Superconductivity

When Thomas Edison went about providing electric power to New York City in the late 1800s, he knew that energy was dissipated as heat in the wires that delivered electric current to his customers. This reduced the amount of power that made it to the homes of his customers, and presented Edison with the problem of trying to minimize this power loss. (I talked about this in a previous blog entry.)

The issue faced by Edison was one of electrical resistance, which is a measure of the degree to which an object opposes an electric current through it. When current flows through an object with resistance, electrical energy is converted to heat at a rate equal to the square of the current times the resistance. This rate is a measure of power loss.

While Edison had means to lessen this loss of power, he couldn't escape it completely. That's because conductors (i.e. materials that conduct electricity) naturally heat up as an electric current moves through them. The electrons that comprise this current, as they snake forward through the material, are constantly bumping into the atoms (ions) of the conductor. At each collision, an electron loses a bit of kinetic energy to an ion, increasing the kinetic energy of the ion, generating heat and increasing the temperature of the conductor. While conductors exhibit less resistance at lower temperatures, ordinary conductors can never be cooled enough to achieve zero resistance.

It was in 1911 that a scientist, Heike Kamerlingh Onnes, discovered that certain unordinary conductors, under certain conditions, do possess zero electrical resistance. That is, passing an electric current through these materials does not result in the heating of the materials and, therefore, no power is lost in them. The reason why no one had seen such behavior before: it only takes place in certain materials, and these materials have to be unimaginably cold. It was only just prior to 1911 that such cold temperatures were achieved in the laboratory (by Onnes). Onnes had taken helium gas and got it so cold (down to 4.2 degrees above absolute zero) that it condensed into a liquid. Using this liquid helium as a refrigerant, he tested the electrical resistance of mercury and was amazed to find that it actually dropped to ZERO! Such behavior was a completely new phenomenon, never before witnessed. Onnes labeled it "superconductivity."

Onnes didn't understand what was going on inside the superconducting material. How could the electrons avoid bumping into the material's ions, passing kinetic energy to them? Why did such behavior occur only below a certain temperature, labeled the critical temperature? Twenty-two years later, in 1933, the answer was still unknown. But in this year, Walter Meissner and Robert Ochsenfeld made an important new discovery about superconducting materials (which, as a class, had expanded to include materials other than mercury). They found that superconductors expelled applied magnetic fields. Magnetic field lines that passed through a sample of material were, in a sense, pushed out of the material (or more accurately, cancelled within the material) when the material was cooled below its critical temperature. This finding, now known as the Meissner effect, provided evidence that superconductivity was, most fundamentally, a magnetic phenomenon. Such a finding also changed the mindset that the fundamental property of a superconductor was zero resistance.

A theory explaining the phenomenon of superconductivity was proposed in 1957 by John Bardeen, Leon Cooper, and Robert Schrieffer. It became known as the BCS Theory, after their initials. It had to do with phonons (not photons) and Cooper pairs. Phonons are quantized crystal lattice vibrations. What does this mean? Certain materials exist as crystals, which means "the constituent atoms, molecules or ions [which are atoms or molecules with a net electric charge] are packed in a regularly ordered, repeating pattern in all three spatial dimensions." (Wikipedia) The graphic below is an example of a unit cell, which is periodically repeated in three dimensions to form a crystal. Each sphere represents an atom and the tubes represent bonds between atoms.


A lattice is a sort of framework upon which, at each point, there exists a unit cell like you see pictured above. So the crystal looks the same when viewed from any lattice point. As an electron moves through a crystal, it exerts a force (i.e. it pulls) on the positively charged lattice ions, distorting them towards its (the electron's) path. As the electron then moves away from that point on the lattice, the lattice ions return to their original position. Because all atoms in a crystal are connected, "the displacement of one or more atoms from their equilibrium positions will give rise to a set of vibration waves propagating through the lattice." (Wikipedia) Finally, these vibration waves are quantized, which means they can't possess just any amount of energy but only certain discrete numerical values.

What happens as an electron moves through a crystal, generating a phonon? Let's picture an electric current flowing through the material. One electron after another. An electron zips past a point in the crystal lattice, distorting the lattice through the creation of a phonon. The lattice is pulled inward towards the negatively-charged electron, but the electron quickly moves away, faster than the lattice can relax back to its original position. This creates a region of positive charge, as the lattice ions that are pulled inward are positively charged. Here's the cool part. A second electron can be attracted to the region of positive charge along the path of the first electron. And these two electrons, which would normally repel one another (because they are both negatively charged), can become bound to one another. "If this binding energy is higher than the energy provided by kicks from oscillating atoms in the conductor (which is true at low temperatures), then the electron pair will stick together and resist all kicks, thus not experiencing resistance." (Wikipedia) These electron pairs are called Cooper pairs, and they lie at the heart of the BCS Theory. They are what allow for superconductivity; they carry the superconducting current. But, as noted just above, the temperature has to be low. Above a critical temperature, the atoms in the crystal are jostling around too much, bumping into the electron pairs with enough force to knock them apart. This breaking apart of the Cooper pairs destroys superconductivity in the material, and the material becomes "normal." What's the highest temperature at which a known material will superconduct? A special ceramic material comprised of many different atoms has been observed to superconduct at -135 degrees C. Notice the negative sign. The holy grail of those working in the field is to find a material that superconducts at room temperature. (Obviously, no material yet identified would have helped Edison ... although there are techniques, which I addressed in a previous blog entry, that lessen the problem.)

The material that superconducts at -135 degrees C (or 138 K), like all materials that superconduct above around -243 degrees C (or 30 K), is called a "high-temperature" superconductor. This is obviously a relative term. Such materials are not consistent with the BCS Theory and there is no good theory to describe how these high-temperature superconductors work.