In
quantum mechanics
Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, q ...
, the Wigner 3-j symbols, also called 3''-jm'' symbols, are an alternative to
Clebsch–Gordan coefficients
In physics, the Clebsch–Gordan (CG) coefficients are numbers that arise in angular momentum coupling in quantum mechanics. They appear as the expansion coefficients of total angular momentum eigenstates in an uncoupled tensor product basis. In ...
for the purpose of adding angular momenta.
While the two approaches address exactly the same physical problem, the 3-''j'' symbols do so more symmetrically.
Mathematical relation to Clebsch–Gordan coefficients
The 3-''j'' symbols are given in terms of the Clebsch–Gordan coefficients by
:
The ''j'' and ''m'' components are angular-momentum quantum numbers, i.e., every (and every corresponding ) is either a nonnegative integer or
half-odd-integer. The exponent of the sign factor is always an integer, so it remains the same when transposed to the left, and the inverse relation follows upon making the substitution :
:
Definitional relation to Clebsch–Gordan coefficients
The CG coefficients are defined so as to express the addition of two angular momenta in terms of a third:
:
The 3-''j'' symbols, on the other hand, are the coefficients with which three angular momenta must be added so that the resultant is zero:
:
Here
is the zero-angular-momentum state (
). It is apparent that the 3-''j'' symbol treats all three angular momenta involved in the addition problem on an equal footing and is therefore more symmetrical than the CG coefficient.
Since the state
is unchanged by rotation, one also says that the contraction of the product of three rotational states with a 3-''j'' symbol is invariant under rotations.
Selection rules
The Wigner 3-''j'' symbol is zero unless all these conditions are satisfied:
:
Symmetry properties
A 3-''j'' symbol is invariant under an even permutation of its columns:
:
An odd permutation of the columns gives a phase factor:
:
Changing the sign of the
quantum numbers (
time reversal) also gives a phase:
:
The 3-''j'' symbols also have so-called Regge symmetries, which are not due to permutations or time reversal. These symmetries are:
:
:
With the Regge symmetries, the 3-''j'' symbol has a total of 72 symmetries. These are best displayed by the definition of a Regge symbol, which is a one-to-one correspondence between it and a 3-''j'' symbol and assumes the properties of a semi-magic square:
:
whereby the 72 symmetries now correspond to 3! row and 3! column interchanges plus a transposition of the matrix. These facts can be used to devise an effective storage scheme.
[
]
Orthogonality relations
A system of two angular momenta with magnitudes and can be described either in terms of the uncoupled basis states (labeled by the quantum numbers and ), or the coupled basis states (labeled by and ). The 3-''j'' symbols constitute a unitary transformation between these two bases, and this unitarity implies the orthogonality relations
:
:
The ''triangular delta'' is equal to 1 when the triad (''j''1, ''j''2, ''j''3) satisfies the triangle conditions, and is zero otherwise. The triangular delta itself is sometimes confusingly called a "3-''j'' symbol" (without the ''m'') in analogy to 6-''j'' and 9-''j'' symbols, all of which are irreducible summations of 3-''jm'' symbols where no variables remain.
Relation to spherical harmonics; Gaunt coefficients
The 3-''jm'' symbols give the integral of the products of three spherical harmonics
In mathematics and physical science, spherical harmonics are special functions defined on the surface of a sphere. They are often employed in solving partial differential equations in many scientific fields.
Since the spherical harmonics form a ...
:
with , and integers. These integrals are called Gaunt coefficients.
Relation to integrals of spin-weighted spherical harmonics
Similar relations exist for the spin-weighted spherical harmonics
In special functions, a topic in mathematics, spin-weighted spherical harmonics are generalizations of the standard spherical harmonics and—like the usual spherical harmonics—are functions on the sphere. Unlike ordinary spherical harmonics, t ...
if :
:
Recursion relations
:
Asymptotic expressions
For a non-zero 3-''j'' symbol is
:
where , and is a Wigner function. Generally a better approximation obeying the Regge symmetry is given by
:
where .
Metric tensor
The following quantity acts as a metric tensor
In the mathematical field of differential geometry, a metric tensor (or simply metric) is an additional structure on a manifold (such as a surface) that allows defining distances and angles, just as the inner product on a Euclidean space allo ...
in angular-momentum theory and is also known as a ''Wigner 1-jm symbol'':[
:
It can be used to perform time reversal on angular momenta.
]
Special cases and other properties
:
From equation (3.7.9) in
:
:
where ''P'' are Legendre polynomials
In physical science and mathematics, Legendre polynomials (named after Adrien-Marie Legendre, who discovered them in 1782) are a system of complete and orthogonal polynomials, with a vast number of mathematical properties, and numerous applicat ...
.
Relation to Racah -coefficients
Wigner 3-''j'' symbols are related to Racah -coefficients by a simple phase:
:
Relation to group theory
This section essentially recasts the definitional relation
in the language of group theory.
