Split Personality: Type 1 and Type 2 Superconductors

Now that you’ve learned about low-temperature and high-temperature superconductivity and the Meissner Effect, you’re ready for us to throw another concept at you: Type 1 and Type 2 superconductors.

All high-temperature superconductors fall into the Type 2 category, as do all superconducting alloys; superconductors made of just one element (mercury and aluminum, for example) are Type 1’s. But these two classifications shouldn’t get confused. Let’s back up a little.

You know already that temperature can determine whether or not a material becomes superconductive. Well, the presence of a magnetic field can also make or break a material’s superconductivity.

You know from the Meissner Effect that a superconductor can expel an outside magnetic force. What we didn’t tell you is that, if that magnetic force gets high enough, the tables turn: The magnetic field gets the better of the superconductor, which then loses its superconductivity and begins conducting electricity normally again.

The strength of the magnetic field required to break the material’s superconductivity is called the critical field.

What separates Type 1 and Type 2 superconductors is the way in which they revert to normal conductivity in the presence of a critical field.

The way Type 1 superconductors operate is pretty cut and dried (as cut and dried as physics can get!). Let’s take our friend mercury. As you know, this element is superconducting at 4.2 K. But expose it to a magnetic field of .041 tesla or higher, and all bets are off; those Cooper pairs break up and the mercury immediately reverts to normal conduction.

Type 2 superconductors are a little more complicated. They have not one, but two critical fields. Let’s take as an example a material widely used in the superconducting magnets of MRI machines: niobium-tin, or Nb₃Sn. This alloy becomes superconducting at 18 K (assuming the absence of any significant magnetic field).

Below Nb₃Sn’s lower critical field of 0.01 tesla, the alloy remains fully superconducting. Above its upper critical field of about 29 tesla, it ceases superconducting entirely. But in between these two fields the material enters what’s known as a mixed state – essentially, part of it conducts electricity normally, part of it is superconducting.

This mixed state comes about because the superconducting material no longer fully excludes the outside magnetic field; as the field strength increases and approaches the upper critical field, the more the field penetrates the superconductor. The closer to the upper critical field you get, the less superconducting the material becomes.

So unlike Type 1 superconductors, which have a take-it-or-leave-it approach to superconductivity, Type 2 superconductors are willing to spend more time in a middle ground.

The long and the short of this, though, is that Type 2 superconductors generally can sustain superconductivity in the presence of much higher magnetic fields. This is of tremendous consequence to Magnet Lab scientists and others who need high magnetic fields for their experiments. All of the superconducting magnets used in high-field research are Type 2.

Superconductivity is a unique and powerful tool in scientific research, and promises great benefits for the planet. As the force that makes MRI possible, superconductivity has already improved – and often saved – the lives of countless patients and their doctors. Other applications of the technology have been under development for years, including maglev “levitating” trains and superconducting power cables that could revolutionize electricity across the planet. You may well reap the benefits of this research in your lifetime. Click on any of the provided links to dig deeper into the topic of superconductivity.

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Thanks to the Magnet Academy's scientific adviser on this article, Dr. Scott Hannahs, Research Associate and Chief of User Research Instrumentation at the Magnet Lab.