Technologies like magnetometry (the measure of magnetism) and magnetic resonance imaging rely on the strength of magnetic fields. With increasing need for experimental precision and control, new physics are sought after to develop stronger electromagnets. One way is to eliminate a material’s electrical resistance. This is known as superconductivity. Herein we discuss what superconductivity is.
In physics we learn that electrons may be excited into higher energy states. We can excite electrons using energy carried by light (known as photoexcitation).
Semiconductors respond to photoexcitations. These are materials that exhibit an electrical resistance unlike that of insulators or conductors. Their electronic behaviour, the way that electrons “move” through the material, is often temperature dependent. All light carries energy. If this energy is absorbed by matter, the matter heats up. It is this heat that excites electrons, so we can use light to change the electronic behaviour of semiconductors. This makes them useful objects of study.
For simplicity, we treat these semiconductors as patterned arrangements of atoms (called a lattice). Where there are “gaps” in the arrangement, for not every lattice is densely packed, we imagine varying densities of electrons. It is reasonable to imagine these electrons as a cloud that pervades the atomic arrangement. If we excite the semiconductor with light, this electron cloud may change, thereby changing the semiconductor’s electronic behaviour.
Where does superconductivity arise? As mentioned, semiconductors are temperature dependent, so they respond to heating (in our example, by way of photoexcitation). What if we cool a semiconductor instead? As a general rule, the electrical resistance of a semiconductor increases as its temperature decreases. However, when cooled to a temperature near absolute zero (the semiconductor’s critical temperature), its electrical resistance vanishes. All magnetic field lines, like those seen with iron filings dropped around a bar magnet, are expelled from the interior of the semiconductor.
“Levitation of a magnet on a superconductor” (source: Wikimedia Commons, available under CC BY-SA 3.0)
This bizarre property of matter, while it is not universal, is the direct consequence of a lack of electron excitations. Recall that excited electrons (like those in an electronic circuit) emit energy as they relax. This energy is absorbed by matter, so energy is lost as heat. Electrons also need somewhere (i.e. a higher energy state) to be excited towards. This “somewhere” is unique to the material, so the material determines how and where electrons are excited.
“Overview of superconducting critical temperatures for a variety of superconducting materials since the first discovery in 1911” (source: Wikimedia Commons, available under CC BY-SA 4.0)
Supercooled materials lose many pathways to these higher energy states, so their electrons are never excited unwillingly, so no energy is lost as heat. Since electrical resistance in semiconductors is temperature dependent, a supercooled and thereby “heat-less” semiconductor has zero electrical resistance. It is superconductive.
– Eric Easthope
The systematic study of candidate superconductors (like bismuth selenide) is ongoing at UBC’s Stewart Blusson Quantum Matter Institute.
Source: General Chemistry: Principles, Patterns, and Applications (2012). Saylor Academy. Available here under CC BY-NC-SA 3.0.