A group representation
In the mathematical field of representation theory, group representations describe abstract groups in terms of bijective linear transformations of a vector space to itself (i.e. vector space automorphisms); in particular, they can be used t ...
of a group
A group is a number of persons or things that are located, gathered, or classed together.
Groups of people
* Cultural group, a group whose members share the same cultural identity
* Ethnic group, a group whose members share the same ethnic ide ...
is a homomorphism
In algebra, a homomorphism is a structure-preserving map between two algebraic structures of the same type (such as two groups, two rings, or two vector spaces). The word ''homomorphism'' comes from the Ancient Greek language: () meaning "sa ...
of the group into
a group of linear transformations
In mathematics, and more specifically in linear algebra, a linear map (also called a linear mapping, linear transformation, vector space homomorphism, or in some contexts linear function) is a Map (mathematics), mapping V \to W between two vect ...
over some vector space. The linear
transformations can be given by a group of matrices with respect to some basis of the vector space.
The group of transformations leaving angular momenta invariant is the three dimensional rotation group SO(3)
In mechanics and geometry, the 3D rotation group, often denoted SO(3), is the group of all rotations about the origin of three-dimensional Euclidean space \R^3 under the operation of composition.
By definition, a rotation about the origin is a ...
.
When "spin" angular momenta are included, the group is its double covering group
In mathematics, a covering group of a topological group ''H'' is a covering space ''G'' of ''H'' such that ''G'' is a topological group and the covering map is a continuous group homomorphism. The map ''p'' is called the covering homomorphism. ...
, SU(2)
In mathematics, the special unitary group of degree , denoted , is the Lie group of unitary matrices with determinant 1.
The more general unitary matrices may have complex determinants with absolute value 1, rather than real 1 in the speci ...
.
A reducible representation is one where a change of basis can be applied to bring all the matrices into block diagonal form. A representation
is irreducible (irrep) if no such transformation exists.
For each value of ''j'', the 2''j''+1 kets form a basis for an irreducible representation (irrep)
of SO(3)/SU(2) over the complex numbers. Given two
irreps, the tensor direct product can be reduced to a
sum of irreps, giving rise to the Clebcsh-Gordon coefficients, or by reduction of the triple product
of three irreps to the trivial irrep 1 giving rise to the 3j symbols.
3j symbols for other groups
The symbol has been most intensely studied in the context of the
coupling of angular momentum. For this, it is strongly related to the
group representation theory
In the mathematical field of representation theory, group representations describe abstract groups in terms of bijective linear transformations of a vector space to itself (i.e. vector space automorphisms); in particular, they can be used to ...
of the groups SU(2) and SO(3)
as discussed above. However, many
other groups are of importance in physics
Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which rel ...
and
chemistry,
and there has been much work on the symbol for these other groups.
In this section, some of that work is considered.
Simply reducible groups
The original paper by Wigner[
was not restricted to SO(3)/SU(2)
but instead focussed on simply reducible (SR) groups.
These are groups in which
* all classes are ambivalent i.e. if is a member of a class then so is
* the Kronecker product of two irreps is multiplicity free i.e. does not contain any irrep more than once.
For SR groups, every irrep is equivalent to its complex conjugate,
and under permutations of the columns the absolute value of the
symbol is invariant and the phase of each can be chosen so that
they at most change sign under odd permutations and remain
unchanged under even permutations.
]
General compact groups
Compact group
In mathematics, a compact (topological) group is a topological group whose topology realizes it as a compact topological space (when an element of the group is operated on, the result is also within the group). Compact groups are a natural ge ...
s form a wide class of groups with topological structure
In mathematics, a topological space is, roughly speaking, a geometrical space in which closeness is defined but cannot necessarily be measured by a numeric distance. More specifically, a topological space is a set whose elements are called poi ...
.
They include the finite groups with added discrete topology
In topology, a discrete space is a particularly simple example of a topological space or similar structure, one in which the points form a , meaning they are ''isolated'' from each other in a certain sense. The discrete topology is the finest t ...
and many of the Lie group
In mathematics, a Lie group (pronounced ) is a group that is also a differentiable manifold. A manifold is a space that locally resembles Euclidean space, whereas groups define the abstract concept of a binary operation along with the addit ...
s.
General compact groups will neither be ambivalent nor multiplicity free.
Derome and Sharp
and Derome examined the symbol
for the general case using the relation to the Clebsch-Gordon coefficients of
:
where wreath product
In group theory, the wreath product is a special combination of two groups based on the semidirect product. It is formed by the action of one group on many copies of another group, somewhat analogous to exponentiation. Wreath products are used ...
of the group with S_3 showed that these correspond to the representations
''Representations'' is an interdisciplinary journal in the humanities published quarterly by the University of California Press. The journal was established in 1983 and is the founding publication of the New Historicism movement of the 1980s. It ...
