Drillingsraum: What are Magnetic Monopoles?

Jonathan Morris: All magnets, for example bar magnets or fridge magnets, have a North and a South pole. If you break a magnet in two, you do not seperate these poles. What happens is that you break it into two smaller magnets each with a North and South pole. This is why magnets can be called dipoles, meaning "two poles". But it was predicted by Paul Dirac, that single magnetic poles could exist. In the same way that electricity has positive and negative charges, magnetic monopoles are magnetic postive (North) and negative (South) charges. This means that they are sources of the magnetic field. Physicists have been searching for magnetic monopoles (seperate North or South poles) for many decades since they were predicted by Dirac, but so far no verifiable and reproducable evidence has been reported.

Drillingsraum: In 2009 the Helmholtz-Centre in Berlin managed a revolutionary discovery: For the first time physicists could observe Magnetic Monopoles as quasiparticles in solid. Can you please explain what exactly happened in this experiment?

Jonathan Morris: We were not looking for these single particle monopoles. What we were looking for was the condensed physics analogue of Dirac's idea of two monopoles being connected by a string along which a magnetic field flows. It was predicted that this could be realised in a special type of material called "spin ice". Spin ice is a magnetic material where the small magnetic dipoles ("spins") that make up the material, are arranged in a specially structure that agrees with the rules that are observed in water ice. The structure is made up of tetrahedra, pyramids whose faces are 4 equilateral triangles. At the corners of the tetrahedra sit the spins. The spins can point into the center of the tetrahedra, or point out away from the center. The "ice rules" mean that two spins point towards the center, and two spins point away from the tetrahedra, in the same way that two hydrogen sit close to the oxygen atom in water ice and two hydrogen sit far away from the oxygen. When the "ice rules" are broken in spin ice we get three spins pointing into, or out of the center and one spin pointing out of, or into the center. If you think of a spin as a dumbbell with a North and South magnetic monopole at either end, then 3 spins pointing into means we have 3 North poles close to the center, and 1 spin point out of means we have 1 South poles close to the center. So we have a net magnetic charge in the center of the tetrahedra. These are the magnetic monopoles we looked for.

The evidence comes from three experiments: First the neutron scattering experiment (done by us at HZB) showed, that we had strings of spins within our material, dysprosium titanium oxide (Dy2Ti2O7). This string seperates the monopoles, in the same way as the Dirac strings in Dirac's idea. We see that these strings follow random walks through the material and can be tilted when a magnetic field that has been applied to the material is also tilted. Second was the magnetisation measurements done by colleagues from La Plata in Argentina and St Andrews in Scotland, which showed that the way that these strings form is understood. They form from a 3D Kasteleyn transition. What that means is that each spin has a magnetic energy from interacting with the applied magnetic field, and also it has an entropy because it has two options about what it can do (point into the center or point away from the center of the tetrahedra). The balance of the energy and entropy means that when the field is reduced below a certain value the entropy becomes more dominant and spins that were aligned along the field direction can flip to point in a direction that would increase it's magnetic energy. Third was a heat capacity experiment (done by colleagues at HZB) and theory (by Oxford and Dresden). The experimental results have a strong dependence with temperature and the theory has shown that we can understand the data from the viewpoint of a gas of magnetic monopoles that interact via the magnetic Coulomb force. That means that the magnetic monopoles interact with themselves in the same manner as electric charges interact with themselves. So the three pieces of evidence provide strong support for the idea of magnetic monopoles in spin ice.

Drillingsraum: What are quasiparticles?

Jonathan Morris: Quasiparticle is a term that describes a pheonomena coming from many body physics. For example, if a row of spins all align along the same direction and one of them is flipped then the position between spins pointing in different directions can be thought of as a particle. Likewise in our research, the point defect of the ice-rules behaves as a magnetic monopole.

Drillingsraum: What does the next steps now look like after the success of this experiment?

Jonathan Morris: The next step is to look at the behaviour of these monopoles in detail. This is the first case where we can look at a gas of magnetic monopoles and how they interact with themselves and with external variables such as magnetic field and pressure.

Drillingsraum: What are Dirac strings?

Jonathan Morris: Dirac's idea was, that magnetic monopoles would be connected by a very thin tube through which magnetic field flows, this tube is the "Dirac string". At the end of the tube the magnetic field sprays out, or disappears in, and these ends are the sources or sinks of the magnetic field, and so are the magnetic monopoles.

Drillingsraum: According to calculations Magnetic Monopoles are supposed to have quite a high mass. What is the reason for that?

