50
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Accepted
Is sound a classical mechanic phenomenon or is it a quantum effect?
The detailed physics of sound wave transmission through air were mathematically worked out and found to be accurate before the invention of QM. No quantum-mechanical effects need to be taken into ...
- 97.8k
31
votes
Why are phonons quantized?
"...But surely the Schrodinger's equation doesn't apply to the Fourier coefficients, right?"
To begin, I have to say that I personally always found to the word "quantization" as ...
- 4,193
22
votes
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Why haven't we found thermal superconductors?
In an electrically insulating crystaline solid, heat is mostly transferred by phonons rather than by photons. What provides thermal resistance is phonon scattering via impurities or umklapp ...
- 56.5k
19
votes
Is sound a classical mechanic phenomenon or is it a quantum effect?
Well, niels answer is correct of course. Sometimes you can have also sound in QM system, it is called phonons where a mechanical classical vibration is traveling along the atomic lattice of some ...
- 4,370
14
votes
How similar are phonons and photons?
One has to keep in mind when studying the use of quantum field theory in modeling physics observations that quantum field theory is a mathematical tool that can be used where quantum phenomena appear, ...
- 235k
14
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Is sound a classical mechanic phenomenon or is it a quantum effect?
Sound is successfully described using continuum mechanics of solids and liquids (that is the theory of elasticity and hydrodynamics). That is, one need not even descend to the level of molecules/atoms,...
- 65k
13
votes
Accepted
Why do we not use the Schrödinger equation/standard QM to describe phonons (or other quasi particles)?
In condensed matter physics, or any field theory where particle numbers are not conserved (like particle physics), the Schrodinger equation does not work. The Schrodinger equation needs the condition ...
- 30.2k
12
votes
Why zero chemical potential does not allow the Bose-Einstein Condensation of Phonons?
There is an important distinction between Bose-Einstein condensation of atoms and quasiparticles, which I will describe below.
Bose-Einstein condensation of an ideal (atomic) Bose gas
For bosons, ...
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12
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How can Cooper pairs form in zero temperature, if there are no phonons?
The electrons create their own virtual phonons. As an electron moves through the lattice of positive ions, the ions are atrracted to the electron and move slighly towards it. The electron is moving ...
- 56.5k
12
votes
Why are phonons quantized?
The surfaces of equal energy for our system are hyper-ellipsoids: for two particles each is an ellipse described by something like $x_1^2+(x_1-x_2)^2+x_2^2=\mathrm{const},$ for three particles it's an ...
- 29.6k
10
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The difference between optical phonons and acoustic phonons in terms of energy absorption
You need to consider both momentum and energy conservation. The speed of sound for most materials is so low that you can't conserve both quantities for a single photon to acoustic phonon absorption.
...
- 8,129
10
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Accepted
Why are phonons bosons if they cannot occupy the same eigenstate?
When the question "what is a quantum particle --- a phonon, a photon, an electron, a Higgs ?" is asked, then the answer should be
--- and here for concreteness I will replace the word "...
- 10.2k
9
votes
How many optical and acoustical branches are in a primitive cell?
TL;DR: We have $3p$ branches in total, corresponding to $3p$ independent modes of vibration. We have $3$ acoustic branches, because we are in $3d$ space, AND, Because we are dealing with an Elastic ...
- 760
9
votes
Accepted
Why phonons are Goldstone modes?
This question is answered in detail in the paper Phonons as Goldstone Bosons. The question What is the difference between a photon and a phonon? is also closely related. Here, I'll just give some ...
- 55k
8
votes
Accepted
Why is crystalline graphite black yet shiny?
Crystalline silicon is, with its diamond-like cubic crystal structure, optically isotropic. Its refractive index at $555\,\mathrm{nm}$ is $4.070+0.0376i$, which results in reflectance at normal ...
- 29.6k
8
votes
Accepted
Why is it possible for us to "hear" acoustic phonons, but not optical phonons?
This is a phonon dispersion plot for silicon carbide:
The $\Gamma$ point refers to $k=0$; the designations of the branches are transverse/longitudinal and acoustic/optical. $X,K,$ and $L$ refer to ...
- 71.3k
8
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Why do we not use the Schrödinger equation/standard QM to describe phonons (or other quasi particles)?
I'm arguing here in a different way than the answer by @josephh. In my opinion, you can definitely apply the Schrödinger equation -- resp. a probably differently named version of it -- to phonons as ...
- 259
7
votes
Accepted
Could gravitons be quasiparticles like phonons?
This idea has been seriously considered by many physicists in the past. What you are proposing sounds very similar to Sakharov's idea of induced gravity.
More recent theories like loop quantum ...
- 49.4k
7
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Why we can use $\frac{1}{V}\sum_{k \in 1BZ} \approx \int_{k\in 1BZ} \frac{dk}{(2\pi)^d}$ for phonons?
That formula comes from the following
$$\frac{1}{V}\sum_k = \frac{1}{Na^3}\frac{(2\pi)^3}{(2\pi)^3}\sum_k = \frac{2\pi}{N_1 a}\frac{2\pi}{N_2 a}\frac{2\pi}{N_3 a}\frac{1}{(2\pi)^3}\sum_k\\
=\frac{1}{(...
- 4,120
7
votes
How similar are phonons and photons?
A part of the question is the distinction between a quasiparticle, such as phonon and a fundamental particle. In this respect it is necessary to note that phonons are not the only quasiparticles in ...
- 65k
6
votes
Phonon density of states from velocity autocorrelation function
I recently stumbled on a more or less similar question. The Fourier transform of the velocity autocorrelation does not give you the phonon DOS but rather the phonon population of your system. In other ...
- 369
6
votes
What is a phonon?
When things get foggy in dealing with quantum concepts, I think it's best to find an experimental question that will give structure to that theoretical idea. Phonons are even trickier, since they ...
- 1,931
6
votes
Why zero chemical potential does not allow the Bose-Einstein Condensation of Phonons?
I think you pretty much have all the right ideas in your question.
In a conventional BEC, particle number is conserved. Therefore, as T goes down towards 0, $\mu$ goes up towards the lowest state to ...
- 16.9k
6
votes
Optical and Acoustic Phonons: Choice of Unit Cell
Your book is correct. In three dimensions, there are 3 acoustic phonons, and 3(M-1) optical phonons. You are also correct, in that since the choice of unit cell is arbitrary, the number of optical ...
- 566
6
votes
Are there phonons in air?
TL;DR: What differs phonons from photons is that phonons require presence of a medium, where the movement of the medium constitutes a field to be quantized. Photons are electromagnetic waves - they do ...
- 65k
5
votes
Accepted
Why are sound waves associated with modes obeying linear dispersion relation?
Your question seems to have the premise: "The speed of sound is constant by definition, therefore, if high-frequency phonons move significantly faster or slower than low-frequency phonons, those high-...
- 16.9k
5
votes
Why is the k=0 phonon neglected when calculating Debye-Waller factor?
The $k=0$ (acoustic) phonon is exactly zero, not approximately, and simply means the entire crystal is translated by a constant. This is a fully coherent motion, as all the atoms move in the same way, ...
- 8,129
5
votes
Accepted
How can phonons possibly be assigned precise locations?
How can you give a phonon a location? The answer is: "The same way you do for an electron."
A phonon is a collective excitation of all the ions. What we typically call an "electron" in a solid isn't ...
- 4,285
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