/math> or ^2/math> of the symmetric group S_2. Cyclic permutations leave the 3j symbol invariant.
* if all three are equivalent, the behaviour is dependant on the representations
''Representations'' is an interdisciplinary journal in the humanities published quarterly by the University of California Press. The journal was established in 1983 and is the founding publication of the New Historicism movement of the 1980s. It ...
of the symmetric groupS_3. Wreath group representations corresponding to /math> are invariant under transpositions of the columns, corresponding to ^3/math> change sign under transpositions, while a pair corresponding to the two dimensional representation 1/math> transform according to that.
Further research into 3j symbols for compact groups has been performed based on these principles.
SU(n)
The Special unitary group
In mathematics, the special unitary group of degree , denoted , is the Lie group of unitary matrices with determinant 1.
The more general unitary matrices may have complex determinants with absolute value 1, rather than real 1 in the speci ...
SU(n) is the Lie group
In mathematics, a Lie group (pronounced ) is a group that is also a differentiable manifold. A manifold is a space that locally resembles Euclidean space, whereas groups define the abstract concept of a binary operation along with the addit ...
of n × n unitary matrices with determinant 1.
The group SU(3)
In mathematics, the special unitary group of degree , denoted , is the Lie group of unitary matrices with determinant 1.
The more general unitary matrices may have complex determinants with absolute value 1, rather than real 1 in the speci ...
is important in particle theory.
There are many papers dealing with the 3j or
equivalent symbol
The 3j symbol for the group SU(4) has been studied
while there is also work on the general SU(n) groups
Crystallographic point groups
There are many papers dealing with the 3j symbols or Clebsch-Gordon coefficients for the finite crystallographic point group
In crystallography, a crystallographic point group is a set of symmetry operations, corresponding to one of the point groups in three dimensions, such that each operation (perhaps followed by a translation) would leave the structure of a crystal un ...
s
and the double point groups
The book by Butler
references these and details the theory along with tables.
Magnetic groups
Magnetic groups include antilinear operators as well as linear operators. They need to be dealt with using
Wigner's theory of corepresentations of unitary and antiunitary groups.
A significant departure from standard representation theory is that the multiplicity of the irreducible corepresentation j_3^*
in the direct product of the irreducible corepresentations j_1 \otimes j_2
is generally smaller than the multiplicity of the trivial corepresentation in the triple
product j_1 \otimes j_2 \otimes j_3, leading to significant differences between the Clebsch-Gordon
coefficients and the 3j symbol.
The 3j symbols have been examined for the grey groups
and for the magnetic point groups
See also
*Clebsch–Gordan coefficients
In physics, the Clebsch–Gordan (CG) coefficients are numbers that arise in angular momentum coupling in quantum mechanics. They appear as the expansion coefficients of total angular momentum eigenstates in an uncoupled tensor product basis. In ...
*Spherical harmonics
In mathematics and physical science, spherical harmonics are special functions defined on the surface of a sphere. They are often employed in solving partial differential equations in many scientific fields.
Since the spherical harmonics form a ...
*6-j symbol
Wigner's 6-''j'' symbols were introduced by Eugene Paul Wigner in 1940 and published in 1965. They are defined as a sum over products of four Wigner 3-j symbols,
:
\begin
j_1 & j_2 & j_3\\
j_4 & j_5 & j_6
\end
= \sum_ (-1)^
\beg ...
*9-j symbol
In physics, Wigner's 9-''j'' symbols were introduced by Eugene Paul Wigner in 1937. They are related to recoupling coefficients in quantum mechanics involving four angular momenta
\sqrt
\begin
j_1 & j_2 & j_3\\
j_4 & j_5 & j_6\\
...
References
*
L. C. Biedenharn and J. D. Louck, ''Angular Momentum in Quantum Physics'', volume 8 of Encyclopedia of Mathematics, Addison-Wesley, Reading, 1981.
* D. M. Brink and G. R. Satchler, ''Angular Momentum'', 3rd edition, Clarendon, Oxford, 1993.
* A. R. Edmonds, ''Angular Momentum in Quantum Mechanics'', 2nd edition, Princeton University Press, Princeton, 1960.
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External links
*
* (Numerical)
*
369j-symbol calculator at the Plasma Laboratory of Weizmann Institute of Science
(Numerical)
* ttp://www.sagemath.org/ Sage (mathematics software)Gives exact answer for any value of j, m
* (accurate; C, fortran, python)
* {{cite web
, first1=H. T.
, last1=Johansson
, title=(FASTWIGXJ)
, url=http://fy.chalmers.se/subatom/fastwigxj/
(fast lookup, accurate; C, fortran)
Rotational symmetry
Representation theory of Lie groups
Quantum mechanics