 Ever wondered why materials need to be so cold in order to become superconductive? This is because superconductivity is believed to require the forming of Cooper pairs, which are pairs of electrons that move through the superconductive material with equal but opposite spin and angular momentum (page 86). These pairs exist throughout superconductors in tightly-knit nets with other pairs.
Now, normally, electrons have too much energy to settle into Cooper pairs (thanks to heat from the surroundings). However, something that's been noticed is that, when materials cool down to near-absolute zero, Cooper pairs form easily (too complicated? read this).
When this happens, almost all the electrons settle down to the Fermi Level of the material. What's interesting about this is that, in essence, the fermi level is the absolute minimum level of potential energy that valence electrons can hold. Think about the energy levels electrons maneuver in order for atoms to emit photons. Now think lower.
The current theory holds that superconductors are only superconductive when they are cold enough for the materials' valence electrons reach the fermi level. Manipulating that, this means that superconductors cannot form at higher temperatures, since the higher temperature will excite the electrons beyond the fermi energy levels.
However, we have clearly seen evidence of room temperature superconductivity. Am I the only one who smells a contradiction? How can anything at room temperature have electrons at the fermi level?
Right now, scientists are only testing for the prized ability of superconductors: to be superconductive. However, it seems that conventional superconductor theories are in for a little twisting and reshaping once scientists manage to fully study fermi levels of higher-temp superconductors.
 MRI, or Magnetic Resonance Imaging, using the magnetic properties of superconductive coils in accordance with other permanent/temporary magnets, produce an image of the human body which is used to help diagnose various conditions and diseases. The superconductive magnet which consists of numerous coils of wire that carries current, is constantly bathed in liquid helium to keep its temperature below the critical point, -269.1 degrees Celsius. This magnet is isolated in a vacuum to eliminate heat loss. The system itself produces a magnetic field of 0.5 - 2.0 Tesla and the magnetic properties of the superconductive coils make it possible to produce such a field and keep MRI cost efficient.
The MRI machine uses three other types of magnets: resistive magnets, permanent magnets, and gradient magnets. Resistive magnets are similar to superconductive magnets except that they do not require the constant bathing in liquid helium to utilize their magnetic properties. Instead, they carry an unusually large amount of current to achieve this. Permanent magnets are also used, but to a much lesser extent than the superconductive magnets for their inability to provide a strong, stable magnetic field over a large enough area. Gradient magnets have a lesser magnetic field than superconductive ones, 0.018 Tesla to 0.027 Tesla, and are used to hone the machine in on key parts of the human body.
These various magnets produce an image of the insides of a human body by utilizing the atomic spins of numerous hydrogen atoms extant in the body, due to the strong magnetic moments of the hydrogen atoms. The strong magnetic field induced by the superconductive coils align the hydrogen atoms either pointing towards the head or pointing towards the feet. There should be a large number of hydrogen atom pairs that negate the spin of each other (one pointing towards the feet cancels the effect of one pointing towards the head). Very miniscule amounts of hydrogen atoms do not cancel each other out, but these exceptions are what allows the MRI machine to map out a picture of the body. The un-negated hydrogen atoms are then exposed to a radio frequency pulse specific to the element hydrogen which causes the atoms to spin at a distinct frequency and direction. This frequency is called the Larmour frequency and is calculated using the type of tissue being examined and the strength of the magnetic field that is being used. Next, the three gradient magnets are turned on and off to change the magnetic field in a manner that helps "visualize" a specific part of the body that is being scanned. The information is sent to a computer that interprets the proton spins using a mathematical formula called the Fourier transform. In doing so, the computer successfully materializes the mathematical data into "images" that are used to diagnose patients.
 So what exactly has to be done to a material so that it becomes one of these mysterious superconductors? Of course one might say that you just have to cool it down enough so it becomes superconductive. But how exactly does that relate to the actual physical properties of the material? One of the main aspects of how a certain material can become superconductive is closely tied with at which point said material can actually start exhibiting these special properties that we have come to know as superconductivity. This point is known as the quantum critical point of the material, the point at which the properties of superconductivity start to appear.
