Phonon

For KDE4's multimedia framework, see Phonon (KDE).

Normal modes of vibration progression through a crystal. The amplitude of the motion has been exaggerated for ease of viewing; in an actual crystal, it is typically much smaller than the lattice spacing.

In physics, a phonon is a quantized mode of vibration occurring in a rigid crystal lattice, such as the atomic lattice of a solid.^[1] The study of phonons is an important part of solid state physics, because phonons play a major role in many of the physical properties of solids, including a material's thermal and electrical conductivities. In particular, the properties of long-wavelength phonons give rise to sound in solids -- hence the name phonon from the Greek φωνή (phonē) = voice.^[2] In insulating solids, phonons are also the primary mechanism by which heat conduction takes place.

Phonons are a quantum mechanical version of a special type of vibrational motion, known as normal modes in classical mechanics, in which each part of a lattice oscillates with the same frequency. These normal modes are important because, according to a well-known result in classical mechanics, any arbitrary vibrational motion of a lattice can be considered as a superposition of normal modes with various frequencies; in this sense, the normal modes are the elementary vibrations of the lattice. Although normal modes are wave-like phenomena in classical mechanics, they acquire certain particle-like properties when the lattice is analysed using quantum mechanics (see wave-particle duality.) They are then known as phonons.

Repeating derivation of normal modes

The equations in this subsection either do not use axioms of quantum mechanics or use relations for which there exists a direct correspondence in classical mechanics.

Mechanics of particles on a lattice

Consider a rigid regular (or "crystalline") lattice composed of N particles. (We will refer to these particles as "atoms", though in a real solid they may actually be ions.) N is some large number, say around 10²³ (on the order of Avogadro's number) for a typical piece of solid. If the lattice is rigid, the atoms must be exerting forces on one another, so as to keep each atom near its equilibrium position. In real solids, these forces include Van der Waals forces, covalent bonds, and so forth, all of which are ultimately due to the electric force; magnetic and gravitational forces are generally negligible. The forces between each pair of atoms may be characterized by some potential energy function V, depending on the separation of the atoms. The potential energy of the entire lattice is the sum of all the pairwise potential energies:

Failed to parse (Missing texvc executable; please see math/README to configure.): \,\sum_{i < j} V(r_i - r_j)

where Failed to parse (Missing texvc executable; please see math/README to configure.): \, r_i

is the position of the Failed to parse (Missing texvc executable; please see math/README to configure.): \, i

^th atom, and Failed to parse (Missing texvc executable; please see math/README to configure.): \, V

is the potential energy between two atoms.

It is extremely difficult to solve this many-body problem in full generality, in either classical or quantum mechanics. In order to simplify the task, we introduce two important approximations. First, we perform the sum over neighboring atoms, only. Although the electric forces in real solids extend to infinity, this approximation is nevertheless valid because the fields produced by distant atoms are screened. Secondly, we treat the potentials Failed to parse (Missing texvc executable; please see math/README to configure.): \, V

as harmonic potentials: this is permissible as long as the atoms remain close to their equilibrium positions. (Formally, this is done by Taylor expanding 

Failed to parse (Missing texvc executable; please see math/README to configure.): \, V

about its equilibrium value, which gives Failed to parse (Missing texvc executable; please see math/README to configure.): \, V
proportional to Failed to parse (Missing texvc executable; please see math/README to configure.): \, x^2

.)

The resulting lattice may be visualized as a system of balls connected by springs. The following figure shows a cubic lattice, which is a good model for many types of crystalline solid. Other lattices include a linear chain, which is a very simple lattice which we will shortly use for modelling phonons. Other common lattices may be found in the article on crystal structure.

The potential energy of the lattice may now be written as

Failed to parse (Missing texvc executable; please see math/README to configure.): \sum_{i \ne j (nn)} {1\over2} m \omega^2 (R_i - R_j)^2

Here, Failed to parse (Missing texvc executable; please see math/README to configure.): \,\omega

is the natural frequency of the harmonic potentials, which we assume to be the same since the lattice is regular. Failed to parse (Missing texvc executable; please see math/README to configure.): \, R_i
is the position coordinate of the Failed to parse (Missing texvc executable; please see math/README to configure.): \, i

^th atom, which we now measure from its equilibrium position. The sum over nearest neighbors is denoted as "(nn)".

