Explained: Phonons

July 8, 2010 by David L. Chandler, Massachusetts Institute of Technology
A computer simulation shows phonons, depicted as color variations, traveling through a crystal lattice. The lattice in this case is broken up by round rods whose spacing has been chosen to block the passage of phonons of certain wavelengths.

For the engineers who design cell phones, solar panels and computer chips, it’s increasingly important to be able to control the way heat moves through the crystalline materials — such as silicon — that these devices are based on. In computer and cell-phone chips, for example, one of the key limitations to increasing speed and memory is the need to dissipate the heat generated by the chips.

To understand how heat spreads through a material, consider that heat — as well as sound — is actually the motion or of atoms and molecules: Low-frequency vibrations correspond to sound, while higher frequencies correspond to heat. At each frequency, principles dictate that the vibrational energy must be a multiple of a basic amount of energy, called a quantum, that is proportional to the frequency. Physicists call these basic levels of energy phonons.

In a sense, then, “” is just a fancy word for a particle of heat.

Phonons are especially relevant in the behavior of heat and sound in , explains Gang Chen, the Rohsenow Professor of Mechanical Engineering at MIT. In a crystal, the atoms are neatly arranged in a uniform, repeating structure; when heated, the atoms can oscillate at specific frequencies. The bonds between the individual atoms in a crystal behave essentially like springs, Chen says. When one of the atoms gets pushed or pulled, it sets off a wave (or phonon) travelling through the crystal, just as sitting down on one edge of a trampoline can set off vibrations through the entire surface.

In practice, most materials are filled with a chaotic mix of phonons that have different frequencies and are traveling in different directions, all superimposed on each other, in the same way that the seemingly chaotic movements of a choppy sea can (theoretically) be untangled to reveal a variety of superimposed waveforms of different frequencies and directions.

But unlike photons (the particles that carry light or other electromagnetic radiation), which generally don’t interact at all if they have different wavelengths, phonons of different wavelengths can interact and mix when they bump into each other, producing a different wavelength. This makes their behavior much more chaotic and thus difficult to predict and control.

Just as photons of a given frequency can only exist at certain specific energy levels — exact multiples of the basic quanta —so, too, can phonons, Chen says. And when working on applied physics relating to the transfer of heat within solids, which is a specific focus of Chen’s research, thinking in terms of phonons has proved to be especially useful.

For example, in the quest for better ways to dissipate heat from — a key requirement as chips get faster and pack in more components — finding ways to manipulate the behavior of the phonons in those chips, so the heat can be removed easily, is the key. Conversely, in designing thermoelectric devices to generate electricity from temperature differences, it’s important to develop materials that can conduct electricity (the motion of electrons) easily, but block the motion of phonons (that is, heat).

“In some cases, you want strong conduction of phonons, and in some cases you want to reduce their propagation,” Chen says. “Sometimes they’re good guys, and sometimes they’re bad guys.”

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4 / 5 (2) Jul 08, 2010
Based on that explaination, I don't see the need for the term phonon. Why not call it an incident wave front like everyone else. Granted, the frequency is probably entirely dependant on the behavior of the electrons which are quantized to energy levels, but calling the wave front a phonon makes me and others think it's a particle when it's really just a quantized vibration in real particles.

Do I got this right now, finally!?
3.3 / 5 (3) Jul 08, 2010
To me, the process they are describing is perhaps analogous to white noise. Each of the atoms that make up the crystal are vibrating at different frequencies and each "emits" random frequencies that then propagate through the medium, mix with each other, absorb variously into other atoms depending on the current state of the atom on which it is incident, etc.

Yes, to a certain extent, I agree that this is a wave-like phenomena. However, it sounds like it is not a coherent process where it can be treated as a propagating wavefront (like in a laser) because it is made up of many different wavefronts all propagating in different directions at the same time.

In the diagram, the spacing of the rods likely corresponds to the wavelengths of the vibrations they wish to block. Perhaps through this blocking, they achieve more coherence in the material as a whole.

Just my $0.02 - I'm not an expert in the field, but I would love to hear from one who is.
not rated yet Jul 16, 2010
phonons are QUANTIZED vibrations of the crystal lattice. This means that they have all the properties of bosons (since they are whole-integer spin) and specifically do NOT behave like classical analogs...

Ivan bosovic, et al. have made a phaser, which has no classical analog.
1 / 5 (2) Jul 16, 2010
Based on that explaination, I don't see the need for the term phonon. Why not call it an incident wave front like everyone else. .. Do I got this right now, finally!?
The article isn't really very illustrative in explanation of the difference between wave front and boson. It's particularly because no one really understand, what the principle of quantization and bosons is.

Actually, such understanding is not difficult, but it requires to introduce concept of extradimensions. Ripples at the 2D water surface are penetrating mutually like ghosts, thus fulfilling Bose-Einstein statistics. But in 3D the situation is slightly different. Water surface is undulating like carpet and every wave makes it actually a little bit larger. It means, every wave is slightly slowed down by another wave and the propagating wavefront is behaving like sparse particle - a soliton.

It means, the secret is hidden in nonlinearity of environment, i.e. in its ability to undulate in another dimensions.
1 / 5 (2) Jul 16, 2010
Waves spreading within atom lattice are highly nonlinear, because such environment is compressible and it's able to accumulate substantial amount of energy in form of thermal excitations of electrons. We can imagine it like spreading of wave through field of gyroscopes. The wave makes electrons rotating faster at the place of traveling wavefront and as the result the atoms are becoming more stiff and dense environment temporarily like PowerBall toy shaken. In this moment such excited environment actually slows down every subsequent wave, which would pass through excited atoms. The wavefront is behaving like fast traveling zone of dense matter, i.e. like blobby particle or boson.

At the case of vacuum we can imagine it composed of nested density fluctuations in recursive fractal way. Such fluctuations therefore are behaving like tiny excited atoms and they change light wave into fast traveling wave packet, i.e a photon.
1 / 5 (2) Jul 16, 2010
Note that the nonlinearity of environment introduces auto-stabilization effect into bosons, which are behaving like solitons. The short wavelength wave is of higher energy density and the resulting wave packet or soliton is more dense, the this one of low frequency. Energy is propagating more slowly through it, which means, such high frequency wave packet is shorter, then this low frequency one.

As the result, every wave packet contains the same amount of energy, corresponding its frequency. Whenever wave packet is formed with higher amount of energy, then corresponds its frequency, it tends to emanate incoherent waves and to literally evaporate into radiation, until it reaches its optimal energy density, corresponding its dominant frequency. We can observe such behavior at the case of coastal waves, which are spreading like solitons due the proximity of the sea bottom.

Now you can understand, why every boson contains the same amount of energy at the given frequency.

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