What is Phonons: Definition and 79 Discussions

In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, specifically in solids and some liquids. Often referred to as a quasiparticle, it is an excited state in the quantum mechanical quantization of the modes of vibrations for elastic structures of interacting particles. Phonons can be thought of as quantized sound waves, similar to photons as quantized light waves.The study of phonons is an important part of condensed matter physics. They play a major role in many of the physical properties of condensed matter systems, such as thermal conductivity and electrical conductivity, as well as play a fundamental role in models of neutron scattering and related effects.
The concept of phonons was introduced in 1932 by Soviet physicist Igor Tamm. The name phonon comes from the Greek word φωνή (phonē), which translates to sound or voice, because long-wavelength phonons give rise to sound. The name is analogous to the word photon.

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  1. raisins

    I How Do Ashcroft and Mermin Derive the Conversion from Equation 22.6 to 22.9?

    Hi all, I'm reading through Chapter 22 of Ashcroft and Mermin and am having difficulty deriving an equation. Could someone please show me (or outline the steps) how Ashcroft and Mermin convert the quadratic term in Eqn. (22.6) to Eqn. (22. 9)? (pictures attached). Thanks in advance :)
  2. shintashi

    Can Phonons and/or Thz+ Acoustic FX cause Ionizing Radiation?

    For example: https://www.laserfocusworld.com/test-measurement/research/article/16566273/scientists-develop-saseran-acoustic-laserthat-produces-terahertz-sound-waves Can these devices produce ionizing radiation? I understand ultrasound traditionally does not produce ionizing radiation, but I am...
  3. C

    I Physical question about Phonons

    Hi, I have a question regarding Phonons and daily experience: Let's say I have a table and I hit it, does it mean Phonons were created where I've hit on the table? Meaning: By hitting the table, I'm giving energy to it, this energy goes to the motion of the table atoms, and this motion of the...
  4. S

    Do photons, phonons and electrons have mass?

    These articles have energy but do they have mass?
  5. L

    A Phonons as a quasiparticles in a quantum LHO

    I am confused with phonons as a quasiparticles in quantum LHO. When I say ##E_n=(n+\frac{1}{2})\hbar \omega## why ##n## is number of phonons? Why no magnons, excitones... And one more question. Why number of maxima of ##|\psi_n(x)|^2## is related to number of phonons, so that number of...
  6. C

    I How can phonons "travel" if they are excitations of normal modes?

    My understanding is that you can describe the complicated motion of atoms in a crystal as a sum of standing waves (normal modes). A phonon is an excitation of a normal mode in the sense that it increases the vibration amplitude of that normal mode and the energy of that mode by a quantized...
  7. C

    A What is the role of transverse optical (TO) phonons?

    Phonons can be acoustic or optical in their character and transverse or longitudinal in their polarization. While the importance of longitudinal acoustic (LA), transverse acoustic (TA), and longitudinal optical (LO) phonons is clear to me for the emergence of different quantum phenomena and for...
  8. AngelFis93

    Minimum wavelength of phonons under the Debye aproximation

    Since in Debye aproximation Debye's frecuency is defined as the maximum frecueny , the corresponding wavelenght should be the minimum one, due to the inverse relation among those λ=v/f=v·2π/ω=5.9 Å , which is higher than the given result. I believe I should be using the information 'cubic...
  9. S

    A Do acoustic phonons disappear in charge density wave states?

    In most standard exposition of (the mean-field theory of) charge density wave (CDW), phase and amplitude fluctuations are introduced as the collective excitations. Kohn anomaly in the acoustic phonon dispersion is also mentioned as temperature goes from the above till the CDW transition...
  10. Solomei

    B Quantum microphone and phonons

    https://phys.org/news/2019-07-physicists-particles-quantum-microphone.html Are phonons fundamental for advanced levitation technology?
  11. L

    I Can you tell which solid a substance is by analyzing the phonons only?

    Can you tell what kind of solids by simply having access to the phonons only? Supposed you join a challenge where you will use any tools just to analyze the phonons (without telling you what kinds of solids). Can you tell what solids is it? Is there any geometric relationship between the...
  12. B

    A Are there phonons in a free electron gas?

