Magnetic Anapoles

Stephen - When you ask yourself what is the property of a material which generate magnetism it is the fact that you have broken the symmetry with respect to the arrow of time. There is another symmetry, and that is the symmetry with respect to the direction of space. You can see this for example when you look at your hand in a mirror, your hand has changed from being left handed to being right handed and this is a direct observation of the fact that you have changed the direction of space. When you combine both the breaking of the direction time with breaking of the direction of space you generate a very special type of magnetism which is called the anapole, they are also called orbital currents. The anapole is thought to be most important in understanding a complex set of materials which are also able to conduct electricity without any resistance, called super conductors.

Meera - Just to step back a little bit there, so there are different types of magnets. So there are the dipoles that people know with the North/South divide and then there are these anapoles.

Stephen - That's quite correct. The dipole moment, for example, your compass needle, has the property that it does break the direction of time, the arrow of time in a material, it is also characterised by the fact that if you change the direction of space it remains unchanged. The characteristic feature of the anapole is that they have the property that they also change their behaviour if you change the direction of space. Now these anapoles can only be observed with light, so if you take a material and illuminate it with light, or X-rays, it gives you a method of imaging these anapoles.

Meera - Now you've been looking for the presence of these anapoles in high temperature superconductors. These are materials that once they reach a certain temperature, they lose their resistance and are able to have electrical currents permanently flowing through them.

Stephen - Yes, as you lower the temperature of a superconducting material, and some of the materials are extremely simple, elemental, then they lose their resistance to passage of an electrical current. Unfortunately, the temperature that this desirable property comes about is very low, so in the practical application, say the transmission of electricity over power lines, so it's not a practical solution to an efficient transmission because you would have to have your cable at an extremely low temperature. The interest in this class of materials which are called high pressure superconductors is that the temperature at which they have this desirable characteristic is much higher, it's almost at room temperature.

Meera - These materials were discovered about 25 years ago, but the workings of them are still very much not understood. This is what you've been studying so it's thought that potentially these anapoles play a role in how they are able to have this property?

Stephen - Well, 25 years on what we have learned is no single unique theory of these high temperature super conductor materials. Each class of compound seems to have its own theory, but we still search for unifying themes and one of the unifying themes that is being proposed is the existence of orbital currents with hitherto I have been referring to as anapole currents. The experiments that have been done at the Swiss Light Source by Valerio Scagnoli and Schtaub have shown very good evidence for the existence of the orbital currents in a prototype high temperature super conducting material which is very simple. It just contains 2 elements - copper and oxygen. And the proof, the experimental proof that the orbital currents exist, now gives us greater confidence in pursuing the types of theory for which the orbital currents are an essential ingredient in the mix.

Meera - So the material that has been studied is copper oxide which is really a parent component of most high temperature super conductors. One example of a high temperature super conductor is itrium barium copper oxide, so copper oxide is in there, and it's also in many others so this a good material to have studied.

Stephen - Yes, it's good for 2 reasons; one it is a parent and the second thing is it is simple.

Meera - So this has provided an understanding of the workings of these high temperature superconductors, or at least one aspect of them?

Stephen -   This is the first experiment of its type and the evidence is extremely solid.

Meera - This finding has repercussions in other materials as well as high temperature superconductors?

Stephen - Yes, the building block that I've been talking about, the anapole, the other area of interest where they appear are so-called multi-ferroic materials. These are materials which are both magnetic, so they respond to an external magnetic field, but they are also polarised electrically. So they have both a definite magnetic polarisation and also a definite electrical polarisation. And these materials too seem to be characterised by the existence of the anapole.

Meera - So what next then having found the presence of this orbital?

Stephen -   We explore copper oxide still further and a the technique will then be used, if you like, on 'real' high temperature super conductors which contain 4 or 5 elements chemically much more complicated because we have a sure-footing, a good understanding of how to do the experiments and how to understand the results from the experiments.

Meera - Stephen Lovesey, Professor of Theoretical Physics from Diamond Light Source.