Lattice waves

Due to the connections between atoms, the displacement of one or more atoms from their equilibrium positions will give rise to a set of vibration waves propagating through the lattice. One such wave is shown in the figure below. The amplitude of the wave is given by the displacements of the atoms from their equilibrium positions. The wavelength Failed to parse (Missing texvc executable; please see math/README to configure.): \,\lambda

is marked.

There is a minimum possible wavelength, given by the equilibrium separation a between atoms. As we shall see in the following sections, any wavelength shorter than this can be mapped onto a wavelength longer than a, due to effects similar to that in aliasing.

Not every possible lattice vibration has a well-defined wavelength and frequency. However, the normal modes (which, as we mentioned in the introduction, are the elementary building-blocks of lattice vibrations) do possess well-defined wavelengths and frequencies. We will now examine it in detail.

Phonon dispersion of a one-dimensional chain of identical atoms

Consider a one-dimensional quantum mechanical harmonic chain of N identical atoms. This is the simplest quantum mechanical model of a lattice, and we will see how phonons arise from it. The formalism that we will develop for this model is readily generalizable to two and three dimensions. The Hamiltonian for this system is

Failed to parse (Missing texvc executable; please see math/README to configure.): \mathbf{H} = \sum_{i=1}^N {p_i^2 \over 2m} + {1\over 2} m \omega^2 \sum_{\{ij\} (nn)} (x_i - x_j)^2

where Failed to parse (Missing texvc executable; please see math/README to configure.): \, m

is the mass of each atom, and Failed to parse (Missing texvc executable; please see math/README to configure.): \, x_i
and Failed to parse (Missing texvc executable; please see math/README to configure.): \, p_i
are the position and momentum operators for the Failed to parse (Missing texvc executable; please see math/README to configure.): \, i

^th atom. A discussion of similar Hamiltonians may be found in the article on the quantum harmonic oscillator.

We introduce a set of Failed to parse (Missing texvc executable; please see math/README to configure.): \, N

"normal coordinates" Failed to parse (Missing texvc executable; please see math/README to configure.): \, Q_k

, defined as the discrete Fourier transforms of the Failed to parse (Missing texvc executable; please see math/README to configure.): \, x 's and Failed to parse (Missing texvc executable; please see math/README to configure.): \, N

"conjugate momenta" Failed to parse (Missing texvc executable; please see math/README to configure.): \,\Pi 
defined as the Fourier transforms of the Failed to parse (Missing texvc executable; please see math/README to configure.): \, p

's:

Failed to parse (Missing texvc executable; please see math/README to configure.): \begin{matrix} x_j &=& {1\over\sqrt{N}} \sum_{n=-N}^{N} Q_{k_n} e^{ik_nja} \\ p_j &=& {1\over\sqrt{N}} \sum_{n=-N}^{N} \Pi_{k_n} e^{-ik_nja} \\ \end{matrix}

The quantity Failed to parse (Missing texvc executable; please see math/README to configure.): \, k_n

will turn out to be the wave number of the phonon, i.e. Failed to parse (Missing texvc executable; please see math/README to configure.): \, 2\,\pi
divided by the wavelength. It takes on quantized values, because the number of atoms is finite. The form of the quantization depends on the choice of boundary conditions; for simplicity, we impose periodic boundary conditions, defining the Failed to parse (Missing texvc executable; please see math/README to configure.): \, (N+1)

^th atom as equivalent to the first atom. Physically, this corresponds to joining the chain at its ends. The resulting quantization is

Failed to parse (Missing texvc executable; please see math/README to configure.): k_n = {n\pi \over Na} \quad \hbox{for}\ n = 0, \pm1, \pm2, ... , \pm N

The upper bound to Failed to parse (Missing texvc executable; please see math/README to configure.): \, n

comes from the minimum wavelength imposed by the lattice spacing Failed to parse (Missing texvc executable; please see math/README to configure.): \, a

, as discussed above.

