Here's part 2 of my post on superconductivity (see previous post for part 1). So, we have a strange material that, when cooled down enough, undergoes a dramatic change: it suddenly starts conducting electricity without loss of energy. That's a superconductor, and we talked a bit about the underlying microscopic mechanism in the previous post. There is however much more to a superconductor in terms of freaky behavior.
During your troubled childhood, you probably have had a chance to play around with magnets. You noticed that magnets can be oriented to repel or attract each other. This is the magnetic force, part of the fundamental electromagnetic force. A magnet projects around it an energy field through empty space. At points nearby the magnets, space literally store a bit of energy due to the proximity of the magnet. This energy field is connected to the repulsion or attraction that you experience when you bring two magnets close to each other. In Nature, energy is money: it is costly to generate energy and Nature would rather not do it unless It really has to…
Now imagine you bring a magnet close to a block of some material. The magnetic energy field will in general penetrate inside materials as well. However, a material could in principle react to the proximity of a magnet by spontaneously generating a current within itself - by moving around its own electrons. This is because an electric current also generates a magnetic energy field, and it is possible for the magnetic field from the current to cancel the one from the magnet within the body of the material. That is a good thing from Nature perspective: the magnetic energy inside the material costs energy, and if it can be canceled by moving the electrons in the material, life would be improved. The problem is that generating a current in a material is also energetically costly… in general, it does not pay off and the magnet's magnetic field does penetrate readily into the material - sometimes slightly modified by the material's lame attempt to put up a fight to the intruding magnetic field.
But a superconductor has a trick up its sleeve: it can conduct electrical current without loss of energy. So, if you get a magnet close to a superconductor, the superconductor can circulate arbitrarily large amounts of electric current within it to make sure the magnetic energy field inside of it is canceled! A superconductor repels magnetic fields! See the first picture attached to this post for a depiction. The end result is that the superconductor acts like a magnet as well in response to the proximity of the external magnet, and superconductor and magnet repel each other: hence, you get magnetic levitation (see videos in previous post).
But Nature has more elaborate tricks up its sleeve… Imagine you ramp up the strength of the external magnet so that it generates more and more magnetic energy fields around it. A superconductor involves a microscopically delicate balance of forces that binds pairs of electrons together (see previous post for the details). If enough magnetic energy field from an external magnet is present, this delicate balance can be disturbed as the electrons feel an external magnetic force. So, it is possible to destroy the superconducting state of a superconductor by putting it in the vicinity of a very strong magnet. But things are much much more interesting than this… When one attempts to destroy a superconducting state with a strong magnet, one finds two very different types of behaviors - depending on the detailed properties of the superconducting material involved. We call these two different types of superconductors Type I and Type II.
Imagine a region of strong magnetic energy field penetrating a superconductor. And imagine that this region of magnetic field penetration is in the shape of a rough cylindrical blob: inside, there's magnetic field and the superconductor is no more, outside there is no magnetic field and the superconductor is still putting up a good fight. The interface between these two regions - the boundary of the blob - is a place of tension and can pack energy. There are two possibilities however. Possibility 1 or Type I: the energy in the interface costs additional net energy, and such regions would rather coalesce and merge to reduce the size of the boundary - imagine droplets of water or mercury merging together to form larger and larger blobs of water or mercury. If you are science fiction fan, imagine instead the liquid metal in the Terminator movies, or the Klingon blood in empty space in Star Trek… The end result is that the region where the magnetic energy field penetrates into the superconductor grows in size quickly and overtakes the whole body of the superconductor; or I should say the late superconductor which is now no more.
Something amazing happens however in Type II superconductors. The contribution of the interface to the total energy balance of the situation is negative! The blob of magnetic energy field penetrating the superconductor would rather maximize the size of the interface… As a results, such blobs fragment into smaller ones - instead of combining as in the Type I case. You then generate a dramatic cascade of magnetic energy regions disintegrating into smaller and smaller regions and the superconductor gets threaded with a lattice of cylindrical vortices of magnetic field (see accompanying actual picture of vortices from the 2003 Noble prize announcement for superconductivity). In between these vortices, the material is still superconducting; within the vortex, it's not. But why don't these regions of magnetic energy fields, these vortices, collapse further - perhaps disappear all together? The reason is a delicate one: there is a minimum amount of magnetic energy field in a vortex that you cannot eliminate without having to rearrange the energy distribution in the entire body of the material; and this costs loads of energy and would rather be avoided. There is then a global aspect of these magnetic vortices that is sensitive to the material's entire state; as if things are held together by a larger order beyond the physics of the local neighborhood of a vortex. What is quite amazing about this is that these vortices behave very much like particles: you can scatter them off each other, they can combine and decay, there are forces between them, there are even anti-vortices around… check out the videos accompanying this post for some entertainment value. The first video is about dynamics of vortices - actual video of vortices in action! The second video is a simulation; focus on the top graph only since that depicts magnetic energy. The lattice of magnetic vortices in a Type II superconductor can also "pin" a levitating magnet as seen in the videos of my previous post. It's like you are putting magnetic nails through a floating superconductor - pinning its orientation in mid-air… beautiful, amazing, elegant, and intricate physics across the board… that's superconductors for you in a nutshell - actually in two nutshells.
Posted on Tuesday, November 9, 2010 at 10:39AM