Superconductivity: Breaking the Resistance

Electronic devices utilise the energy stored in electric currents as they flow through a series of wires, or circuit. Electrons are obstructed due to collisions with vibrating metal ions within the wire, resulting in energy losses and giving rise to the phenomenon of resistance. Wires at higher temperatures contain ions that vibrate more violently and will therefore exhibit a higher resistance.

Whilst there are plenty of electronic devices that use resistance constructively to perform clever operations, resistance in wires and transmission lines results in decreased efficiency, increasing costs for electricity companies and users. Engineering materials with low resistance is one of the main steps in pioneering effective ways to power the world. And, it turns out, we can go one better than creating materials with low resistance.

What is Superconductivity?

Superconductors are a class of materials that, at very low temperatures, exhibit zero resistance, allowing an electric current to flow indefinitely without being impeded. Superconductivity was discovered by Heike Kamerlingh Onnes in 1911, three years after he succeeded in creating liquid helium, making the low temperatures necessary for observing superconductivity accessible.

There are elements (as it happens a lot of them) that, when cooled below a certain temperature, undergo a sudden transition into the superconducting state. The temperature at which this transition takes place is known as the critical temperature, and, whilst different for each element, is usually less than 10 Kelvin (-263 degrees Celsius) for metals. 

The dependence of resistance on temperature for a normal conductor and superconductor. At the critical temperature, the resistance of the superconductor drops rapidly to zero. The critical temperature varies for different elements but is usually between 0 and 10 Kelvin.

BCS Theory

How do electrons travel through a superconductor without losing energy during collisions with the wire? The answer was found by Bardeen, Cooper and Schrieffer in 1957 when they discovered the theory that now holds their initials. BCS theory proposes that the electrons in a superconductor pair up to form Cooper pairs

A lone electron is a fermion, meaning it cannot occupy the same energy level as other electrons nearby. A pair of electrons, however, forms a composite boson, allowing neighbouring electrons to occupy a single energy level.

It turns out that Cooper pairs cannot interact with the metal ions in the wire (provided the ions have low enough thermal energies) and thus pass without colliding. The interaction between the paired electrons is mediated by a phonon, a small vibration within the wire, and can only be broken by a certain amount of energy. The pairing energy between electrons is large enough to overcome mutual electric repulsion, meaning Cooper pairs can remain intact over long periods of time.

As long as the superconductor remains below its critical temperature, there isn’t enough thermal energy to separate the paired electrons and the supercurrent persists. Heating the superconductor above its critical temperature means vibrations in the wire become energetic enough to separate the Cooper pairs, and the material will no longer exhibit superconductivity.

Type I and Type II

There are two classifications of superconductors. Type I superconductors can exist in strictly two states – superconducting, or normal, depending on if the material is above or below the critical temperature. This type of superconductivity is normally exhibited by pure metals such as aluminium or lead.

Type II superconductors are less binary; they can assume intermediate states, where sections of the material remain superconducting whilst others begin to return to normal – such sections are known as vortices. As long as there is still one path through the material with zero resistance, superconductivity will continue to be observed. Alloys are often deployed; the different materials within the alloy have different electrical properties and different critical temperatures, meaning sections of the material exhibit superconductivity whilst others transition back to normal.

Higher Temperatures

Recently, there have been a number of breakthroughs in creating superconductors that have much higher critical temperatures. Such materials tend to be oxides and are known as the cuprates, with critical temperatures of between 100 and 175 Kelvin (-173 and -98 degrees Celsius).

There is currently a huge amount of research being conducted into the possibility of engineering room temperature superconductors, which would, in principle, allow us to deploy such materials in everyday electronics. The cost of electricity would decrease dramatically resulting in savings for many across the globe. In October, a team successfully observed superconductivity at around 12 degrees Celsius. 

The downside to these studies is that the material is required to be placed under extremely high pressure to become superconducting. Further studies aim to address this issue and work towards a future containing room temperature and pressure superconductors.

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