Introduction to Superconductors

In 1908, Dutch Physicist Heike Kammerlingh Onnes became the first man to cool helium to its liquid state, 4K. Though he initially only liquefied a few milliliters, it opened up new doors to low-temperature experimentation. In 1911, he began to experiment with many metals at low temperatures. It had been known that the resistivity of a metal decreased as its temperature decreased, but it was unknown whether it would decrease indefinitely or reach some small limiting value. Some scientists, William Kelvin included, believed that the flow of electrons would cease as the conductor approached absolute zero. Onnes took a rod of very pure mercury and measured its resistivity as he gradually reduced its temperature. To his surprise, once the temperature reached a certain point, the resistivity vanished completely! As he noted, "Mercury has passed into a new state, which on account of its extraordinary electrical properties may be called the superconductive state." Onnes was awarded the Nobel prize for this discovery in 1913.

Resistivity in Normal Conductors

In order to understand superconductivity, it's helpful to understand some of the basic properties of electricity in normal conductors. Without a difference in potential, there is no net flow of electrical charge in a conductor. Once a potential is introduced across a wire, an electric field is established, and electrons are "pushed" by this field. The electrons flowing through a conductor in such a way is called a current. However, these electrons do not move unimpeded; they collide and interact with the atoms of the conductor. These collisions and interactions impede (or resist) current flow; the collective result of which is called resistance.

 A Mechanical Analogy of Resistance- Visualize the orange balls as electrons, flowing through a normal conductor. The nails in the board are like the atoms of the conductor, and the tilt of the board represents the voltage applied across the conductor (gravitational potential represents electrical potential). Obviously, these orange balls are going to bounce around a bit on the nails (like a pachinko machine), rather than simply falling straight through. This bouncing is analogous to electron interactions with conductor atoms which is the cause of resistance.

The resistance of a given material is a function of several factors: its length, cross-sectional area, the material's properties (including crystal structure), impurities in the material, and its temperature. For this thought experiment, we will assume a wire of given length and area, so those factors will be disregarded. Considering temperature as atomic vibrations, it seems natural that this would impede current; the more energetically atoms vibrate, the more likely they are to interact and collide with passing electrons. Now, turn that argument around: the less energetically atoms vibrate, the less likely they are to interact or collide with passing electrons. Furthermore, as Onnes discovered, in some materials it is possible for electrons to flow without interacting or colliding with atoms at all, given the temperature is below a critical point. This is the fundamental idea of superconductivity.

However, that idea can not be extrapolated to all materials; that is, not all conductors become superconductive past a critical temperature. For example, Gold, Silver, and Copper- some of the best conductors at room temperature- do not become superconductive at all. As mentioned above, this is due to the material's properties and crystal structure. To get a better understanding of this, think again of the mechanical analogy. If the nails on our inclined board were lined up in such a way that the orange balls had free, unimpeded paths, it is likely that they would roll down the board without bouncing on the nails at all. Electronically, if the crystal structure of a material has analogous ideal alignment, electrons can flow with little or no resistance!

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TJ Barry MMIII