What is Superconductivity? (Part 1)

The incredible phenomenon of superconductivity was discovered in 1911 and celebrated its centenary last year. The breakthrough led to a great number of further discoveries and exotic phenomena, leading in turn to technical applications such as superconductive magnets for particle accelerators, magnetic resonance imaging, maglev trains that can reach speeds of 581 km/h, transformers for power grids, and SQUIDs (superconducting quantum interference devices) for detecting mines and submarines.

Despite the recent technological explosion, mysteries still surround the phenomenon. Here I have taken advantage of a fictional centenary celebration at Leyden, in the Netherlands, to interview some of the protagonists in this marvellous adventure and have them guide us through a reconstruction of superconductivity’s history up to its open-ended finale.

Good Morning, Prof. Onnes. So, how did everything start?

Onnes: Good Morning. Yes, well … it was in April of 1911, when I was carrying out measurements of the electrical resistivity of mercury at low temperatures. You know, I had managed to get liquid helium reaching the temperature of 4 Kelvin just two years before … We are talking about –269 Celsius: terribly cold! Cryogenics was such a fashion in those days.

Yes, as often occurs in science, no?

Onnes: Of course. The idea was to study mercury at very low temperatures – when solid – but something astonishing happened when I reached the temperature of liquid helium.

To do with the resistivity?

Onnes: It suddenly dropped to zero, I mean … it utterly vanished. I repeated many times, but that was it: amazing!

Why was this?

Onnes: As you know, resistivity is a physical parameter that measures the resistance of a material to the flow of an electric current. The fact that it vanished implies that the electric flow is completely free within the material. This is something that conducting materials, such as metals, normally do not have. They always have a certain resistance to the electric current and this is manifest.

How does this show up?

Onnes: Through the so called Joule effect: the metal usually gets hot because of the resistivity against the electric current. It simply generates heat and the electric current decreases with time. But when we let an electric current pass through a mercury coil at around 4 Kelvin, expecting it to diminish progressively, it instead persisted invariably, like perpetual motion. It was amazing!

So, in summary: mercury is a metal and it conducts electric current, usually losing a bit of it. But at extremely low temperatures, like -269 Celsius, it conducts without any current loss. Does this happen in other metals too?

Onnes: Oh yes. We later found that lead also exhibits the same property at 7 Kelvin and I called the property superconductivity.

And you got the Nobel prize for this in 1913? Remarkable indeed. Congratulations!

Onnes: Thank you! I am very proud of it.

The new science progressed in the following years until the discovery in 1941 that not only pure metals but also metal alloys could become superconductors at low temperatures. Am I right Dr. Aschmann?

Aschmann: Yes, we found that niobium-based alloys, in particular, are superconductors at around 16 Kelvin. We looked for metal alloys, because soft metals such as tin and lead deteriorated easily under a thermal cycle between room temperature and that of liquid helium.

That’s interesting. I also notice that the temperature for superconductivity starts to increase …

Aschmann: Yes, the so-called critical temperature is higher for alloys than for pure metals. Our niobium nitride alloy is just one example. Later on in 1954, Americans found that niobium-tin can be superconductive at 18 K”.

I see. Prof. Meissner and Dr. Ochsenfeld, you found out something very important about superconductors in 1933. What was it?

Meissner: We realised that superconductors are not only perfect conductors in virtue of losing resistance to electric current. They are something more. When we put superconductive lead and tin in a magnetic field, this was pushed out of the materials. This means that a magnet on top of the superconductor will levitate, or float on top of it. The superconductor expels nearly all the magnetic flux. It does this by setting up electric currents at its surface. These currents generate a magnetic field which cancels the external magnetic field in the bulk.

This is very interesting. Is this valid whatever the strength of the magnetic field?

Ochsenfeld: Actually, no. There is a critical magnetic field for a superconductor, like a critical temperature. For magnetic fields higher than the critical point the superconductivity ceases.

London: I described a model in 1935, which explains the Meissner-Ochsenfeld effect. The magnetic field is not completely cancelled near the surface of the superconductor, but it is within the bulk. The effect is due to the system seeking a stable energy within a magnetic field.

Mmm … you also found something else curious, didn’t you, London?

