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Includes fun things like:
As of 2020, this is the other "fundamental branch of physics" besides to particle physics/nuclear physics.
Condensed matter is basically chemistry but without reactions: you study a fixed state of matter, not a reaction in which compositions change with time.
Just like in chemistry, you end up getting some very well defined substance properties due to the incredibly large number of atoms.
Just like chemistry, the ultimate goal is to do de-novo computational chemistry to predict those properties.
And just like chemistry, what we can actually is actually very limited in part due to the exponential nature of quantum mechanics.
Also since chemistry involves reactions, chemistry puts a huge focus on liquids and solutions, which is the simplest state of matter to do reactions in.
Condensed matter however can put a lot more emphasis on solids than chemistry, notably because solids are what we generally want in end products, no one likes stuff leaking right?
But it also studies liquids, e.g. notably superfluidity.
One thing condensed matter is particularly obsessed with is the fascinating phenomena of phase transition.
AMO is a slightly more general area than condensed matter physics, including related phenomena with smaller numbers atoms and optics. The two terms are however sometimes used as synonyms. The term AMO has gained wide usage and acceptability, see e.g.:
If Ciro had had greater foresight, this might have been what he studied at university!
How are the bands measured experimentally?
Why are there gaps? Why aren't bands infinite? What determines the width of gaps?
The basis of 1970-20XX computers, gotta understand them I guess.
Most notable example: gallium arsenide, see also: gallium arsenide vs silicon.
An important class of semiconductors, e.g. there is a dedicated III-V lab at: École Polytechnique:
  • "An introduction to superconductivity" by Alfred Leitner originally published in 1965, source:
  • Isotope effect on the critical temperature. 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.
  • 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, 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.
  • 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.
As of 2020, basically means "liquid nitrogen temperature", which is much cheaper than liquid helium.
Figure 1. Timeline of superconductivity from 1900 to 2015. Source.
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:
TODO, come on, Internet!
Video 3. Superconducting Short Circuits across Batteries by Eugene Khutoryansky (2020) Source. Well, internal battery resistance acts as the only resistor, and voltage drops to zero immediately outside of the battery. And you get a huge current.
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 two cases 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
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.
Figure 2. I-V curve of the AC Josephson effect. Source.
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?
Video 4. Superconducting Transition of Josephson junction by Christina Wicker (2016) Source. Amazing video that presumably shows the screen of a digital oscilloscope doing a voltage sweep as temperature is reduced and superconductivity is reached.
Figure 3. I-V curve of a superconducting tunnel junction. So it appears that there is a zero current between and . Why doesn't it show up on the oscilloscope sweeps, e.g. Video 4. "Superconducting Transition of Josephson junction by Christina Wicker (2016)"?
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: 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 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:
  • : 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:
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.
Figure 4. Electron microscope image of a Josephson junction its I-V curve. Source.
The inverse of the magnetic flux quantum.
The most practical/precise volt standard.
It motivated the definition of the ampere in the 2019 redefinition of the SI base units
The wiki page contains amazing schematics of the device, apparently made by the US Government.
Figure 5. Source.
Can be used as a very precise magnetometer.
There are high temperature yttrium barium copper oxide ones that work on liquid nitrogen.
Video 5. Superconducting Quantum Interference Device by Felipe Contipelli (2019) Source. Good intuiotionistic video. Some points deserved a bit more detail.
Video 6. Mishmash of SQUID interviews and talks by Bartek Glowaki. Source.
The videos come from: Vintage.
Mentions that the SQUID device is analogous to a double-slit experiment.
One of the segments is by John Clarke.
Video 7. Superconducting Quantum Interference Devices by UNSW Physics (2020) Source.
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.
Video 8. The Ubiquitous SQUID by John Clarke (2018) Source.
Two parallel Josephson junctions.
In Ciro's ASCII art circuit diagram notation:
|   |
X   X
|   |
Specific type of Josephson junction. Probably can be made tiny and in huge numbers through photolithography.
Figure 6. Illustration of a thin-film superconducting tunnel junction (STJ) Source. The superconducting material is light blue, the insulating tunnel barrier is black, and the substrate is green.
Video 9. Quantum Transport, Lecture 14: Josephson effects by Sergey Frolov (2013) Source. mentions maskless electron beam lithography being used to produce STJs.
Video 10. Alfred Leitner - Liquid Helium II the Superfluid by Alfred Leitner (1963) Source. Original source:
Video 11. Ben Miller experiments with superfluid helium by BBC (2011) Source. Just quickly shows the superfluid helium climbing out o the cup, no detailed setup. With professor Robert Taylor from the University of Oxford.
Inward Bound by Abraham Pais (1988) page 282 shows how this can be generalized from the Maxwell-Boltzmann distribution 7 Different Types of Plastic and Their Uses by Orange Plastics Academy (2018) Does not mention packaging foams.
The wiki comments:
The Bohr–van Leeuwen theorem, discovered in the 1910s, showed that classical physics theories are unable to account for any form of magnetism, including ferromagnetism. Magnetism is now regarded as a purely quantum mechanical effect. Ferromagnetism arises due to two effects from quantum mechanics: spin and the Pauli exclusion principle.
Also has some funky relations to renormalization TODO.
A tiny idealized magnet! Likely a good approximation for electron spin in many cases.
