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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.
Bibliography:
How are the bands measured experimentally?
Why are there gaps? Why aren't bands infinite? What determines the width of gaps?
Bibliography:
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: http://www.3-5lab.fr/contactus.php
Experiments:
  • "An introduction to superconductivity" by Alfred Leitner originally published in 1965, source: http://www.alfredleitner.com/
  • Isotope effect on the critical temperature. http://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, https://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.
Media:
Transition into superconductivity can be seen as a phase transition, which happens to be a second-order phase transition.
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!
Bibliography.
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
discretes quantum effect observed in superconductors with a small insulating layer.
TODO: curves showing the effect.
The effect is likely mentioned in Leitner's video at: https://www.youtube.com/watch?v=BFdq6IecUJc&t=2290s
Bibliography:
A device that exhibits the Josephson effect.
The inverse of the Magnetic flux quantum.
Can be used as a very precise magnetometer.
There are high temperature yttrium barium copper oxide ones that work on liquid nitrogen.
Specific type of Josephson junction. Probably can be made tiny and in huge numbers through photolithography.
Figure 2. 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 3. Quantum Transport, Lecture 14: Josephson effects by Sergey Frolov (2013) Source. https://youtu.be/-HUVGWTfaSI?t=878 mentions maskless electron beam litograpy being used to produce STJs.
Video 4. Alfred Leitner - Liquid Helium II the Superfluid by Alfred Leitner (1963) Source. Original source: http://www.alfredleitner.com.
Video 5. 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
https://www.youtube.com/watch?v=PbuiIhr0LVA 7 Different Types of Plastic and Their Uses by Orange Plastics Academy (2018) Does not mention packaging foams.
The wiki comments: https://en.wikipedia.org/w/index.php?title=Ferromagnetism&oldid=965600553#Explanation
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.
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 6. 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 7. Black-body Radiation Experiment by sciencesolution (2008) Source. A modern version of the experiment with a PASCO scientific EX-9920 setup.
Video 8. What is the Ultraviolet Catastrophe? by Physics Explained (2020) Source.
Video 9. 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
Video 10. How Lasers Work - A Complete Guide 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.

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