# Uses of type I superconductors

## Main Question or Discussion Point

Does anyone know of any practical uses for a type I superconductor, where it isn't possible to use a type II (which typically have higher critical temperatures so need less cooling etc)?

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Gokul43201
Staff Emeritus
Gold Member
There must be a reason that Type I SCs are used in SQUIDs (resolution?).

f95toli
Gold Member
Whether or not a superconductor is type I or II is generally of little importance in most applications. All high-Tc superconductors are type II so if you want to work at high temperatures there is no choice.
There are only a few low-Tc superconductors that are actually used and most of them are type II. Niobium and its superconducting alloys are all type II and are used in almost all real-life applications of superconductivity (including MRI magnets) since the Tc is rather high and they can carry reasonably high currents.
Anyway; the most important low-Tc superconductor for fundamental research is aluminium but that has little to do with the fact that it is type I; the main advantage is instead that aluminium-oxide is a very good insulator and can be created by simply exposing aluminium to an oxygen atmosphere during fabrication. This means that high-quality junctions can be created which in turn means that we can fabricate multilayer structures such as Josephson junctions, SQUIDs, SETs etc using relatively straightforward methods (shadow evaporation). It is MUCH harder to fabricate good Nb structures (and even then aluminium-oxide is used as the insulator).
Aluminium also has a fairly high Tc (1.2-1.6K) meaning even a simply He-3 cryostat will often do.

Most low-Tc SQUIDs are fabricated from niobium and aluminium SQUIDs are rarely used as actual magnetometers; Al SQUIDs are usually just used as "tunable Josephson junctions" since we can control the critical current (and therefore Ej) using an external magnetic field (this is used in e.g. split Cooper-pair boxes etc).

Gokul43201
Staff Emeritus
Gold Member
Most low-Tc SQUIDs are fabricated from niobium and aluminium SQUIDs are rarely used as actual magnetometers; Al SQUIDs are usually just used as "tunable Josephson junctions" since we can control the critical current (and therefore Ej) using an external magnetic field (this is used in e.g. split Cooper-pair boxes etc).
I have no idea what a split Cooper-pair box is, but thanks for the clarification anyway.

Aluminium also has a fairly high Tc (1.2-1.6K) meaning even a simply He-3 cryostat will often do.
Or even a He-4 cryostat! (or was that a typo?)

f95toli