How do impurities affect superconductivity?

Submitted by Infferno122 t3_11o5rkm in askscience

I'm currently in the second year of my Physics degree and had a question about superconductors, which we have not covered in much detail. So a superconductor at a sufficiently low temperature should exhibit 0 resistance but would introducing impurities in the superconductor change this? I imagine it would reduce the critical temperature, but could impurities get rid of the superconducting property of this material completely?

Thanks in advance!

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SwitchingtoUbuntu t1_jbrq3ya wrote

It totally depends on the superconducting metal and the impurity.

Superconductors have a tendency of "proximatizing" other materials, making them superconducting by being near to them.

For example, a 1nm thin film of a normal metal on top of a 100nm thick film of superconducting Niobium will likely superconduct.

Similarly, some superconducting metals when deposited with non-metals actually can become better superconductors. For example, Aluminum that has a little oxygen in it (granular aluminum, or dirty aluminum) actually has a higher superconducting critical temperature than clean aluminum.

That said, if you get too much copper or gold in your superconducting film, it just won't superconduct at all.

The interactions are all really complex and involve the coupling between the lattice of the superconducting metal and the charge carriers.

Look up "BCS theory" for more info!

Source: PhD working in superconducting qubits.

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ZakeDude t1_jbrvfx6 wrote

Really depends on the impurity and the material.

Some increase or decrease the critical temperature. Some affect the critical magnetic field (see Ta or Ti in Nb3Sn) by changing the normal state resistance. You can also increase the amount of current that can pass through the material by increasing the number of microstructural features -- smaller grains, say, or bits of non-superconducting material.

Source: working on a degree in superconducting wires

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MisterKyo t1_jbsdvrw wrote

Broadly speaking, the robustness of the superconducting state depends on the specific material, its pairing mechanism (for Cooper pair creation), type of impurity (e.g. chemical doping, site disorder, magnetic ions, etc.).

I won't comment on specifics (and take some of this with a grain of salt) bc I'm a bit out-of-date and don't work on superconductivity directly, but I have sat through many talks and work on electronic transport phenomena. For conventional BCS superconductors (SC), the superconducting state can be fairly robust to impurities as long as they are not magnetic, although I believe they tend to decrease the superconducting transition temperature (T_c).

With unconventional SC, the phase diagram is a bit more complex due to pairing mechansims that are still an area of research. In the superconducting cuprates, the archetypal phase diagram is a superconducting dome (or domes I hear...) as a function of hole-doping - this means that there exists a minimum amount of chemical impurity (substitutional doping) that creates a superconducting state, an optimal one with the highest T_c, and then a maximally allowed doping where competing states seem to interfere with the SC state. I believe this is a common occurrence in doping phase diagrams, although the interpretation of the underlying physics may differ: e.g. heavy fermion systems and quantum criticality, versus Fe-based SC, versus SrRuO-214. Once again tho, SC is a tangential topic for me, so please do give this a fact check. The idea here is that chemical impurities do not necessarily destroy the SC.

What i believe is true generally for the unconventional SC is the idea of competing ground states. Charge and magnetic order (both long-range and short-range) tend to surround SC areas in the phase diagram. Adding impurities then mess up this delicate balance and tip the system towards one type of order or another.

If you were to dig further into this vast topic, I'd probably suggest going the route of looking up 3 broad topics: 1) effect of chemical substitution on elemental superconductors (old and purity is suspect in some data but well-understood phonon-mediated models), 2) strontium doping in the lanthanum cuprates (La2-xSrxCuO4) and oxygen doping in La2CuO4+d (simplest cuprate systems but still rich in physics), and 3) effects on magnetic and transport properties in metallic alloys (to understand effect of impurities on the normal state of metals).

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SwitchingtoUbuntu t1_jbskuge wrote

Replying to CrazyisNSFW (#2,207,643)

It depends! There is often a difference between bulk and thin film superconductors, and it will depend on loads of details relating to fabrication and the frequency of signals present.

For example, a resonator made of thin film Nb has zero DC resistance but will have dielectrics and other contaminants that give it some loss tangent when in the presence of microwave fields.

We usually talk about "Quality factor" which is sort of a way of saying how many times can a microwave photon bounce back and forth in the circuit before it leaves due to some loss mechanism.

Qs of tens of millions are achievable.

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Leading_Study_876 t1_jbsxe33 wrote

Basic answer is yes it can. The original superconductor was mercury, which had to be distilled before it could be pure enough to go superconductive.

This is covered in this very informative discussion from BBC radio.

I hope this is available in your area. This series is great for curious people wanting to learn about many subjects.

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luckyluke193 t1_jbuu6ht wrote

Replying to ChemicalRain5513 (#2,208,089)

The superconducting coils that generate the magnetic fields for MRI and NMR spectroscopy systems have zero DC resistance.

They usually operate in persistent mode, meaning that there is no power supply attached to them. As long as you keep them superconducting, you can have hundreds of amps circulating in the coils without any change in the current over several years.

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