In physics

Physics

Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...

, Feshbach resonance, named after Herman Feshbach

Herman Feshbach

Herman Feshbach was an American physicist. He was an Institute Professor Emeritus of physics at MIT. Feshbach is best known for Feshbach resonance and for writing, with Philip M...

, is a resonance of a many-body system in which a bound state is achieved if the coupling(s) between an internal degree(s) of freedom and the reaction coordinate

Reaction coordinate

In chemistry, a reaction coordinate is an abstract one-dimensional coordinate which represents progress along a reaction pathway. It is usually a geometric parameter that changes during the conversion of one or more molecular entities....

s which lead to dissociation vanish. The opposite situation, when a bound state is not formed, is a shape resonance

Shape resonance

A shape resonance is a metastable state in which an electron is trapped due the shape of a potential barrier.Serhan describes a state as being a shape resonance if, "the internal state of the system remains unchanged upon disintegration of the quasi-bound level."A more general discussion of...

.

Feshbach resonances have become important in the study of fermi gas

Fermi gas

A Fermi gas is an ensemble of a large number of fermions. Fermions, named after Enrico Fermi, are particles that obey Fermi–Dirac statistics. These statistics determine the energy distribution of fermions in a Fermi gas in thermal equilibrium, and is characterized by their number density,...

es, as these resonances allow for the creation of Bose–Einstein condensates (BEC

BEC

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s). In the context of a BEC, the Feshbach resonance occurs when the energy of a bound state of an interatomic potential is equal to the kinetic energy

Kinetic energy

The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...

 of a colliding pair of atoms, which have hyperfine structure

Hyperfine structure

The term hyperfine structure refers to a collection of different effects leading to small shifts and splittings in the energy levels of atoms, molecules and ions. The name is a reference to the fine structure which results from the interaction between the magnetic moments associated with electron...

 coupled via Coulomb or exchange interactions. This condition is rare in our local conditions, but can be satisfied in ultracold

Ultracold atom

Ultracold atoms is a term used to describe atoms that are maintained at temperatures close to 0 kelvins , typically below some tenths of microkelvins , where their quantum-mechanical properties become important...

 alkali

Alkali metal

The alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, along with hydrogen. The alkali metals are lithium , sodium , potassium , rubidium , caesium , and francium...

atoms.