By inverting the discrete Fourier transforms to express the Failed to parse (Missing texvc executable; please see math/README to configure.): \, Q 's in terms of the Failed to parse (Missing texvc executable; please see math/README to configure.): \, x 's and the Failed to parse (Missing texvc executable; please see math/README to configure.): \,\Pi 's in terms of the Failed to parse (Missing texvc executable; please see math/README to configure.): \, p 's, and using the canonical commutation relations between the Failed to parse (Missing texvc executable; please see math/README to configure.): \, x 's and Failed to parse (Missing texvc executable; please see math/README to configure.): \, p 's, we can show that

Failed to parse (Missing texvc executable; please see math/README to configure.): \left[ Q_k , \Pi_{k'} \right] = i \hbar \delta_{k k'} \quad ;\quad \left[ Q_k , Q_{k'} \right] = \left[ \Pi_k , \Pi_{k'} \right] = 0

In other words, the normal coordinates and their conjugate momenta obey the same commutation relations as position and momentum operators! Writing the Hamiltonian in terms of these quantities,

Failed to parse (Missing texvc executable; please see math/README to configure.): \mathbf{H} = \sum_k \left( { \Pi_k\Pi_{-k} \over 2m } + {1\over2} m \omega_k^2 Q_k Q_{-k} \right)

where

Failed to parse (Missing texvc executable; please see math/README to configure.): \omega_k = \sqrt{2 \omega^2 (1 - \cos(ka))}

Notice that the couplings between the position variables have been transformed away; if the Failed to parse (Missing texvc executable; please see math/README to configure.): \, Q 's and Failed to parse (Missing texvc executable; please see math/README to configure.): \,\Pi 's were Hermitian (which they are not), the transformed Hamiltonian would describe Failed to parse (Missing texvc executable; please see math/README to configure.): \, N

uncoupled harmonic oscillators.

Three-dimensional phonons

It is straightforward, though tedious, to generalize the above to a three-dimensional lattice. One finds that the wave number k is replaced by a three-dimensional wave vector k. Furthermore, each k is now associated with three normal coordinates.

The new indices s = 1, 2, 3 label the polarization of the phonons. In the one dimensional model, the atoms were restricted to moving along the line, so all the phonons corresponded to longitudinal waves. In three dimensions, vibration is not restricted to the direction of propagation, and can also occur in the perpendicular plane, like transverse waves. This gives rise to the additional normal coordinates, which, as the form of the Hamiltonian indicates, we may view as independent species of phonons.

Dispersion relation

In the above discussion, we have obtained an equation that relates the frequency of a phonon, Failed to parse (Missing texvc executable; please see math/README to configure.): \,\omega_k , to its wave number Failed to parse (Missing texvc executable; please see math/README to configure.): \, k

Failed to parse (Missing texvc executable; please see math/README to configure.): \omega_k = \sqrt{2 \omega^2 (1 - \cos(ka))} = 2 \omega | sin(ka/2) |

This is known as a dispersion relation.

The speed of propagation of a phonon, which is also the speed of sound in the lattice, is given by the slope of the dispersion relation, Failed to parse (Missing texvc executable; please see math/README to configure.): \,\tfrac{\partial\omega_k}{\partial k}

(see group velocity.) At low values of Failed to parse (Missing texvc executable; please see math/README to configure.): \, k
(i.e. long wavelengths), the dispersion relation is almost linear, and the speed of sound is approximately Failed to parse (Missing texvc executable; please see math/README to configure.): \,\omega a

, independent of the phonon frequency. As a result, packets of phonons with different (but long) wavelengths can propagate for large distances across the lattice without breaking apart. This is the reason that sound propagates through solids without significant distortion. This behavior fails at large values of Failed to parse (Missing texvc executable; please see math/README to configure.): \, k , i.e. short wavelengths, due to the microscopic details of the lattice.