    It is well-known that the electron gas of volume V has an equation of state p=p(V) and thus has a bulk modulus $$B=-V(dp/dV)$$. Suppose the electron gas had no underlying lattice but was confined. Do phonons emit and absorb in such an electron gas at finite temperature? The reason I ask is...
  13. W

    I Phonons and Crystal Momentum: Conservation and Quantization

    Hi all, I have a few questions. Thanks in advance! Phonons: Can phonons be thought of as a quantised wave of sound, in the same manner photons being a quantised light wave? Also, can they be taken as particles of sound in the same way that photons can be particles of light? Crystal momentum...
  14. fluidistic

    I How come electrons and phonons lead to the heat equation?

    Phonons on their own lead to the common heat equation. One sees that for example in insulators or non doped semiconductors. However in metals (or conductors), the electrons are the ones that are mostly responsible for the heat transfer, which extremely surprisingly to me, is also of the form of...
  15. H

    Light in a medium: do phonons store light energy?

    When light travels through a transparent medium and is absorbed then re-emitted, do the phonons just move to a higher energy state after absorption? Is it the same as absorption and re-emission of light by electrons, aside from having a continuous range of available energies?
  16. C

    I Wave particle duality and phonons

    I was reading the thread about wave particle duality linked from the newsletter, and I noticed it said (to use my own words) that the conflict between wave and particle dynamics can be avoided by using operator dynamics instead. Unfortunately, in the case of phonons, I've never seen a...
  17. A

    I Can the Planck equation be derived by considering phonons?

    As far as I understand, phonons are just thermal vibrations of atoms in a lattice and blackbody radiation is just the radiation emittied due to thermal oscillations accelerating the atoms back and forth. Is there any example of a derivation of the Planck equation from considering black body...
  18. otaKu

    Solid State A good book to learn about phonon dispersion relation

    So I see them in the books labelled as accoustic and optical phonons but I don't seem to find a comprehensive treatment of the matter for a beginner who doesn't know a thing about the dispersion curves. I'd prefer not to dwell too much into the mechanical treatment if possible since I just need...
  19. chikou24i

    A Acoustic and optical branches in phonons dispersion curves

    What is the physical meaning of acoustic and optical branches in phonons dispersion curves ?
  20. J

    Phonons driven at high frequency

    Homework Statement For one-dimensional monatomic crystals, the dispersion plot for phonons has a maximum frequency ωmax. If a driving force oscillates the crystal beyond this frequency, the phonon will no longer propagate and will instead decay as it gets further from the external source of...
  21. P

    What factors affect the mean free path of phonons in Germanium at 300K?

    Homework Statement If all the heat transport is by phonons, estimate the mean free path of phonons in Germanium at 300K using the following data. Thermal conductivity=80W/K m; Debye temperature=360K; atomic weight=72.6; sound velocity=4500m/s; density=5500kg m−3 Homework Equations K=CVvsl l...
  22. J

    Phonon dispersion Points/Modes question

    Hey all, I am trying to recreate the phonon dispersion plot of the paper below http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-730-physics-for-solid-state-applications-spring-2003/projects/ProjP21.pdf The problem is I do not know what the Tau, L, and X on the x-axis...
  23. Jimster41

    Phonons and classical non-locality?

    I just discovered the concept of phonons and the story of Brian Josephson (from reading "Sync" by Steven Strogatz). What the h...? So Phonon's are collective excitations of classical stuff made from QM ensembles, excitations made possible by the QM properties of the ensemble members? Are...
  24. NaOH

    Phonons & Collective Oscillations: Locality Experiments

    I am struggling with the concept of phonons and if they should be considered (experimentally) to be treated as collective oscillations of a certain region instead of the whole crystal. I know the title comes off as a little strange -- since phonons are not localised to anyone atom, but consider...
  25. J

    Optical phonons in fcc-lattice

    Aluminium has an fcc structure, which is a simple cubic lattice with four Al-atoms in the basis. On the other hand, diamond has a diamond structure, which is a simple cubic lattice with 8 atoms in basis. Now, diamond has optical modes in addition to acoustical, while Aluminium does not. What...
  26. U

    Number of phonons - Which length should I use?

