Experiments:
- "An introduction to superconductivity" by Alfred Leitner originally published in 1965, source: www.alfredleitner.com/
- Isotope effect on the critical temperature. hyperphysics.phy-astr.gsu.edu/hbase/Solids/coop.html mentions that:
If electrical conduction in mercury were purely electronic, there should be no dependence upon the nuclear masses. This dependence of the critical temperature for superconductivity upon isotopic mass was the first direct evidence for interaction between the electrons and the lattice. This supported the BCS Theory of lattice coupling of electron pairs.
Lectures:
Video 1. 20. Fermi gases, BEC-BCS crossover by Wolfgang Ketterle (2014) Source. Part of the "Atomic and Optical Physics" series, uploaded by MIT OpenCourseWare. Actually goes into the equations.Notably, youtu.be/O_zjGYvP4Ps?t=3278 describes extremely briefly an experimental setup that more directly observes pair condensation.Video 2. Superconductivity and Quantum Mechanics at the Macro-Scale - 1 of 2 by Steven Kivelson (2016) Source. For the Stanford Institute for Theoretical Physics. Gives a reasonable basis overview, but does not go into the meat of BCS it at the end.
Lecture notes:
Media:
- Cool CNRS video showing the condensed wave function, and mentioning that "every pair moves at the same speed". To change the speed of one pair, you need to change the speed of all others. That's why there's not energy loss.
Transition into superconductivity can be seen as a phase transition, which happens to be a second-order phase transition.
andor.oxinst.com/learning/view/article/measuring-resistance-of-a-superconducting-sample-with-a-dry-cryostat Not a video, but well done, by Oxford Instruments.
TODO!!! Even this is hard to find! A clean and minimal one! Why! All we can find are shittly levitating YBCO samples in liquid nitrogen! Maybe because liquid helium is expensive?
We know that superfluidity happens more easily in bosons, and so electrons joins in Cooper pairs to form bosons, making a superfluid of Cooper pairs!
Isn't that awesome!
As of 2020, basically means "liquid nitrogen temperature", which is much cheaper than liquid helium.
The dream of course being room temperature and pressure superconductor.
Upside: superconducting above 92K, which is above the 77K of liquid nitrogen, and therefore much much cheaper to obtain and maintain than liquid helium.
Downside: it is brittle, so how do you make wires out of it? Still, can already be used in certain circuits, e.g. high temperature SQUID devices.
Discovered in 1988, the first high-temperature superconductor which did not contain a rare-earth element.
Superconductivity is one of the key advances of 21st century technology:
- produce powerful magnetic fields with superconducting magnets
- the Josephson effect, applications listed at: Section "Applications of Josephson Junctions"
As of 2023 the most important ones economicaly were:The main application is Magnetic resonance imaging. Both of these are have to be Liquid helium, i.e. they are not "high-temperature superconductor" which is a pain. One big strength they have is that they are metallic, and therefore can made into wires, which is crucial to be able to make electromagnetic coils out of them.
- Nb-Ti: the most widely used one. Used e.g. to create the magnetic fields of the Large Hadron Collider Up to 15 T.
- Nb-Sn: more expensive than Nb-Ti, but can reach up to 30 T.
TODO, come on, Internet!
Bibliography.
No, see: superconductor I-V curve.
Bibliography:
- physics.stackexchange.com/questions/62664/how-can-ohms-law-be-correct-if-superconductors-have-0-resistivity on Physics Stack Exchange
- physics.stackexchange.com/questions/69222/how-can-i-put-a-permanent-current-into-a-superconducting-loop
- www.quora.com/Do-superconductors-produce-infinite-current-I-V-R-R-0-How-do-they-fit-into-quantum-theory
- www.reddit.com/r/askscience/comments/dcgdf/does_superconductivity_imply_infinite_current/
- www.reddit.com/r/askscience/comments/7xhb46/what_would_happen_if_a_voltage_was_applied_to_a/
Main theory to explain Type I superconductors very successfully.
TODO can someone please just give the final predictions of BCS, and how they compare to experiments, first of all? Then derive them.
High level concepts:
- the wave functions of pairs of electrons (fermions) get together to form bosons. This is a phase transition effect, thus the specific sudden transition temperature.
- the pairs form a Bose-Einstein condensate
- once this new state is reached, all pairs are somehow entangled into one big wave function, and you so individual lattice imperfections can't move just one single electron off trajectory and make it lose energy
Discrete quantum effect observed in superconductors with a small insulating layer, a device known as a Josephson junction.
To understand the behaviour effect, it is important to look at the Josephson equations consider the following Josephson effect regimes separately:
A good summary from Wikipedia by physicist Andrew Whitaker:
at a junction of two superconductors, a current will flow even if there is no drop in voltage; that when there is a voltage drop, the current should oscillate at a frequency related to the drop in voltage; and that there is a dependence on any magnetic field
Bibliography:
- www.youtube.com/watch?v=cnZ6exn2CkE "Superconductivity: Professor Brian Josephson". Several random excerpts from Cambridge people talking about the Josephson effect
Some golden notes at True Genius: The Life and Science of John Bardeen page 224 and around. Philip W. Anderson commented:
We were all - Josephson, Pippard and myself, as well as various other people who also habitually sat at the Mond tea and participated in the discussions of the next few weeks - very much puzzled by the meaning of the fact that the current depends on the phase
As part of the course Anderson had introduced the concept of broken symmetry in superconductors. Josephson "was fascinated by the idea of broken symmetry, and wondered whether there could be any way of observing it experimentally."