For a crystal that has at least two atoms in a unit cell (which may or may not be different), the dispersion relations exhibit two types of phonons, namely, optical and acoustic modes corresponding to the upper and lower sets of curves in the diagram, respectively. The vertical axis is the energy or frequency of phonon, while the horizontal axis is the wave-vector. The boundaries at -k_m and k_m are those of the first Brillouin zone. The blue, violet, and brown curves are those of longitudinal acoustic, transverse acoustic 1, and transverse acoustic 2 modes, respectively.

In some crystals the two transverse acoustic modes have exactly the same dispersion curve. It is also interesting that for a crystal with N ( > 2) different atoms in a primitive cell, there are always three acoustic modes. The number of optical modes is 3N - 3. Many phonon dispersion curves have been measured by neutron scattering.

The physics of sound in fluids differs from the physics of sound in solids, although both are density waves: sound waves in fluids only have longitudinal components, whereas sound waves in solids have longitudinal and transverse components. This is because fluids can't support shear stresses. (but see viscoelastic fluids, which only apply to high frequencies, though).

Acoustic and optical phonons

In solids with more than one atom in the smallest unit cell, there are two types of phonons: "acoustic" phonons and "optical" phonons. "Acoustic phonons", which are the phonons described above, have frequencies that become small at the long wavelengths, and correspond to sound waves in the lattice. Longitudinal and transverse acoustic phonons are often abbreviated as LA and TA phonons, respectively.

"Optical phonons," which also arise in crystals with more than one atom in the smallest unit cell, always have some minimum frequency of vibration, even when their wavelength is large. They are called "optical" because in ionic crystals (like sodium chloride) they are excited very easily by light (in fact, infrared radiation). This is because they correspond to a mode of vibration where positive and negative ions at adjacent lattice sites swing against each other, creating a time-varying electrical dipole moment. Optical phonons that interact in this way with light are called infrared active. Optical phonons which are Raman active can also interact indirectly with light, through Raman scattering. Optical phonons are often abbreviated as LO and TO phonons, for the longitudinal and transverse varieties respectively.

Phonons

In fact, the above derived Hamiltonian looks like the classical Hamilton function, but if it is interpreted as an operator, then it describes a quantum field theory of non-interacting bosons. This leads to new physics.

The energy spectrum of this Hamiltonian is easily obtained by the method of ladder operators, similar to the quantum harmonic oscillator problem. We introduce a set of ladder operators defined by

Failed to parse (Missing texvc executable; please see math/README to configure.): \begin{matrix} a_k &=& \sqrt{m\omega_k \over 2\hbar} (Q_k + {i\over m\omega_k} \Pi_{-k}) \\ a_k^\dagger &=& \sqrt{m\omega_k \over 2\hbar} (Q_{-k} - {i\over m\omega_k} \Pi_k) \end{matrix}

The ladder operators satisfy the following identities:

Failed to parse (Missing texvc executable; please see math/README to configure.): \mathbf{H} = \sum_k \hbar \omega_k \left(a_k^{\dagger}a_k + 1/2\right)

Failed to parse (Missing texvc executable; please see math/README to configure.): [a_k , a_{k'}^{\dagger} ] = \delta_{kk'}

Failed to parse (Missing texvc executable; please see math/README to configure.): [a_k , a_{k'} ] = [a_k^{\dagger} , a_{k'}^{\dagger} ] = 0.

As with the quantum harmonic oscillator, we can then show that Failed to parse (Missing texvc executable; please see math/README to configure.): \, a_k^\dagger

and Failed to parse (Missing texvc executable; please see math/README to configure.): \, a_k
respectively create and destroy one excitation of energy Failed to parse (Missing texvc executable; please see math/README to configure.): \,\hbar\omega_k

. These excitations are phonons.

We can immediately deduce two important properties of phonons. Firstly, phonons are bosons, since any number of identical excitations can be created by repeated application of the creation operator Failed to parse (Missing texvc executable; please see math/README to configure.): \, a_k^\dagger . Secondly, each phonon is a "collective mode" caused by the motion of every atom in the lattice. This may be seen from the fact that the ladder operators contain sums over the position and momentum operators of every atom.