The quantum critical point of a superconductive material is similar to the phase changes of ordinary matter. Materials such as water exhibit different thermodynamic phases according to what pressure and temperature is exerted on it, and switches to the appropriate phase accordingly, such as ice or vapor. But these physical phase changes rely on the ordering and position of atoms. The quantum critical point however relies on the interactions that exist between the different electron carriers that exist within the material. As the material reaches the quantum critical point, which it normally does at very low temperatures, its physical properties start to change very rapidly according to different quantum fluctuations that arise from the rapid scattering of electrons within the material. This quantum fluctuation causes the electrons of the material to allow magnetic fields to pass through the superconductor, effectively making the material what we consider to be superconductive.
By studying the different levels at which different materials exhibit their quantum critical point, researchers may be able to come up with a way of fixing this quantum critical point to higher temperatures by somehow separating the different superpositions, or physical stages, to effectively create higher-temperature superconductors. Alas, we still do not know enough of to actually separate these stages and be able to control the defining factor of superconductivity: the quantum critical point.
"We do not claim yet a local correlation between the pseudogap and superconductivity. We don't have experimental evidence strong enough to prove such a correlation. But establishing this connection will be an important direction of future study."
Read more at: http://phys.org/news/2012-11-atomic-resolution-images-fresh-insights-mysterious.html#jCp
 For anyone well-acquainted with the idea of "high-temp" superconductors, cuprate compounds quickly come to mind. As of yet, cuprate superconductors are able to exist at relatively high temps (-100 degrees Celsius or below, yikes!). Of course, research is being done about possible superconductors that could work at, say, room temperature. That's where the pseudogap comes into play. As mentioned in the linked article, the pseudogap phase is a state of non-superconducting behavior found in superconductors (mostly cuprate) near the point when the superconductor should be in a proper environment for superconductivity.
The interesting part of the pseudogap phase is what was found by this international research team. The researchers noticed that, when studying samples (which become superconductors after sufficient doping) under a scanning tunneling microscope, the sample underwent interesting physical changes just as the superconductor moved from the pseudogap state to superconductivity. The researchers saw various nano-scale clusters of atoms accumulate as doping brought the sample closer and closer to the point of superconductivity. As the samples transitioned from pseudogap state to superconductivity, the researchers saw these clusters of atoms start to join together.
What does this remind you of? For me, it reminds me about the unit cells of crystalline solids. Crystalline solids form the unique shapes that they do because, on the atomic level, atoms or molecules line up into lattice structures, with the smallest repeated pattern called the unit cell.
Now, when the sample compounds reached superconductivity, the researchers saw the "clusters" of atoms fully connect with one another. A possible thought is that these clusters, when fully connected, form a lattice similar to that of crystalline solids. For crystalline solids, this kind of lattice led to rigid shapes in the macroscopic world. What if, for superconductors, the atoms form a similar lattice that allows for superconductivity? I am not saying that the lattice would need to be identical to those of crystalline solids, but I think it is reasonable to believe that they could be similar in concept. Frustratingly enough, this can all only be speculation, since very little is actually known about the pseudogap and superconductivity.
 Ever heard of levitation? You know, that floating trick that you see in Star Wars or those other science fiction pieces? Well, guess what. It's no longer science fiction. At a TED Talk, Boaz Almog shows how a 3-inch thick disk can levitate over a magnet. Even more stunning, he explains that a similar disk could be used to carry a small car on top, while levitating above the ground. How? Through a process called quantum locking.
The disk in the video (click the picture to watch the TED Talk) has a superconductor layer about half a micron thick. To put that into perspective, if 2,000 identical layers were put on top of each other, that would just barely make a millimeter. Anyways, that disk, hardly 3 inches thick, can flip, spin, and slide in midair through a process called "quantum locking", a result of the Meissner effect. I'll try to summarize it as best I can, but you really need to look at the video, quantum locking is pretty amazing.
Superconductors are, well, super good at conducting electrons. In the face of a magnetic field, this causes the surface of the superconductor to form currents, which nullify the magnetic field. To look at it another way, there is absolutely no way for a magnet to force a magnetic field into a superconductor. This is, in essence, the Meissner effect: the superconductor resists any magnetic field, so it is repelled by the force of the field.
Now, in a perfect superconductor, this would essentially be the same as pointing the same sides of two magnets together: they would push in opposite directions. This is far from stable, and thus a perfect superconductor is not perfect for levitation. Rather, the keys to levitation are the superconductors which aren't perfect. Super-thin superconductors, for instance, have small "weak spots" where the field from a magnet can just barely pierce through, forming something called a flux tube. When these flux tubes form, they essentially anchor the superconductor into its space. This is why you can tilt a levitating superconductor and it will remain motionless in midair.
