Effective mass

In, a particle's effective mass (often denoted $m^*$ ) is the that it seems to have when responding to forces, or the mass that it seems to have when interacting with other identical particles in a. One of the results from the of solids is that the movement of particles in a periodic potential, over long distances larger than the lattice spacing, can be very different from their motion in a vacuum. The effective mass is a quantity that is used to simplify band structures by modeling the behavior of a free particle with that mass. For some purposes and some materials, the effective mass can be considered to be a simple constant of a material. In general, however, the value of effective mass depends on the purpose for which it is used, and can vary depending on a number of factors.

For s or s in a solid, the effective mass is usually stated in units of the, me (9.11×10−31 kg). In these units it is usually in the range 0.01 to 10, but can also be lower or higher—for example, reaching 1,000 in exotic s, or anywhere from zero to infinity (depending on definition) in. As it simplifies the more general band theory, the electronic effective mass can be seen as an important basic parameter that influences measurable properties of a solid, including everything from the efficiency of a solar cell to the speed of an integrated circuit.

Simple case: parabolic, isotropic dispersion relation
At the highest energies of the valence band in many semiconductors (Ge, Si, GaAs, ...), and the lowest energies of the conduction band in some semiconductors (GaAs, ...), the band structure $E(k)$ can be locally approximated as
 * $$E(\mathbf k) = E_0 + \frac{\hbar^2 \mathbf k^2}{2 m^*}$$

where $E(k)$ is the energy of an electron at $k$ in that band, $E_{0}$ is a constant giving the edge of energy of that band, and $m^{*}$ is a constant (the effective mass).

It can be shown that the electrons placed in these bands behave as free electrons except with a different mass, as long as their energy stays within the range of validity of the approximation above. As a result, the electron mass in models such as the must be replaced with the effective mass.

One remarkable property is that the effective mass can become negative, when the band curves downwards away from a maximum. As a result of the, the electrons respond to electric and magnetic forces by gaining velocity in the opposite direction compared to normal; even though these electrons have negative charge, they move in trajectories as if they had positive charge (and positive mass). This explains the existence of, the positive-charge, positive-mass s that can be found in semiconductors.

In any case, if the band structure has the simple parabolic form described above, then the value of effective mass is unambiguous. Unfortunately, this parabolic form is not valid for describing most materials. In such complex materials there is no single definition of "effective mass" but instead multiple definitions, each suited to a particular purpose. The rest of the article describes these effective masses in detail.

Intermediate case: parabolic, anisotropic dispersion relation
In some important semiconductors (notably, silicon) the lowest energies of the conduction band are not symmetrical, as the constant-energy surfaces are now s, rather than the spheres in the isotropic case. Each conduction band minimum can be approximated only by
 * $$E\left(\mathbf{k}\right) =

E_0 + \frac{\hbar^2}{2 m_x^*}\left(k_x - k_{0,x}\right)^2 + \frac{\hbar^2}{2 m_y^*}\left(k_y - k_{0,y}\right)^2 + \frac{\hbar^2}{2 m_z^*}\left(k_z - k_{0,z}\right)^2 $$

where x, y, and z axes are aligned to the principal axes of the ellipsoids, and $m_{x}^{*}$, $m_{y}^{*}$ and $m_{z}^{*}$ are the inertial effective masses along these different axes. The offsets $k_{0,x}$, $k_{0,y}$, and $k_{0,z}$ reflect that the conduction band minimum is no longer centered at zero wavevector. (These effective masses correspond to the principal components of the inertial effective mass tensor, described later.)

In this case, the electron motion is no longer directly comparable to a free electron; the speed of an electron will depend on its direction, and it will accelerate to a different degree depending on the direction of the force. Still, in crystals such as silicon the overall properties such as conductivity appear to be isotropic. This is because there are multiple s (conduction-band minima), each with effective masses rearranged along different axes. The valleys collectively act together to give an isotropic conductivity. It is possible to average the different axes' effective masses together in some way, to regain the free electron picture. However, the averaging method turns out to depend on the purpose:

General case
In general the dispersion relation cannot be approximated as parabolic, and in such cases the effective mass should be precisely defined if it is to be used at all. Here a commonly stated definition of effective mass is the inertial effective mass tensor defined below; however, in general it is a matrix-valued function of the wavevector, and even more complex than the band structure itself. Other effective masses are more relevant to directly measurable phenomena.