Wikipedia has done well for once:
The current to the coil windings is provided by a high current, very low voltage DC power supply, since in steady state the only voltage across the magnet is due to the resistance of the feeder wires. Any change to the current through the magnet must be done very slowly, first because electrically the magnet is a large inductor and an abrupt current change will result in a large voltage spike across the windings, and more importantly because fast changes in current can cause eddy currents and mechanical stresses in the windings that can precipitate a quench (see below). So the power supply is usually microprocessor-controlled, programmed to accomplish current changes gradually, in gentle ramps. It usually takes several minutes to energize or de-energize a laboratory-sized magnet.
Used to explain the black-body radiation experiment.
The Quantum Story by Jim Baggott (2011) page 9 mentions that Planck apparently immediately recognized that Planck constant was a new fundamental physical constant, and could have potential applications in the definition of the system of units (TODO where was that published):
Planck wrote that the constants offered: 'the possibility of establishing units of length, mass, time and temperature which are independent of specific bodies or materials and which necessarily maintain their meaning for all time and for all civilizations, even those which are extraterrestrial and nonhuman, constants which therefore can be called "fundamental physical units of measurement".'
This was a visionary insight, and was finally realized in the 2019 redefinition of the SI base units.
Video 12. Quantum Mechanics 2 - Photons by ViaScience (2012) Source. Contains a good explanation of how discretization + energy increases with frequency explains the black-body radiation experiment curve: you need more and more energy for small wavelengths, each time higher above the average energy available.
Derived from classical first principles, matches Planck's law for low frequencies, but diverges at higher frequencies.
  • The Quantum Story by Jim Baggott (2011) page 10 mentions:
    Early examples of such cavities included rather expensive closed cylinders made from porcelain and platinum.
    and the footnote comments:
    The study of cavity radiation was not just about establishing theoretical principles, however. It was also of interest to the German Bureau of Standards as a reference for rating electric lamps.
  • 1859-60 Gustav Kirchhoff demonstrated that the ratio of emitted to absorbed energy depends only on the frequency of the radiation and the temperature inside the cavity
  • 1896 Wien approximation seems to explain existing curves well
  • 1900 expriments by Otto Lummer and Ernst Pringsheim show Wien approximation is bad for lower frequencies
  • 1900-10-07 Heinrich Rubens visits Planck in Planck's villa in the Berlin suburb of Grünewald and informs him about new experimental he and Ferdinand Kurlbaum obtained, still showing that Wien approximation is bad
  • 1900 Planck's law matches Lummer and Pringsheim's experiments well. Planck forced to make the "desperate" postulate that energy is exchanged in quantized lumps. Not clear that light itself is quantized however, he thinks it might be something to do with allowed vibration modes of the atoms of the cavity rather.
  • 1900 Rayleigh-Jeans law derived from classical first principles matches Planck's law for low frequencies, but diverges at higher frequencies.
Video 13. Black-body Radiation Experiment by sciencesolution (2008) Source. A modern version of the experiment with a PASCO scientific EX-9920 setup.
Video 14. What is the Ultraviolet Catastrophe? by Physics Explained (2020) Source.
Video 15. Time-Correlated Single Photon Counting (TCSPC) with the Fluorolog Fluorimeter by Yale CBIC (2011) Source.
One important quantum mechanics experiment, which using quantum effects explain the dependency of specific heat capacity on temperature, an effect which is not present in the Dulong-Petit law.
This is the solid-state analogue to the black-body radiation problem. It is also therefore a quantum mechanics-specific phenomenon.
Observation that all solids appear to have the same constant heat capacity per mole.
It can be seen as the limit case of an Einstein solid at high temperatures. At lower temperatures, the heat capacity depends on temperature.
Wikipedia mentions that it is completely analogous to Planck's law, since both are
The key advantages of lasers over other light sources are:
  • it emits a very narrow range of frequencies (small linewidth), which for many purposes can be considered a single frequency
    It does however have a small range of frequencies. The smaller the range, the better the laser quality.
  • it can be efficient collimated, while still emitting a lot of output power: Section "Why can't you collimate incoherent light as well as a laser?"
  • can be phase and polarization coherent, though it is not always the case? TODO.
Video 16. How Lasers Work by Scientized (2017) Source.
An extremely good overview of how lasers work. Clearly explains the electron/photon exchange processes involved, notably spontaneous emission.
Talks about the importance of the metastable state to achieve population inversion.
Also briefly explains the imperfections that lead to the slightly imperfect non punctual spectrum seen in a real laser.
The type of laser described at: Video 16. "How Lasers Work by Scientized (2017)", notably Mentioned at: That point also mentions that 4-level lasers also exist and are more efficient. TODO dominance? Alternatives?
You could put an LED in a cavity with a thin long hole but then, most rays, which are not aligned with the hole, will just bounce inside forever producing heat.
So you would have a very hot device, and very little efficiency on the light output. This heat might also behave like a black-body radiation source, so you would not have a single frequency.
The beauty of lasers is the laser cavity (two parallel mirrors around the medium) selects parallel motion preferentially, see e.g.: from Video 16. "How Lasers Work by Scientized (2017)"
Sample usages:
Video 17. Optical Tweezers Experiment by Alexis Bishop. Source. Setup on a optical table. He drags a 1 micron ball of polystyrene immersed in water around with the laser. You look through the microscope and move the stage. Brownian motion is also clearly visible when the laster is not holding the ball.