The Wigner D-matrix is a unitary matrix in an irreducible representation of the groups SU(2) and SO(3). The complex conjugate of the D-matrix is an eigenfunction of the Hamiltonian of spherical and symmetric rigid rotors. The matrix was introduced in 1927 by Eugene Wigner. D stands for Darstellung, which means "representation" in German.

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Definition of the Wigner D-matrixEdit

Let Jx, Jy, Jz be generators of the Lie algebra of SU(2) and SO(3). In quantum mechanics, these three operators are the components of a vector operator known as angular momentum. Examples are the angular momentum of an electron in an atom, electronic spin, and the angular momentum of a rigid rotor.

In all cases, the three operators satisfy the following commutation relations,

 

where i is the purely imaginary number and Planck's constant ħ has been set equal to one. The Casimir operator

 

commutes with all generators of the Lie algebra. Hence, it may be diagonalized together with Jz. This defines the spherical basis used here.

That is, in this basis, there is a complete set of kets with

 

where j = 0, 1/2, 1, 3/2, 2,... for SU(2), and j = 0, 1, 2, ... for SO(3). In both cases, m= -j, -j + 1,..., j.

A 3-dimensional rotation operator can be written as

 

where α, β, γ are Euler angles (characterized by the keywords: z-y-z convention, right-handed frame, right-hand screw rule, active interpretation).

The Wigner D-matrix is a unitary square matrix of dimension 2j + 1 in this spherical basis with elements

 

where

 

is an element of the orthogonal Wigner's (small) d-matrix.

That is, in this basis,   is diagonal, like the γ matrix factor, but unlike the above β factor.

Wigner (small) d-matrixEdit

Wigner[1] gave the following expression

 

The sum over s is over such values that the factorials are nonnegative.

Note: The d-matrix elements defined here are real. In the often-used z-x-z convention of Euler angles, the factor   in this formula is replaced by  , causing half of the functions to be purely imaginary. The realness of the d-matrix elements is one of the reasons that the z-y-z convention, used in this article, is usually preferred in quantum mechanical applications.

The d-matrix elements are related to Jacobi polynomials   with nonnegative   and  .[2] Let

 
 

Then, with  , the relation is

 

where  

Properties of the Wigner D-matrixEdit

The complex conjugate of the D-matrix satisfies a number of differential properties that can be formulated concisely by introducing the following operators with  ,

 

which have quantum mechanical meaning: they are space-fixed rigid rotor angular momentum operators.

Further,

 

which have quantum mechanical meaning: they are body-fixed rigid rotor angular momentum operators.

The operators satisfy the commutation relations

 

and the corresponding relations with the indices permuted cyclically. The   satisfy anomalous commutation relations (have a minus sign on the right hand side).

The two sets mutually commute,

 

and the total operators squared are equal,

 

Their explicit form is,

 

The operators   act on the first (row) index of the D-matrix,

 

and

 

The operators   act on the second (column) index of the D-matrix

 

and because of the anomalous commutation relation the raising/lowering operators are defined with reversed signs,

 

Finally,

 

In other words, the rows and columns of the (complex conjugate) Wigner D-matrix span irreducible representations of the isomorphic Lie algebras generated by   and  .

An important property of the Wigner D-matrix follows from the commutation of   with the time reversal operator  ,

 

or

 

Here we used that   is anti-unitary (hence the complex conjugation after moving   from ket to bra),   and  .

Orthogonality relationsEdit

The Wigner D-matrix elements   form a set of orthogonal functions of the Euler angles  ,   and  :

 

This is a special case of the Schur orthogonality relations.

Crucially, by the Peter–Weyl theorem, they further form a complete set.

The group characters for SU(2) only depend on the rotation angle β, being class functions, so, then, independent of the axes of rotation,

 

and consequently satisfy simpler orthogonality relations, through the Haar measure of the group,[3]

 

The completeness relation (worked out in the same reference, (3.95)) is

 

whence, for β' =0,

 

Kronecker product of Wigner D-matrices, Clebsch-Gordan seriesEdit

The set of Kronecker product matrices

 

forms a reducible matrix representation of the groups SO(3) and SU(2). Reduction into irreducible components is by the following equation:[4]

 

The symbol   is a Clebsch-Gordan coefficient.

Relation to spherical harmonics and Legendre polynomialsEdit

For integer values of  , the D-matrix elements with second index equal to zero are proportional to spherical harmonics and associated Legendre polynomials, normalized to unity and with Condon and Shortley phase convention:

 

This implies the following relationship for the d-matrix:

 

A rotation of spherical harmonics   then is effectively a composition of two rotations,

 

When both indices are set to zero, the Wigner D-matrix elements are given by ordinary Legendre polynomials:

 

In the present convention of Euler angles,   is a longitudinal angle and   is a colatitudinal angle (spherical polar angles in the physical definition of such angles). This is one of the reasons that the z-y-z convention is used frequently in molecular physics. From the time-reversal property of the Wigner D-matrix follows immediately

 

There exists a more general relationship to the spin-weighted spherical harmonics:

 

Relation to Bessel functionsEdit

In the limit when   we have   where   is the Bessel function and   is finite.

List of d-matrix elementsEdit

Using sign convention of Wigner, et al. the d-matrix elements for j = 1/2, 1, 3/2, and 2 are given below.

for j = 1/2

  •  
  •  

for j = 1

  •  
  •  
  •  
  •  

for j = 3/2

  •  
  •  
  •  
  •  
  •  
  •  

for j = 2 [5]

  •  
  •  
  •  
  •  
  •  
  •  
  •  
  •  
  •  

Wigner d-matrix elements with swapped lower indices are found with the relation:

 .

See alsoEdit

ReferencesEdit

  1. ^ Wigner, E. P. (1931). Gruppentheorie und ihre Anwendungen auf die Quantenmechanik der Atomspektren. Braunschweig: Vieweg Verlag. Translated into English by Griffin, J. J. (1959). Group Theory and its Application to the Quantum Mechanics of Atomic Spectra. New York: Academic Press.
  2. ^ Biedenharn, L. C.; Louck, J. D. (1981). Angular Momentum in Quantum Physics. Reading: Addison-Wesley. ISBN 0-201-13507-8.
  3. ^ Schwinger, J. "On Angular Momentum", Harvard University, Nuclear Development Associates, Inc., United States Department of Energy (through predecessor agency the Atomic Energy Commission) (January 26, 1952)
  4. ^ Rose, M. E. Elementary Theory of Angular Momentum. New York, JOHN WILEY & SONS, 1957.
  5. ^ Edén, M. (2003). "Computer simulations in solid-state NMR. I. Spin dynamics theory". Concepts Magn. Reson. 17A (1): 117–154. doi:10.1002/cmr.a.10061.

External linksEdit