# What exactly is superconductivity?

Does superconductivity mean that the coulomb force or some magnetic force has gone up? I guess that it applies only to wires which get less resistance due to cooling... Is this wrong?

Also, Are there different kinds of superconductive states?
Is there a kind of superconductivity only for magnetism?

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Hi mick to Physics.SE! Currently your text contains five questions, in most it is not really clear what you want to know. A good starting point is the Wikipedia article on superconductivity. If I try to guess, the answers to your questions would be: no, no, no, yes plenty, no. – Alexander Sep 13 '12 at 15:23
Part of learning is asking what is the correct question ;) Also i find wiki not always clear. ( sometimes even wrong although i dont know about physics but thats another discussion ) I will consider restating my question(s) , but i need some time. Thanks for comment. – mick Sep 13 '12 at 15:39

Before thinking about Superconductivity, let's see Resistivity. Current flow appears in a direction when free electrons travel in opposite direction. During the traverse of free electrons, there would be a lot of collisions with atoms in the metal lattice. Hence, "Atoms experience vibrations about their mean position due to the passing waves of free electrons depending upon the temperature." (Thermal activity plays some role here)

By Ohm's law, resistance $R=\frac{V}{I}$ and also $R=\rho\frac{l}{A}$ where $\rho$ is the Specific resistance or Resistivity. Note that, these are only for normal conductors...

But for superconductors, this is not applicable for sure. Because, something happens differently in superconductors. At extremely low temperatures, the vibrations of atoms slow down so much that they synchronize with those passing waves of electrons..! (i.e.) The free electrons pass unobstructed through the complex metal lattice. Hence, there would be no wastage of electrical energy in the form of heat.

The Superconductivity was very well explained by the well-known BCS Theory. On cooling certain materials below a certain temperature called Transition temperature or Critical temperature $T_c$, three changes happen in a material which makes it a superconductor. Its resistivity becomes absolute zero. Its conductivity becomes Infinity. It's perfectly diamagnetic and excludes Magnetic flux lines (as a result of Meissner Effect). Hence none of your forces are emitted from superconductors. They always repel magnets (i.e) They develop a magnetic polarity opposite to that of the applied field and hence, they don't allow magnetic fields to pass through them.

I would also add that, it's just an overhead view. For a brief zoom, refer the links in Wikipedia. It gives an extra detail in terms of Fermi levels, Cooper pairs and phonon which would be very helpful for this Novel-like question. Perhaps @Alexander's comment provides the answer...

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Superconductivity the property of a material to induce no resistance on electric currents when they are passed throughout the material. We see that no material is a complete insulator — all materials conduct electricity when cooled down to a certain temperature. So no material is, say, 100 percent resistant, resistance is just a measure of how much a material can conduct at room temperature.

However, there’s a really interesting relation between the increase of temperature and the conductivity of a material. Let’s take gold, for example. Gold is one of the best conductors there is — but it shows more resistance as it is heated. This relation is shown throughout all materials.

Now, where does this factor into superconductivity? We see that the cooler we make a material, the more conductive it gets. The physicist Heike Kamerlingh Onnes tested this, and found that when he cooled a wire of Mercury to -269 degrees Celsius, the resistance dissapeared.

The BCS Theory also helps explain this phenomenon. The theoory explains that materials suddenly become extreme conductors when the electrons inside them join forces to make what are called Cooper pairs (or BCS pairs). Normally, the electrons that carry electricity through a material are scattered about by impurities, defects, and vibrations of the material’s crystal lattice (its scaffold-like inner structure). That’s what we know as electrical resistance. But at low temperatures, when the electrons join together in pairs, they can move more freely without being scattered in the same way. [1]

Not all materials show superconductivity. Apart from mercury, the original superconductor, you can find the effect in about 25 other elements (mostly metals, semimetals, or semiconductors), though it’s also been discovered in thousands of compounds and alloys. Each different material becomes a superconductor at a slightly different temperature.

However, some materials are superconductive at high temperatures. It was thought that materials were only superconductive at low temperatures, but in 1986, two European scientists working for IBM, German physicist J. Georg Bednorz (1950–) and Swiss physicistK. Alex Muller (1927–), discovered a ceramic cuprate (a material containingcopper and oxygen) that could became a superconductor at much higher temperatures [1]

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