As we know, metals and alloys, the resistivity (the degree of difficulty encountered materials on the go) of the material increases with temperature, and decreases with it, almost linear. However, in superconductors at temperatures near absolute zero (0 K) there is a sudden drop in resistivity. The resistivity is then zero. Despite the zero resistivity, conductivity is not infinite (something mathematical formula deduced from conductivity).
That is, the ease being the conductivity of a material to the movement of electrons through it, superconductivity will occur when the electrons are not opposed to its passage through the conductor material (or just do it, is theoretically impossible to achieve a conductivity of 100%).
Superconductivity is therefore a property found in many metals and some ceramics, which appears at low temperatures, characterized by the loss of resistivity at a certain temperature characteristic of each material, called temperature critical.
Superconductors also have a marked diamagnetism, ie, they are repelled by magnetic fields. Which cause the levitation effect we see in the image:
Superconductor at temperatures near 0 K
On the other hand, we lack the explanation of how superconductors. There are two theories. One is the BCS theory, briefly explained in the first section of the paper, and one theory Ginzburg - Landau.
Figure Resistivity - temperature, where we see clearly the decline in the first low temperature in a superconductor.
Notice to mariners: the explanation of these theories do not simplify or for work, talking about really complicated concepts. If you read on, you risk not understand almost nothing about the writing. However, only two paragraphs, then return to discuss issues more "simple."
BCS theory tells us that "superconductivity can be explained as an application of Bose-Einstein condensate. However, the electrons are fermions , so that they can apply this theory directly. The idea is based on the BCS theory is that electrons are paired into a pair of fermions behaves like a boson . This pair is called a Cooper pair and the link is justified in the interactions of electrons with each other mediated by the crystal structure of the material. " As we can see, requires knowledge of quantum mechanics than would have like to know when assessing the application of Bose - Einstein conductivity at temperatures near 0 K. But by having a brief affair, and I quote his explanation and was done with the BCS theory, we say that "Bose-Einstein condensate is the aggregation of matter that occurs in certain materials at very low temperatures . The property that characterizes it is that a macroscopic number of particles of the material passed to the lowest energy level, called the ground state. The condensate is a quantum property that has no classical analogue . Due to the Pauli exclusion principle, only bosonic particles can have this state of aggregation. This means that the atoms are separated and form ions. A grouping of particles at that level is called Bose-Einstein condensate. "That is, the BCS theory is based on the application of Bose - Einstein on the conductivity and resistivity of a material.
Table periodic elements according to their superconductivity
The other theory, the Ginzburg - Landau, is much better at predicting the qualities if the materials to study are not homogeneous, since BCS theory only works when the energy band to study homogeneous. This theory tries to explain the phenomenon of macroscopic form based on the breaking of symmetries in the phase transition. "The theory is based on a variational calculation which attempts to minimize the Helmholtz free energy regarding the electron density found in the superconducting state. The conditions for applying the theory are
- temperatures have managed to be near the critical temperature, since it is based on a Taylor series expansion around T c .
- The wave pseudofunción Ψ and the vector potential
, must vary smoothly.
This theory predicts two characteristic lengths:
- penetration length is the distance that the magnetic field penetrates the superconductor material
- coherence length: is the approximate size of the Cooper pair "
We find also different classifications between superconductors. According to the theory that best explains them, are classified into conventional (explainable by the BCS theory) and unconventional (can not be explained by BCS theory or its derivatives).
I believe, however, that the classification more important for superconductors is that the divide in type I superconductors, which pass abruptly from the superconducting to normal and type II, which have an intermediate or mixed between the superconductor and normal.
Superconductivity is one of the fields of physics fascinating twentieth century
. He belongs to that small group of scientific advances capable of changing the way of life of mankind. Its range of applications is broad, but includes essentially three types: the generation of strong magnetic fields, making driving cables electricity and electronics. The first type we use as spectacular as the manufacture of levitated mass transit systems, that is, trains that float above their tracks without having friction with them, making it possible to achieve speeds that develop airplanes. The second is the possibility of transmitting electricity from the production centers, such as dams or nuclear reactors to centers of consumption without any losses in transit. For the third type we may mention the possibility of manufacturing extremely fast supercomputers, or any electronic gadget with a range and capabilities far greater than today. Certainly superconductor applications in almost any field of technology, but how do they work? How do we explain their behavior? At this point we will explain the key theoretical behavior of superconductors.
The use of superconductors, as mentioned before, allows the transmission of electrical energy without loss (or minimization) or the production of magnetic fields. It's where I want to focus now.
As you know, the LHC suffered numerous delays in its originally planned, even several months, due to cooling problems. These problems were due to that they could not maintain the low temperature superconductors low enough to operate at full capacity. Keep in mind that a superconducting temperature of one degree above the right can lead to considerable energy losses with all that that entails, not only economic losses but damage from the heat released in the rest of the system.
magnetic train in Shanghai, the "maglev"
With this example I set, we can see the main problem superconductors have today: the need to reach temperatures low enough to make them work.
Although these temperatures are considerably higher today than 100 years ago, when HK Onnes discovered superconductivity of mercury to 4 K, it remains expensive to achieve temperatures so low, about -100 º C in the most advanced compounds, to achieve superconductivity. That is why superconductivity is considered one of the fields of science, along with nuclear fusion, which can cause an industrial and technological revolution with the highest incidence in the world.
If nuclear fusion is not difficult to imagine why. A virtually inexhaustible energy source (operation is the same as the one with the Sun), relatively clean (there are contaminants, derived mainly from the need to maintain a temperature of more than 1 million degrees) and would have multiple applications.
In the case of superconductors, the application will be valid for any field of technology: improvements in power transmission and the elimination of losses as heat will make a global improvement of any system that works by electricity. Will also no doubt to improved performance in electric motors, or even as already used today in building magnetic levitation trains. That is, we help you make better energy we have, at lower cost, especially once it becomes a superconductor at room temperature so that we can achieve more efficient mills and even brand new they are unthinkable today.
NOTE: This entry is part of a work I did recently, as the next on the history of superconductivity. The language is more complicated than usual, although I have simplified and explained the terms "rare" as much as it has been possible.
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