‘Beyond Resistance’: The Story of Superconductors

Like their name, superconductors are the superman of physics, defying the laws and norms we study in school about resistance and conduction. Moreover, they are a relatively new discovery and a generous one, awarding 5 Nobel Prizes in Physics to 12 scientists, so far. Let’s learn about this almost magical substance, through dance, levitation and extreme cold.

Conduction is the property of letting current pass through. It is found in metals and some other materials (like graphite) which contain delocalised electrons (electrons free to move within the structure). However, no matter how high the conductivity of a material is, it also has some resistance, known as its resistivity. Resistance is the force which hinders current and thus leads to loss of energy as heat, a phenomenon you may notice when an electrical appliance or wire gets hot. But what if I told you that there were materials with zero resistance; materials which ensure that no current is lost as heat and electrons are able to move freely forever. Presenting to you: superconductors.

Superconductors were discovered in April 1911 by Dutch physicist Heike Kamerlingh Onnes (1853-1926), as he was studying the resistance of solid mercury at very low temperatures, using liquid helium as a coolant (which he first made). He noticed that at 4.2 K (-268.95 C), the resistance of mercury became zero. This phenomenon was later in 1957 explained by 3 American physicists (John Bardeen, Leon N. Cooper and J. Robert Schrieffer) at the University of Illinois, using quantum mechanics.

You see, when electrons move through a material, they frequently or occasionally collide with nuclei in their paths. These collisions result in the loss of energy from the electrons, and subsequently become resistance of the materials. According to the Bardeen-Cooper-Schrieffer (BCS) Theory, below the critical temperature, electrons formed into Cooper pairs (named after Leon Cooper), held by atomic-level vibrations (phonons). Electrons in a Cooper pair repel each other and spin in opposite directions at same speeds. They thus navigate through the waves between electrons and the material’s structure, thus avoiding collisions with nuclei. You can think of them as two dancers, dancing away from one another but with the same sync and avoiding collisions with others meanwhile.  The cooler the particles, the more organised their ‘dance’. Thus resistance becomes zero.

Superconductors are materials which are able to conduct electricity with no resistance, below a certain temperature, known as critical temperature (Tc). These temperatures are usually very low, and thus require much labour, cost and energy to be enacted. With regards to critical temperatures, superconductors are divided into 2 types:

1. Low Tc Superconductors:

These include niobium and titanium. At normal pressures, their critical temperatures vary from 0.000325-7.8 K (-273.15- -265.35 C). Some of them require high pressures, like sulphur which needs 9.3 atmospheric pressures and 17 K (-256.15 C) to become superconductive.

• High Tc Superconductors:
  These include copper, lead, iron-based materials or heavy metal oxides. They have relatively high critical temperatures, which doesn’t quite follow the Bardeen-Cooper-Schrieffer (BCS) Theory. Their mechanisms are not fully known. The ceramic copper oxides, for example, are superconductive at 77 K (-196.15 C).

Research is constantly underway to discover or invent a superconductor which can be superconductive at room-temperatures and normal atmospheric pressures, so that we can have more efficient wires and appliances. In 2023, researchers in South Korea claimed to have discovered such a superconductor, which they named “LK-99”. However, it was rejected as a superconductor by the Condensed Matter Theory Centre at the University of Maryland.

The Meissner Effect

Superconductors also do another cool thing. Imagine dropping a magnet over a superconductor. You would expect the two to be attracted, as magnets attract metals. But no! The magnet you dropped stays in mid-air above the superconductor, without you having to say Wingardium Leviosa.

Superconductors suspend other magnetic objects in mid-air by cancelling their magnetic fields. This is done by creating their own magnetic field from the flowing current (current creates a magnetic field, according to the Oersted law), which is powerful enough to oppose gravity itself. This process is known as ‘diamagnetic levitation” or the Meissner Effect as it was discovered in 1933 by German physicists Walther Meissner and Robert Ochsenfeld.

Applications

Despite our inability to build an easily usable superconductor, we still use them in lots of ways. In medicine, superconductive wiring is used in MRI (Magnetic Resonance Imaging) machines, to generate powerful magnetic fields which increase the ability of radio waves to penetrate the human skull, and detect accurate images of the brain. Due to the complete lack of resistance, superconductors are able to do this at low power inputs.

Superconductors are also being used in nanowires in particle accelerators to detect photons (light particles). Quantum computers also use superconductive qubits (basic unit of information; quantum counterpart of bit) to channel current through.

Maglev trains also use two sets of superconductor systems. One of them repels the train upwards into the air (using the Meissner effect), while the other accelerates it forward. Thus the all-electric trains are able to move at 300 miles per hours (482.8 km/h) on a frictionless path of a sort of ‘air cushion’.