    Homework Statement (a) Find debye frequency. (b) Find number of atoms. Homework EquationsThe Attempt at a Solution Part(a) [/B] Density of states is given by g(\omega) = \frac{3V\omega^2}{2 \pi^2 c^3} = N \left[ \frac{12 \pi \omega^2}{(2\pi)^2 n c^3} \right] = 9N \frac{\omega}{\omega_D^3}...
  27. Einj

    Longitudinal and transverse phonons velocities

    Hello everyone! Does anyone know how to extract the velocities of longitudinal and transverse phonons starting from their Lagrangian and/or equation of motion? Thanks!
  28. Spinnor

    Mössbauer effect at low T versus high T, making phonons.

    We read that when a crystal used in the Mössbauer effect is cooled we enhance the odds of a nucleus emitting a gamma ray with no crystal phonons being produced. When the crystal is heated there are more phonons around then at lower temperatures. How does the presents of more phonons enhance the...
  29. ShayanJ

    Are Phonons Particles? Exploring the Wave-Particle Duality

    Phonons are said to be the result of the quantization of crystal waves.Let the function u=u_0 e^{i(\vec{k}\cdot\vec{r}-\omega t)} describe such a wave. Is it right to say that if we assume u to be the quantum mechanical wave function of a particle, that particle is a phonon? Thanks
  30. L

    Phonons: For oscillator wave function

    For oscillator wave function ##\frac{1}{\sqrt 2}(y-\frac{d}{dy})\psi_n(y)=\sqrt{n+1}\psi_{n+1}(y)## ##\frac{1}{\sqrt 2}(y+\frac{d}{dy})\psi_n(y)=\sqrt{n}\psi_{n-1}(y)## and I interpretate ##n## as number of phonons. Of course ##\psi_n(y)=C_ne^{-\frac{y^2}{2}}H_n(y)##. And ##C_n=f(n)##...
  31. H

    The role of phonons in momentum conservation

    In an indirect transition from the valence band maximum to conduction band minimum, the momentum of electron and hole would not change but the crystal momentum would change and this change is supplied by phonons.I have two questions here: 1) phonons don't carry momentum so how they can transfer...
  32. N

    Does amorphous solids support phonons or nomal modes of vibration.?

    Does amorphous solids support phonons or nomal modes of vibration.?? Can we apply the concept of phonons and Debye Model in case of amorphous and poly-crystalline solids?? http://en.wikipedia.org/wiki/Phonon http://en.wikipedia.org/wiki/Debye_model The links I have attached above...
  33. T

    Brillouin Zones, Phonons, Energy Propagation through a Crystal

    Hello, I am having quite a bit of trouble really grasping Brillouin Zones and their relation to phonons, energy propagation, etc. I've got a few questions, and there will probably be a number of misconceptions, but I figure they'll clarify what I exactly don't understand. I think a lot of the...
  34. C

    What's with 'virtual' phonons in BCS pairing?

    Can someone explain why virtual phonons are necessary to explain BCS pairing in type I superconductors. I thought phonons originated with the lattice vibrations. I read that it s the 'virtual" phonons that explains the pairing; how so? what am I missing ?
  35. A

    Classical Phonons: Solving Differential/Difference Equation

    The below picture is from my book's derivation on the equations describing waves in matter. But problem is: I don't understand the solution of the differential equation - or "difference" equation (whatever that is). How is it solved with the proposed solution? If I plug it in I don't get...
  36. H

    Indirect absorption and phonons

    In an indirect absorption process that an electron jumps from the top of the valence band to the bottom of the conduction band,why a phonon must be involved to conserve the momentum? At these two points (top of the valence band and bottom of the conduction band) the electron momentum is zero so...
  37. L

    Solid State - Phonons at Brillouin Zone Boundary

    Homework Statement Homework Equations {3.9b} A[2\mu -m\omega ^2 ]=2\mu Bcos(\frac{ka}{2}) B[2\mu -M\omega ^2 ]=2\mu Acos(\frac{ka}{2}) The Attempt at a Solution All I can think of is setting k =\frac{\pi}{a} so that B[2\mu -M\omega ^2 ]=A[2\mu -m\omega ^2 ] solve for omega...
  38. H

    Phonons in Crystals: Standing or Traveling Waves?

