Quasi Particles

How to Imagine a Quasi Particle

	Protons, neutrons and electrons are particles. Nobody has a problem with that because most everybody just imagines them as some little ball that can exist by itself even in the absolute vacuum of space. While the "little ball" part of that imagination is faulty, the "can exist by itself" is correct.
		Now let's look at photons. Definitely a particle, but the "little ball" picture is now completely off. A photon can exist be itself but must keep moving.
	If we move one step away from the "little ball" image we can perceive all those particles in a more abstract way as more or less localized carriers of fixed and immutable properties like rest mass, charge, spin, and some others that we don't need to worry about.
		Then we have another set of properties tied to what those particle "do", expressed foremost in energy and momentum - quantities that are usually coupled by some dispersion relation and containing other parameters like wavelengths.
		The important point is that all of the above is covered by quantum theory and that means that all properties are usually quantized and that interactions between those particles must take into account the Pauli principle. This leads to the Fermi-Dirac or Bose-Einstein distributions if more than one particle needs to be "filled" into a system with defined energy levels.
	Now we make another leap of imagination and consider any entity that can be perceived as a carrier of defined properties in the sense alluded to above. This entity then could be either a "real" particle or a quasi-particle. Discounting the many elementary particles of high energy physics that are not stable, whatever we are discussing now must have protons, neutrons, electrons and photons as building blocks. So let's see what kind of "quasi" particles that we have already encountered we can compose with these basic ingredients:
		Atoms obviously. We can easily perceive atoms as real particles in their own right; nothing "quasi" about them. So we don't have to dwell on this.
		Molecules and crystals. Now we have a problem. We can certainly "see" a Cl₂ molecule as a "real" particle, but not a DNA molecule consisting of some 100.000 atoms or a 300 mm Si wafer. But this is not a real problem, it is just the old tiring story that as things get larger, it is more convenient to switch from particle-oriented quantum mechanics to good old classical mechanics. We don't think of big things as particles anymore (but still as "mass points" for certain questions!).
		Holes, behaving for all purposes like a positively charged electron. Now we have a "real" quasi particle in the sense that you can't take it out of the crystal in which it dwells and look at it. You cannot, to belabor this point somewhat more, make a fine point to a crystal and by applying a high field strength extract a hole beam which you focus and scan a cross on a specimen, running a scanning hole microscope. With electrons this is everyday technology. Holes in "reality" are still collectives of electrons that follow certain rules.
		Phonons or quantized vibrations of a crystal. The term "elastic waves" is actually better than the term "vibrations", because that's what phonons are: Waves running through the crystal with amplitudes resulting from a local elastic deformation - the atoms are somewhat off their equilibrium position. Being a wave, it has a wavelength and thus momentum and some energy; always quantized, of course. So what is the difference to a photon? Only that the term "A" in the basic wave equation A(r,t) = A₀ · exp(kr – wt) has a different unit (distance instead of electrical field strength). If a photon is a particle in vacuum, so is a phonon inside a crystal!
	Holes and phonons are thus quasi particles that can only exist inside crystals (or matter). They describe some collective behavior of electrons and atoms, respectively, in a simple and consistent way.
		It should come as no surprise that we will find more "collectives" that behave in a fashion with defined properties that we can ascribe to a suitable quasi-particle. Let's first look at list of what we have and then discuss those quasi particles very briefly

The Quasi Particle Zoo

Here is a list of crystal-dwelling quasi particles:

Quasi particle	Constituents	Remarks
Phonons	Crystal atoms	large momentum little energy Smallest wave length = lattice constants
Holes	Electrons in Valence band	"Define" semiconductors
Polarons	e^- + phonon	Essential to organic semiconductors
Plasmons	Electron collective	Determines optical properties of nanocrystals
Excitons	Bound electron hole pair	Produce the light in GaP LED's
Polaritons	Photon + phonon	Pretty strange - but those things do exist.
	Photon + electron
	Photon + exciton
Magnons	Crystal magnetic moments (= spins)	The "phonons" of spin waves
..	.....	........
Cooper pairs	2 electrons coupled by phonons	The quasi-particle responsible for superconductivity

Now we would need a closer look (= large part of a advanced solid state physics lecture). I'm not doing that here but only give a minimal glimpse at:

Plasmons: Imagine a nanoparticle of Au, for example. There is only a small number of atoms and accordingly a small number of free electrons. Now imagine an (oscillating) electrical field acting on that particle - e.g. a photon "coming in".

If a photon homes in on a large piece of Au the electrons in surface near regions feel a force, move a bit and by doing this screen the electrical field - it will not penetrate in the interior of the metal.

However, if the particle is small enough, all electrons feel the same force, and all electrons behave as one, as an ensemble called "plasma" for reasons easy to guess. What this ensemble of electrons can do is, of course, subject to quantum mechanics, i.e. we must expect that there is some quantization of the energy. The plasmon then is nothing but the quantized excitation states of an electron collective - the "plasma". Excitations are more or less restricted to longitudinal oscillations of the plasma. In other words, our electrons in the nanocrystal swing from "left" to "right" in unison at some quantized "Eigenfrequencies" and then can be described just as well as a (standing wave like) plasmon.

Quantization of energy means quantization of frequencies and that means that only "light" with the right frequency can excite a plasmon in our Au nanoparticles. Only this light then will be absorbed, transferring its energy to the plasmon.

This is easy to show: Put some of your Au nanoparticles in otherwise fully transparent glass - it will now look colored because your nanoparticles, depending on their size, take out some wavelengths since light with wavelengths just right to excite plasmons will be absorbed. But you don't have to do this experiment, just look at stained glass windows in medieval churches. The beautiful dark red you see there is obtained in exactly this way! Our forbearers actually knew how to get Au nanoparticles in Glass. They just had no idea why and how it worked

What you can imagine for Au, you can imagine for everything else, of course.

	Finally, just to show that I'm not making this up, the beginning of a collection of articles etc. where quasi particles come up as being useful.
	First we have polaritons



		It can't get much more exotic than that: putting exciton-polaritons to use! The blue lines indicate a part of the article where you will find a lot of the vocabulary used here in this context.

	Now let's look at plasmons:

With frame

9.1.1 Semiconductors and Light