London: Yes, try to spin a superconductor and you will observe a magnetic field generating. It aligns exactly along the spinning axis. This effect was used in the “Gravity probe B” experiment to measure the space-time curvature generated by the Earth, but that is another story …

Fantastic! Let us go back to the critical factors of superconductivity. So, there are a critical temperature and a critical magnetic field, am I right, Prof. Landau?

Landau: You are approximately right. In truth, the scenario is a bit more complex. In 1950 Ginzburg and I developed a theory from quantum mechanics, which demonstrated the existence of two types of superconductors. At least, this is what Prof. Abrikosov spotted.

Abrikosov: Both types have a unique critical temperature above which superconductivity is completely destroyed, but while Type I superconductors have also a unique critical magnetic field, Type II have two critical magnetic fields. In Type I the magnetic field is completely expelled when below the critical value.

However, in the intermediate range of magnetic fields of Type II it’s possible for there to be a partial penetration of magnetic field into the superconductor while maintaining the flow of an electric current without resistance – as long as the current is small. Only when the magnetic field overcomes the higher critical value does superconductivity disappear completely.

The intermediate state in type II is called the “vortex state”, because it corresponds to the occurrence of quantized vortices in the electronic current.

Essentially the two types correspond to two different behaviours of the superconductor under the increase of the magnetic field but not to the temperature of superconductivity, right? What does this exotic phenomenon really consist of at the microscopic level?

Maxwell: Well, the experiment I did with Reynolds in 1950 gave some fundamental hints. We observed an isotopic effect on the critical temperature, which means this varies with the variation of the mass of some atoms. This pointed to the role of crystal lattice vibrations, also called phonons, in determining the phenomenon.

Bardeen: Indeed, their experiments were very helpful for constructing a quite complex theory which explained the superconductivity. The basic idea is that superconductivity arises from the coupling of electrons in pairs. The microscopic explanation of the occurrence of resistivity in conductors is the presence of lattice vibrations in a crystal. The atoms in a crystal lattice vibrate even at very low temperatures and electrons in movement across the crystal can be scattered by the atomic vibrations.

I see, so why is this different for a superconductor?

Cooper: Because in a superconductor the electrons do not move alone but in pairs. These pairs form thanks to the atomic vibrations in the crystal. The electrons being in pairs makes a huge difference. They become ‘bosons’ and they no longer obey the Pauli principle. In essence, this prevents each pair from interfering with other pairs. Moreover, the pairs do not see the crystal lattice, because the crystal lattice actually help the pairs to form continuously.

How does this happen?

Schrieffer: The phonons act as ‘bridges’ between two electrons, making them couple. In virtue of the same electric charge, two electrons would normally repel each other, but if the temperature is sufficiently low an electron approaching a metallic ion induces its movement, or a phonon vibration, which in turn ‘calls’ another electron to it, acting as an attractive point of the lattice. The result of the two combined actions is that the two electrons chase each other, rather than repelling. A Cooper’s pair is formed.

Of course, this is only possible at sufficiently low temperatures: how low?

Bardeen: We predicted about 30 Kelvin as the limit, above which no superconductivity can exist …but we were eventually wrong.

Well, Bardeen, Cooper, and Schrieffer – all three of you still got the Nobel prize in 1972 for your BCS theory. Congratulations! And superconductivity continued to amaze us with exotic phenomena, while the Westinghouse company started to produce the first superconductive magnets from niobium-titanium alloy.

Josephson: Yes, if you construct a junction with two adjacent superconductors separated by a thin layer of insulating material, you can observe a super-current flowing across the junction.

The Josephson effect! You predicted it in 1962 for which you later won the Nobel prize.

Josephson: Correct! It is extremely important in technological applications, such as superconductive quantum interference devices (SQUID), which use my junctions and allow us to measure extremely weak magnetic fields, like that of the Earth.

Then, in 1964, Bill Little predicted the possibility of carbon-based organic superconductors and Neil Ashcroft hypothesised that metallic hydrogen could be a superconductor at room temperature when squeezed under extremely high pressures, due to the extremely high speed of sound and the strong electron-phonon coupling. So far, though, nobody has managed to do such an experiment.

Continued in Part 2.

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