separated by considerable distances, and they examined the dynamics of this process.

It is a very neat and elegant idea and, in retrospect, should have occurred to earlier authors (there have been dozens of papers on these systems in the last decade). But if you built a classical model with wires and constrained magnets, with the same geometry, it would behave the same way. So all the buzzwords should be taken with a grain of salt.

Rahul Siddharthan said...

I've written a slightly longer take on the thing here.