Effective mass (solid-state physics)

In <a href="/facts/Solid_state_physics/Cmc6hvyR">solid state physics</a>, a particle's effective mass (often denoted 
 
 
 
 
 m
 
 ∗
 
 
 
 
 {\textstyle m^{*}}
 
) is the <a href="/facts/Mass/HHdc8XmF">mass</a> 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 <a href="/facts/Maxwell%25E2%2580%2593Boltzmann_distribution/KAuVfdKJ">thermal distribution</a>. One of the results from the <a href="/facts/Band_theory/W5o7oE2R">band theory</a> 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 <a href="/facts/Vacuum/BTk3goyV">vacuum</a>. 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 <a href="/facts/Electron/L0KBo5cW">electrons</a> or <a href="/facts/Electron_hole/YymdTv9d">electron holes</a> in a solid, the effective mass is usually stated as a factor multiplying the <a href="/facts/Electron_rest_mass/xlG83aQl">rest mass of an electron</a>, me (9.11 × 10−31 kg). This factor is usually in the range 0.01 to 10, but can be lower or higher—for example, reaching 1,000 in exotic <a href="/facts/Heavy_fermion_material/MT1jnClt">heavy fermion materials</a>, or anywhere from zero to infinity (depending on definition) in <a href="/facts/Graphene/QDGUBaPt">graphene</a>. 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.

Effective mass (solid-state physics) open-in-new

Effective mass (solid-state physics)