Inertial effective mass tensor
A classical particle under the influence of a force accelerates according to, $g$. This intuitive principle appears identically in semiclassical approximations derived from band structure. However, each of the symbols has a slightly modified meaning; acceleration becomes the rate of change in :


 * $$\mathbf{a} =

\frac{\operatorname{d}}{\operatorname{d}t}\,\mathbf{v}_\text{g} = \frac{\operatorname{d}}{\operatorname{d}t}\left(\nabla_k\,\omega\left(\mathbf{k}\right)\right) = \nabla_k\frac{\operatorname{d}\omega\left(\mathbf{k}\right)}{\operatorname{d}t} = \nabla_k\left(\frac{\operatorname{d}\mathbf{k}}{\operatorname{d}t}\cdot\nabla_k\,\omega(\mathbf{k})\right), $$

where $g = 6$ is the in, and force gives a rate of change in  $a = m^{−1}F$:


 * $$\mathbf{F} =

\frac{\operatorname{d}\mathbf{p}_{\text{crystal}}}{\operatorname{d}t} = \hbar\frac{\operatorname{d}\mathbf{k}}{\operatorname{d}t}, $$

where $∇_{k}$ is the . Combining these two equations yields


 * $$\mathbf{a} = \nabla_k\left(\frac{\mathbf{F}}{\hbar}\cdot\nabla_k\,\omega(\mathbf{k})\right).$$

Extracting the $p_{crystal}$th element from both sides gives


 * $$a_i =

\left(\frac{1}{\hbar}\,\frac{\partial^2 \omega\left(\mathbf{k}\right)}{\partial k_i \partial k_j}\right)\!F_j = \left(\frac{1}{\hbar^2}\,\frac{\partial^2 E\left(\mathbf{k}\right)}{\partial k_i \partial k_j}\right)\!F_j, $$

where $ħ = h/2π$ is the $i$th element of $ai$, $i$ is the $a$th element of $Fj$, $j$ and $F$ are the $ki$th and $kj$th elements of $i$, respectively, and $j$ is the total energy of the particle according to the. The index $k$ is contracted by the use of (there is an implicit summation over $E$). Since Newton's second law uses the (not the ), we can identify the inverse of this mass in the equation above as the tensor


 * $$\left[M_{\text{inert}}^{-1}\right]_{ij} = \frac{1}{\hbar^2} \frac{\partial^2 E}{\partial k_i \partial k_j}\,.$$

This tensor expresses the change in group velocity due to a change in crystal momentum. Its inverse, $j$, is known as the effective mass tensor.

The inertial expression for effective mass is commonly used, but note that its properties can be counter-intuitive:
 * The effective mass tensor generally varies depending on $j$, meaning that the mass of the particle actually changes after it is subject to an impulse. The only cases in which it remains constant are those of parabolic bands, described above.
 * The effective mass tensor diverges (becomes infinite) for linear dispersion relations, such as with photons or electrons in . (These particles are sometimes said to be massless, however this refers to their having zero ; rest mass is a distinct concept from effective mass.)

Cyclotron effective mass
Classically, a charged particle in a magnetic field moves in a helix along the magnetic field axis. The period T of its motion depends on its mass m and charge e,
 * $$T = \left\vert\frac{2\pi m}{e B}\right\vert$$

where B is the.

For particles in asymmetrical band structures, the particle no longer moves exactly in a helix, however its motion transverse to the magnetic field still moves in a closed loop (not necessarily a circle). Moreover, the time to complete one of these loops still varies inversely with magnetic field, and so it is possible to define a cyclotron effective mass from the measured period, using the above equation.

The semiclassical motion of the particle can be described by a closed loop in k-space. Throughout this loop, the particle maintains a constant energy, as well as a constant momentum along the magnetic field axis. By defining $M_{inert}$ to be the $k$ area enclosed by this loop (this area depends on the energy $A$, the direction of the magnetic field, and the on-axis wavevector $k-space$), then it can be shown that the cyclotron effective mass depends on the band structure via the derivative of this area in energy:
 * $$m^*\left(E, \hat{B}, k_{\hat{B}}\right) = \frac{\hbar^2}{2\pi} \cdot \frac{\partial}{\partial E} A\left(E, \hat{B}, k_{\hat{B}}\right)$$

Typically, experiments that measure cyclotron motion (,, etc.) are restricted to only probe motion for energies near the.

In es, the cyclotron effective mass is defined only for one magnetic field direction (perpendicular) and the out-of-plane wavevector drops out. The cyclotron effective mass therefore is only a function of energy, and it turns out to be exactly related to the density of states at that energy via the relation $$\scriptstyle g(E) \;=\; \frac{g_v m^*}{\pi \hbar^2}$$, where $E$ is the valley degeneracy. Such a simple relationship does not apply in three-dimensional materials.