This is what happens when you apply a DC voltage across a Josephson junction.
It is called "AC effect" because when we apply a DC voltage, it produces an alternating current on the device.
By looking at the Josephson equations, we see that a positive constant, then just increases linearly without bound.
Therefore, from the first equation:
we see that the current will just vary sinusoidally between .
This meas that we can use a Josephson junction as a perfect voltage to frequency converter.
Wikipedia mentions that this frequency is , so it is very very high, so we are not able to view individual points of the sine curve separately with our instruments.
Also it is likely not going to be very useful for many practical applications in this mode.
An I-V curve can also be seen at: Figure "Electron microscope image of a Josephson junction its I-V curve".
Voltage is horizontal, current vertical. The vertical bar in the middle is the effect of interest: the current is going up and down very quickly between , the Josephson current of the device. Because it is too quick for the oscilloscope, we just see a solid vertical bar.
The non vertical curves at right and left are just other effects we are not interested in.
TODO what does it mean that there is no line at all near the central vertical line? What happens at those voltages?

If you shine microwave radiation on a Josephson junction, it produces a fixed average voltage that depends only on the frequency of the microwave. TODO how is that done more preciesely? How to you produce and inject microwaves into the thing?
It acts therefore as a perfect frequency to voltage converter.
The Wiki page gives the formula: en.wikipedia.org/wiki/Josephson_effect#The_inverse_AC_Josephson_effect You get several sinusoidal harmonics, so the output is not a perfect sine. But the infinite sum of the harmonics has a fixed average voltage value.
And en.wikipedia.org/wiki/Josephson_voltage_standard#Josephson_effect mentions that the effect is independent of the junction material, physical dimension or temperature.
All of the above, compounded with the fact that we are able to generate microwaves with extremely precise frequency with an atomic clock, makes this phenomenon perfect as a Volt standard, the Josephson voltage standard.
TODO understand how/why it works better.
Two equations derived from first principles by Brian Josephson that characterize the device, somewhat like an I-V curve:
where:
- : Josephson current
- : the Josephson phase, a function defined by the second equation plus initial conditions
- : input voltage of the system
- : current across the junction, determined by the input voltage
Note how these equations are not a typical I-V curve, as they are not an instantaneous dependency between voltage and current: the history of the voltage matters! Or in other words, the system has an internal state, represented by the Josephson phase at a given point in time.
To understand them better, it is important to look at some important cases separately:
- AC Josephson effect: V is a fixed DC voltage
Maximum current that can flow across a Josephson junction, as can be directly seen from the Josephson equations.
Is a fixed characteristic value of the physical construction of the junction.
A function defined by the second of the Josephson equations plus initial conditions.
It represents an internal state of the junction.
A device that exhibits the Josephson effect.

The inverse of the magnetic flux quantum.
As mentioned in True Genius: The Life and Science of John Bardeen page 224, the idea of symmetry breaking was a major motivation in Josephson's study of the Josephson effect.
- the basis for the most promising 2019 quantum computing implementation: superconducting quantum computer
- Josephson voltage standard: the most practical/precise volt standard, which motivated the definition of the ampere in the 2019 redefinition of the SI base units
- SQUID devices, which are:
- very precise magnetometer
- the basis for superconducting quantum computers
The most practical/precise volt standard.
It motivated the definition of the ampere in the 2019 redefinition of the SI base units
Quick NIST article about it: www.nist.gov/news-events/news/2013/04/primary-voltage-standard-whole-world (archive)
The wiki page en.wikipedia.org/wiki/Josephson_voltage_standard contains amazing schematics of the device, apparently made by the US Government.
Can be used as a very precise magnetometer.
There are high temperature yttrium barium copper oxide ones that work on liquid nitrogen.
The videos come from: www.ascg.msm.cam.ac.uk/lectures/. Vintage.
Mentions that the SQUID device is analogous to a double-slit experiment.
One of the segments is by John Clarke.
An experimental lab video for COVID-19 lockdown. Thanks, COVID-19. Presented by a cute and awkward Adam Stewart.
Uses a SQUID device and control system made by STAR Cryoelectronics. We can see Mr. SQUID EB-03 written on the probe and control box, that is their educational product.
As mentioned on the Mr. SQUID specs, it is a high-temperature superconductor, so liquid nitrogen is used.
He then measures the I-V curve on an Agilent Technologies oscilloscope.
Unfortunately, the video doesn't explain very well what is happening behind the scenes, e.g. with a circuit diagram. That is the curse of university laboratory videos: some of them assume that students will have material from other internal sources.
- youtu.be/ql2Yo5LgU8M?t=211 shows the classic voltage oscillations, presumably on a magnetic field sweep, and then he puts a magnet next to the device from outside the Dewar
- youtu.be/ql2Yo5LgU8M?t=253 demonstrates the formation of Shapiro steps. Inserts a Rohde & Schwarz signal generator into the Dewar to vary the flux. The result is not amazing, but they are visible somewhat.
Two parallel Josephson junctions.
In Ciro's ASCII art circuit diagram notation:
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