Now let me ask you: what can't superconductors do?
 Superconductivity, as defined by Merriam-Webster, is a complete disappearance of electrical resistance in a substance especially at very low temperatures. That is a very basic definition that is not completely accurate (which will be explained later). But, then what is a superconductor? In order to create a better, more detailed definition, the characteristics of superconducting materials should be understood. Superconductors are characterized by a repulsion of magnetic fields, which was discovered by W. Meissner and R. Ochsenfeld in 1933. This discovery occurred 22 years after the initial discovery of superconductors in 1911 by H. K. Onnes, when he observed a second characteristic of superconductors, a complete lack of electrical resistance. For a more detailed history of superconductors and their discoverers, click here.
As you've probably seen, the first definition did not make mention of the repulsion of magnetic fields, called the Meissner effect, but that's not why that definition is flawed, as this is a result of the lack of resistance. Rather it is the second part, the very low temperature part, which is wrong. As scientists are able to synthesize superconductors, higher temperature ones have been created. High being relative, since the temperature are still cold, but they are much higher than the 4 K Mercury that Onnes discovered superconductivity with.
So, superconductors, what are they? They are materials, usually metals, that when brought down below a certain temperature experiences a complete lack of resistance. This definition (and brief history lesson) should make understanding superconductivity a bit easier.
 Water doping has been able to increase the critical points of various materials so that they observe superconductor-like properties. An example of this is reflected in Esquinazi and his team's work at the University of Leipzig with water doping in graphite to give the material some strange superconductor-like properties. Water doping is simply soaking the material in water to provide the material with extra electrons so that the material can become a better conductor.
Graphite, after soaked in water, "superconducted" at temperatures above 100 degrees Celsius, which drastically outmatches normal conductors who need to be refrigerated with liquid nitrogen or liquid helium, which the highest possible temperature 77 K. The samples of graphite remained magnetic after being exposed to a magnetic field, but the team is not sure if this characteristic is a direct result of its superconductive properties or ferromagnetism. The team believes that high electron concentrations between layers of graphite allow the conduction of a current to be much easier in water-doped graphite and provide the solution as to why this material observes the properties that it showcases.
Another example is shown through the work of Sasaki and his colleagues from Japan Science and Technology Corporation and the National Institute for Material Science in Tsukuba on water doping cobalt oxides. The cobalt oxide configuration includes layers of cobalt and oxygen, separated by a layer of metal atoms. Water-doping altered this configuration so that a layer of water molecules separated the metal atoms and the cobalt oxides. This change increased the size of the unit cell and the material's physical properties. At 5 Kelvin, its resistance decreased and it became more difficult to alter the material's magnetic properties with an external magnetic field, both of which are superconductive properties. The figure above shows the structure change of cobalt oxide when after water-doping. Get more information here.
 Iron-based superconductors have been a hot topic ever since their discovery 4 years ago. This is because of the many practical advantages iron-based superconductors hold over, for example, copper based superconductors which are brittle and difficult to work with. Earlier this year, a team from the National Institute of Standards and Technology and the University of Maryland found an iron based superconductor that operated at the highest temperature in its class (47 Kelvin). Although still extremely cold, its critical temperature, or the point it takes a material to reach its superconductivity, was relatively high compared to others in its 1:2:2 class.
This class consisted of its crystals surrounded by a group of a calcium atom, two iron atoms, and two arsenic atoms. This diversity of atoms that make up the unit cell hold promise to the superconductor's ability to serve functions, for these common atoms could be substituted with different atoms to suit different situations. The superconductor that the team from the University of Maryland and the NIST discovered held a strange atomic property. When a calcium atom was replaced by a smaller atom, it unit cell structure was shrunk by 10%. Avoiding this size change, which is considerably large in the atomic level, would be key to this iron based superconductor's utility in electronics and other practical implications. The picture on the left shows this "shrinking" characteristic of the iron based superconductor; when the calcium on the far, front, right corner of the unit cell was replaced with praseodymium, the structure itself shrunk. In order to re-stabilize the structure, much more praseodymium was needed in the unit cell.
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