Jonathan Morris: Magnetic monopoles are predicted by some theories, and normally require a huge mass. Their mass is predicted to be in the range of milligrams, which is huge for a single particle. This is because the Universe would have to focus a lot of energy at a single point to create a source of magnetic field out of the vacuum. These individual particle magnetic monopoles have been searched for in high energy physics experiments, cosmic ray observations, lunar dust, and the ocean bed, but no reproducable results have been reported. Our monopoles don't cost the Universe so much energy, because the magnetic structure of spin-ice gives a "vacuum" where the conditions already exist.

Drillingsraum: What experimental possibilities do physicists have to verify Magnetic Monopoles, are there different ways to detect them?

Jonathan Morris: Other groups have also reported magnetic monopoles in spin ice. In the same edition of the journal Science that we appeared in, a French/British collaboration reported different neutron results which they argue are evidence for monopoles. The same group later reported in the journal Nature that they had measured the magnetic charge on these monopoles using a technique modified from a technique that allows electric charge to be measured. Also a Japanese group reported, in the Journal for the Physical Society of Japan, different neutron scattering results that they argue is evidence for monopoles. So there are other groups working on this area, and trying different techniques.

Drillingsraum: What's the today's idea of the composition of a Magnetic Monopole, what does the inner structure looks like?

Jonathan Morris: Particle physicists who are thinking of monopoles as single particles see these as particles in the same way that electrons or quarks. Condensed matter physicists, like outselves, are looking for emergent magnetic monopoles. "Emergent" means that we look for effects in a many body system that look like and behave like magnetic monopoles, where the individual constituent parts that make up the system contains no monopoles.

Drillingsraum: What would physicists do with Magnetic Monopoles if they could create and stabilize them? Is there any application?

Jonathan Morris: Future discoveries will tell us if there are applications. The French/British collaboration reported "magnetricity" which is the magnetic analogue of electricity.

Drillingsraum: Could Magnetic Monopoles play a role in developing new materials and technologies?

Jonathan Morris: Our material has to be kept at very low temperatures for the ice-rules to be obeyed, and so the monopoles to exist as excitations. That temperature is below 1K, so below 1 centigrade above absolute zero. If materials could be found where monopoles exist at higher temperature then maybe there could be applications. That maybe one of the problems facing scientists in the future.

Drillingsraum: It wouldn't be a challenge to adapt Magnetic Monopoles to the Maxwell Equations. But how would our picture of electrodynamics and physics in general change, if we would find Magnetic Monopoles one day?

Jonathan Morris: Maxwell's equations do not change with our observations. The reason for this is the Dirac string. This contains the magnetic field that flows from the monopoles, and so the Maxwell equation that says that the magnetic field is continuous, and has no sources, still stands. We can call the defects in the spin-ice structure "magnetic monopoles" since the heat capacity show that they behave as magnetic charges in a vacuum.

Drillingsraum: What is the exciting part by investigating Magnetic Monopoles, what fascinates you about it?

Jonathan Morris: This is the one of the first times that a fundemental physical entity has been seen to exist in a fraction within 3D. For example in 2D the electron has been observed to exist in fractions, but not many cases exist for 3D. We have shown a method where physical properties could be fractionalised in 3D, and this could lead to interesting physics and possible applications. What those applications are, if any, will be discovered in the future - what discoveries lie ahead? Looking for the answer to that question is exciting.

Drillingsraum: Is the investigation of Magnetic Monopoles a major task in today's physics or is it more a small section?

Jonathan Morris: I'm not sure you'd describe the attempt to look for monopoles in condensed matter physics as a major task, it is not on the same scale as the Large Hadron Collider (LHC) for example. But there maybe people on the LHC that are employed to keep an eye out for monopoles.

Drillingsraum: How many scientists work at the exploration of Magnetic Monopoles?

Jonathan Morris: Our colloboration contains people from Germany (HZB and PTB in Berlin, MPI in Dresden), UK (Oxford, St Andrews and Edinburgh), Argentina (La Plata). But there are other groups looking for these monopoles in spin-ice the French/British collaboration and the Japanese collaboration. There could be other groups aswell. But that is just the groups looking for monopoles in spin-ice. I have no idea about teams looking for cosmic magnetic monopoles.

Drillingsraum: How is your research funded?

Jonathan Morris: I work for the Helmholtz-Zentrum Berlin which is funded by the Helmholtz Gemeinschaft. Other people in the collaboration are funded from other funding bodies.