It is not a priori obvious that these excitations generated by the Failed to parse (Missing texvc executable; please see math/README to configure.): \, a

operators are literally waves of lattice displacement, but one may convince oneself of this by calculating the position-position correlation function. Let Failed to parse (Missing texvc executable; please see math/README to configure.): \, |k\rangle
denote a state with a single quantum of mode Failed to parse (Missing texvc executable; please see math/README to configure.): \, k
excited, i.e.

Failed to parse (Missing texvc executable; please see math/README to configure.): \begin{matrix} | k \rangle = a_k^\dagger | 0 \rangle \end{matrix}

One can show that, for any two atoms Failed to parse (Missing texvc executable; please see math/README to configure.): \, j

and Failed to parse (Missing texvc executable; please see math/README to configure.): \,\ell

,

Failed to parse (Missing texvc executable; please see math/README to configure.): \langle k | x_j(t) x_{\ell}(0) | k \rangle = \frac{\hbar}{Nm\omega_k} \cos \left[ k(j-\ell)a - \omega_k t \right] + \langle 0 | x_j(t) x_\ell(0) |0 \rangle

which is exactly what we would expect for a lattice wave with frequency Failed to parse (Missing texvc executable; please see math/README to configure.): \,\omega_k

and wave number Failed to parse (Missing texvc executable; please see math/README to configure.): \, k

.

In three dimensions the Hamiltonian has the form

Failed to parse (Missing texvc executable; please see math/README to configure.): \mathbf{H} = \sum_k \sum_{s=1}^3 \hbar \, \omega_{k,s} \left( a_{k,s}^{\dagger}a_{k,s} + 1/2 \right)

Crystal momentum

Main article: Crystal momentum

It is tempting to treat a phonon with wave vector Failed to parse (Missing texvc executable; please see math/README to configure.): \, k

as though it has a momentum Failed to parse (Missing texvc executable; please see math/README to configure.): \,\hbar k

, by analogy to photons and matter waves. This is not entirely correct, for Failed to parse (Missing texvc executable; please see math/README to configure.): \,\hbar k

is not actually a physical momentum; it is called the crystal momentum or pseudomomentum. This is because Failed to parse (Missing texvc executable; please see math/README to configure.): \, k
is only determined up to multiples of constant vectors, known as reciprocal lattice vectors. For example, in our one-dimensional model, the normal coordinates Failed to parse (Missing texvc executable; please see math/README to configure.): \, Q
and Failed to parse (Missing texvc executable; please see math/README to configure.): \,\Pi
are defined so that

Failed to parse (Missing texvc executable; please see math/README to configure.): Q_k \ \stackrel{\mathrm{def}}{=}\ Q_{k+K} \quad;\quad \Pi_k \ \stackrel{\mathrm{def}}{=}\ \Pi_{k + K} \quad

where

Failed to parse (Missing texvc executable; please see math/README to configure.): \, K = 2n\pi/a

for any integer Failed to parse (Missing texvc executable; please see math/README to configure.): \, n . A phonon with wave number Failed to parse (Missing texvc executable; please see math/README to configure.): \, k

is thus equivalent to an infinite "family" of phonons with wave numbers Failed to parse (Missing texvc executable; please see math/README to configure.): \, k\pm\tfrac{2\,\pi}{a}

, Failed to parse (Missing texvc executable; please see math/README to configure.): \, k\pm\tfrac{4\,\pi}{a} , and so forth. Physically, the reciprocal lattice vectors act as additional "chunks" of momentum which the lattice can impart to the phonon. Bloch electrons obey a similar set of restrictions.

It is usually convenient to consider phonon wave vectors Failed to parse (Missing texvc executable; please see math/README to configure.): \, k

which have the smallest magnitude Failed to parse (Missing texvc executable; please see math/README to configure.): \, (|k|)
in their "family". The set of all such wave vectors defines the first Brillouin zone. Additional Brillouin zones may be defined as copies of the first zone, shifted by some reciprocal lattice vector.