Density of states effective masses (lightly doped semiconductors)
In semiconductors with low levels of doping, the electron concentration in the conduction band is in general given by
 * $$n_e = N_C \exp\left(-\frac{E_\text{C} - E_\text{F}}{kT}\right)$$

where $k_{B}$ is the, $g_{v}$ is the minimum energy of the conduction band, and $E_{F}$ is a concentration coefficient that depends on temperature. The above relationship for $E_{C}$ can be shown to apply for any conduction band shape (including non-parabolic, asymmetric bands), provided the doping is weak ($N_{C}$); this is a consequence of limiting towards.

The concept of effective mass is useful to model the temperature dependence of $n_{e}$, thereby allowing the above relationship to be used over a range of temperatures. In an idealized three-dimensional material with a parabolic band, the concentration coefficient is given by
 * $$\quad N_C = 2\left(\frac{2\pi m_e^* kT}{h^2}\right)^\frac{3}{2}$$

In semiconductors with non-simple band structures, this relationship is used to define an effective mass, known as the density of states effective mass of electrons. The name "density of states effective mass" is used since the above expression for $E_{C}-E_{F} >> kT$ is derived via the for a parabolic band.

In practice, the effective mass extracted in this way is not quite constant in temperature ($N_{C}$ does not exactly vary as $N_{C}$). In silicon, for example, this effective mass varies by a few percent between absolute zero and room temperature because the band structure itself slightly changes in shape. These band structure distortions are a result of changes in electron-phonon interaction energies, with the lattice's thermal expansion playing a minor role.

Similarly, the number of holes in the valence band, and the density of states effective mass of holes are defined by:
 * $$n_h = N_V \exp\left(-\frac{E_\text{F} - E_\text{V}}{kT}\right), \quad N_V = 2\left(\frac{2\pi m_h^* kT}{h^2}\right)^\frac{3}{2}$$

where $N_{C}$ is the maximum energy of the valence band. Practically, this effective mass tends to vary greatly between absolute zero and room temperature in many materials (e.g., a factor of two in silicon), as there are multiple valence bands with distinct and significantly non-parabolic character, all peaking near the same energy.

Experimental
Traditionally effective masses were measured using, a method in which microwave absorption of a semiconductor immersed in a magnetic field goes through a sharp peak when the microwave frequency equals the cyclotron frequency $$\scriptstyle f_c \;=\; \frac{eB}{2\pi m^*}$$. In recent years effective masses have more commonly been determined through measurement of s using techniques such as angle-resolved  or, most directly, the. Effective masses can also be estimated using the coefficient γ of the linear term in the low-temperature electronic at constant volume $$\scriptstyle c_v$$. The specific heat depends on the effective mass through the density of states at the and as such is a measure of degeneracy as well as band curvature. Very large estimates of carrier mass from specific heat measurements have given rise to the concept of materials. Since carrier depends on the ratio of carrier collision lifetime $$\tau$$ to effective mass, masses can in principle be determined from transport measurements, but this method is not practical since carrier collision probabilities are typically not known a priori. The is an emerging technique for measuring the free charge carrier density, effective mass and mobility parameters in semiconductors. The optical Hall effect measures the analogue of the quasi-static electric-field-induced electrical Hall effect at optical frequencies in conductive and complex layered materials. The optical Hall effect also permits characterization of the anisotropy (tensor character) of the effective mass and mobility parameters.

Theoretical
A variety of theoretical methods including, , and are used to supplement and support the various experimental measurements described in the previous section, including interpreting, fitting, and extrapolating these measurements. Some of these theoretical methods can also be used for predictions of effective mass in the absence of any experimental data, for example to study materials that have not yet been created in the laboratory.

Significance
The effective mass is used in transport calculations, such as transport of electrons under the influence of fields or carrier gradients, but it also is used to calculate the carrier density and in semiconductors. These masses are related but, as explained in the previous sections, are not the same because the weightings of various directions and wavevectors are different. These differences are important, for example in, where high conductivity, generally associated with light mass, is desired at the same time as high , generally associated with heavy mass. Methods for assessing the electronic structures of different materials in this context have been developed.

Certain group - compounds such as (GaAs) and  (InSb) have far smaller effective masses than   materials like  and. In the simplest of electronic transport, the maximum obtainable charge carrier velocity is inversely proportional to the effective mass: $$\scriptstyle \vec{v} \;=\; \left\Vert \mu \right\Vert \cdot \vec{E}$$ where $$\scriptstyle \left\Vert \mu \right\Vert \;=\; \frac{e \tau}{\left\Vert m^* \right\Vert}$$ with $$\scriptstyle e$$ being the. The ultimate speed of s depends on the carrier velocity, so the low effective mass is the fundamental reason that GaAs and its derivatives are used instead of Si in high- applications like.

In April 2017, researchers at Washington State University claimed to have created a fluid with negative effective mass inside a, by engineering the.