    Are phonons in crystals standing waves or traveling waves?
  39. J

    Why can't optical phonons travel far?

    I can't figure this out/find the answer. Why are acoustic phonons very low loss (i.e. earthquake P and S waves), but optical phonons die out rapidly? Thanks, Joel
  40. L

    Thermal phonons @ low temperatures

    Homework Statement a) estimate the number of phonons in a 1mm cube of a crystal at 10K, taking a sound speed of 1000m/s. b) estimate the frequency ω for which you will find the highest phonon nrg content in the range of ω to ω+dω. (Again, assume low temps). Give your solution in the form...
  41. N

    Do phonons conserve momentum or not?

    I'm reading Kittel and on p101 he says Aren't the first and second part contradicting each other? The first part implies that for example the momentum of an electron can be (partially) transferred to the momentum of a phonon (as Kittel also states), but if the phonon momentum is not real...
  42. S

    Phonons and the Dulong-Petit Law

    Hey, My question is displayed below: Now I know for large T, β approaches zero. I have used the relation of the heat capacity and the partition function and have found the derivative of lnZ with respect to β twice. I think this derivative is just the derivative of the logarithm...
  43. S

    Phonons and the Dulong-Petit Law

    Hey, My question is on Phonons, the Einstein & Debye models and the Dulong Petit Law. The question is displayed below: I am told how to get to the heat capacity by using the logarithm of the partition function 'Z', and so I set about differentiating the logarithm of Z with respects to Beta...
  44. M

    Can heat (Phonons) generate an electric field in Piezocrystals?

    I was thinking. Since phonons are lattice vibrations, and are affected by the temperature of the lattice, can heating a piezocrystal cause it to vibrate and therefore put a small stress on it, causing it's electron density to shift, creating an electric field?
  45. S

    Meaning of normal coordinates and normal modes, in relation to phonons

    I am trying to find a succinct definition of normal coordinates and normal modes - relating to numerous coupled harmonic oscillators. What exactly do the normal coordinates describe about these coupled harmonic oscillators? I know that for normal modes, all parts of the system move with...
  46. N

    Phonon vs Photon: What's the Difference?

    What is the difference between the photon and the phonon ?
  47. J

    Light waves, photons; sound waves, phonons; water waves, hydrons ?

    Light waves, photons; sound waves, phonons; water waves, "hydrons"? Light waves are made of photons; sound waves are made of phonons; so are water waves made of "hydrons", and if so, how would they behave, and would it be possible to make a water laser or something similar based on these particles.
  48. T

    Phonons (kintetic waves) in a plasma?

    What I am imagining is a thruster in space igniting a fuel to create an explosion to push a spaceship and phonons (mechanical waves) pushing the spent fuel to the sides of the thruster where vents vacuum it back into the spaceship for recycling. What do you think of this?
  49. D

    Phonon Modes and Vanishing Eigenvalues in Periodic Lattices

    Dear all, in these http://pages.unibas.ch/comphys/comphys/TEACH/SS04/course.pdf" lecture notes, the author says on page (0-120): http://img15.imageshack.us/img15/615/capturena.png It is not obvious to me, why due to the translation invariance of the energy 3 eigenvalues of the D_IJ matrix have...
  50. K

    Wave Vectors of Light vs. Phonons: A Puzzling Paradox

    In his paper, "Radiative exchange of heat between nanostructures" Pendry makes this statement : "In general phonons have much smaller wave vectors at a given frequency than does light " Isn't wave vector given by 2*Pi /Lambda? So how can wave vectors be smaller for phonons than for...