It is interesting that similar consideration is needed in analog-to-digital conversion where aliasing may occur under certain conditions.

Thermodynamic properties

A crystal lattice at zero temperature lies in its ground state, and contains no phonons. According to thermodynamics, when the lattice is held at a non-zero temperature its energy is not constant, but fluctuates randomly about some mean value. These energy fluctuations are caused by random lattice vibrations, which can be viewed as a gas of phonons. (Note: the random motion of the atoms in the lattice is what we usually think of as heat.) Because these phonons are generated by the temperature of the lattice, they are sometimes referred to as thermal phonons.

Unlike the atoms which make up an ordinary gas, thermal phonons can be created or destroyed by random energy fluctuations. In the language of statistical mechanics this means that the chemical potential for adding a phonon is zero. It is very important to note that this behaviour takes us away from the harmonic potential mentioned earlier, and into the anharmonic regime. The behaviour of thermal phonons is similar to the photon gas produced by an electromagnetic cavity, wherein photons may be emitted or absorbed by the cavity walls. This similarity is not coincidental, for it turns out that the electromagnetic field behaves like a set of harmonic oscillators; see Black-body radiation. Both gases obey the Bose-Einstein statistics: in thermal equilibrium and within the harmonic regime, the probability of finding phonons (or photons) in a given state with a given angular frequency is:

Failed to parse (Missing texvc executable; please see math/README to configure.): n(\omega_{k,s}) = \frac{1}{\exp(\hbar\omega_{k,s}/k_BT) - 1}

where Failed to parse (Missing texvc executable; please see math/README to configure.): \,\omega_{k,s}

is the frequency of the phonons (or photons) in the state, Failed to parse (Missing texvc executable; please see math/README to configure.): \, k_B
is Boltzmann's constant, and Failed to parse (Missing texvc executable; please see math/README to configure.): \, T
is the temperature.

See also

Physics Portal

• Fracton
• Linear elasticity
• Rayleigh wave
• Surface acoustic wave
• Rigid Unit Modes a phonon where polyhedra move, by translation and/or rotation, without distorting
• Phononic crystal
• a vibron is for a molecule what a phonon is for a crystal

References

1. ^ International Union of Pure and Applied Chemistry. "phonon". Compendium of Chemical Terminology Internet edition.
2. ^ Although φόνον (phonon) is literally the accusative case of φόνος (phonos) = murder!

External links

• PHONONS 2007: 12th International Conference on Phonon Scattering in Condensed Matter [1].

  v • d • e Particles in physics
  Elementary particles	Fermions:  Quarks: u · d · c · s · t · b • Leptons: e− · e+ · μ− · μ+ · τ− · τ+ · νe · νμ · ντ Bosons:  Gauge bosons: γ · g · W± · Z0  Other:  Ghosts
  Composite particles	Hadrons:  Baryons(list)/Hyperons/Nucleons: p · n · Δ · Λ · Σ · Ξ · Ω · Ξb • Mesons(list)/Quarkonia: π · K · ρ · J/ψ · Υ Other:  Atomic nuclei • Atoms • Exotic atoms: Positronium • Molecules
  Hypothetical elementary particles	Superpartners: Axino · Dilatino · Chargino · Gluino · Gravitino · Higgsino · Neutralino · Sfermion · Slepton · Squark Other: Axion · Dilaton · Goldstone boson · Graviton · Higgs boson · Tachyon · X · Y · W' · Z' · Sterile neutrino
  Hypothetical composite particles	Exotic hadrons:  Exotic baryons: Pentaquark • Exotic mesons: Glueball · Tetraquark Other:  Mesonic molecule
  Quasiparticles	Davydov soliton · Exciton · Magnon · Phonon · Plasmon · Polariton · Polaron

  ar:فونون

bg:Фонон ca:Fonó cs:Fonon de:Phonon el:Φωνόνιο et:Foonon es:Fonón fr:Phonon he:פונון it:Fonone ms:Fonon nl:Fonon ja:フォノン pl:Fonon ru:Фонон fi:Fononi sv:Fonon uk:Фонон