A series of intermetallic compounds that have a particular crystal structure and the chemical formula A₃B, where A represents a transition element and B can be either a transition element or a nontransition element. Many A15 compounds (see figure) exhibit the phenomenon of superconductivity at relatively high temperatures in the neighborhood of 20 K (−253°C; −424°F) and in high magnetic fields on the order of several tens of teslas (several hundred kilogauss). High-temperature/high-field superconductivity has a number of important technological applications and is a challenging fundamental research area in condensed-matter physics. See also: High magnetic fields; Solid-state physics; Superconductivity; Temperature

A departure of an optical image-forming system from ideal behavior. Ideally, such a system will produce a unique image point corresponding to each object point. In addition, every straight line in the object space will have as its corresponding image a unique straight line. A similar one-to-one correspondence will exist between planes in the two spaces.

The temperature at which matter reaches a lowest energy state and all thermal activity theoretically ceases. Absolute zero is popularly conceived as the coldest possible temperature that a given system of matter, such as an atom, may reach. According to classical physics, all particle motion stops at absolute zero. However, according to the uncertainty principle in quantum mechanics, which states that the velocity and position of a particle cannot be known with total precision simultaneously, a small amount of residual particle motion—known as zero-point motion—would continue even at absolute zero. See also: Classical mechanics; Kinetic theory of matter; Physics; Quantum mechanics; Temperature; Uncertainty principle

Either the taking up of energy from radiation by the medium through which radiation is passing, or the taking up of matter in bulk by other matter. A simple example of the absorption of energy is how sunlight warms a tree's leaves (see illustration). Wavelengths of green light are reflected by molecules in the leaves, making them appear green, while other wavelengths of light are absorbed for use in photosynthesis, with some energy being converted into heat. A simple example of absorption by matter of other matter is the dissolving of carbon dioxide gas into water to create soda water. See also: Carbon dioxide; Chemistry; Energy; Photosynthesis; Physics; Tree; Water

The reception of electromagnetic radiation by particles with electric charge, resulting in transference of the radiation's energy and momentum. Electromagnetic radiation is also referred to as electromagnetic waves, photons, and light. Photons are the elementary particles that carry electromagnetic energy, manifesting as light across a spectrum of energies, or wavelengths. Light can be absorbed by any particle that carries electric charge, such as an electron. The light that is absorbed is destroyed, in the sense that its energy and momentum are transferred to the charged particle, with individual photons of light ceasing to exist. The absorbed energy can increase the charged particle’s kinetic energy, potential energy, or both, leading to effects such as heating (Fig. 1) and atomic transitions. The absorbed momentum can change the particle’s motion, leading to effects such as ionization and radiation pressure. See also: Electric charge; Electromagnetic radiation; Electromagnetism; Electron; Elementary particle; Energy; Heat; Ionization; Light; Momentum; Motion; Photon

The time rate of change of velocity. Acceleration is a physics term that measures the rate of change of velocity per unit of time. Because velocity is a directed or vector quantity involving both magnitude and direction, a velocity may change by a change of magnitude (speed) or by a change of direction, or both. It follows that acceleration is also a directed, or vector, quantity (Fig. 1). See also: Acceleration analysis; Acceleration measurement; Accelerometer; Speed; Velocity

The technique of measuring the magnitude and direction of acceleration. Measurement of acceleration (see illustration) includes translational acceleration with tangential and radial components and angular acceleration. See also: Acceleration

The use of a combination of mass spectrometers and an accelerator to measure the natural abundances of very rare radioactive isotopes. These abundances are frequently lower than parts per trillion. The most important applications of accelerator mass spectrometry are in archeological, geophysical, environmental, and biological studies, such as in radiocarbon dating by the counting of the rare carbon-14 (radiocarbon; ¹⁴C) isotope. See also: Mass spectroscope; Particle accelerator

A mechanical or electromechanical instrument that measures acceleration. The two general types of accelerometers measure either the components of translational acceleration or angular acceleration. See also: Acceleration measurement

An impurity atom in a semiconductor which can accept or take up one or more electrons from the crystal and become negatively charged. An atom which substitutes for a regular atom of the material but has one less valence electron may be expected to be an acceptor atom. For example, atoms of boron, aluminum, gallium, or indium are acceptors in germanium and silicon (illus.a), and atoms of antimony and bismuth are acceptors in tellurium crystals. Acceptor atoms tend to increase the number of holes (positive charge carriers) in the semiconductor (illus.b). The energy gained when an electron is taken up by an acceptor atom from the valence band of the crystal is the ionization energy of the atom. See also: Donor atom; Semiconductor