The general notion of scalar, vector, and tensor operators
In quantum mechanics, physical observables that are scalars, vectors, and tensors, must be represented by scalar, vector, and tensor operators, respectively. Whether something is a scalar, vector, or tensor depends on how it is viewed by two observers whose coordinate frames are related to each other by a rotation. Alternatively, one may ask how, for a single observer, a physical quantity transforms if the state of the system is rotated. Consider, for example, a system consisting of a molecule of mass $M$, traveling with a definite center of mass momentum, $p{\mathbf {\hat {z}} }$, in the $z$ direction. If we rotate the system by $90^{\circ }$ about the $y$ axis, the momentum will change to $p{\mathbf {\hat {x}} }$, which is in the $x$ direction. The center-of-mass kinetic energy of the molecule will, however, be unchanged at $p^{2}/2M$. The kinetic energy is a scalar and the momentum is a vector, and these two quantities must be represented by a scalar and a vector operator, respectively. By the latter in particular, we mean an operator whose expected values in the initial and the rotated states are $p{\mathbf {\hat {z}} }$ and $p{\mathbf {\hat {x}} }$. The kinetic energy on the other hand must be represented by a scalar operator, whose expected value must be the same in the initial and the rotated states.
In the same way, tensor quantities must be represented by tensor operators. An example of a tensor quantity (of rank two) is the electrical quadrupole moment of the above molecule. Likewise, the octupole and hexadecapole moments would be tensors of rank three and four, respectively.
Other examples of scalar operators are the total energy operator (more commonly called the
Hamiltonian), the potential energy, and the dipole-dipole interaction energy of two atoms. Examples of vector operators are the momentum, the position, the orbital angular momentum, ${\mathbf {L} }$, and the spin angular momentum, ${\mathbf {S} }$. (Fine print: Angular momentum is a vector as far as rotations are concerned, but unlike position or momentum it does not change sign under space inversion, and when one wishes to provide this information, it is said to be a pseudovector.)
Scalar, vector and tensor operators can also be formed by products of operators. For example, the scalar product ${\mathbf {L} }\cdot {\mathbf {S} }$ of the two vector operators, ${\mathbf {L} }$ and ${\mathbf {S} }$, is a scalar operator, which figures prominently in discussions of the
spin–orbit interaction. Similarly, the quadrupole moment tensor of our example molecule has the nine components
Here, the indices $i$ and $j$ can independently take on the values 1, 2, and 3 (or $x$, $y$, and $z$) corresponding to the three Cartesian axes, the index $\alpha$ runs over all particles (electrons and nuclei) in the molecule, $q_{\alpha }$ is the charge on particle $\alpha$, and $r_{\alpha ,i}$ is the $i$-th component of the position of this particle. Each term in the sum is a tensor operator. In particular, the nine products $r_{\alpha ,i}r_{\alpha ,j}$ together form a second rank tensor, formed by taking the outer product of the vector operator ${\mathbf {r} }_{\alpha }$ with itself.
and let ${\widehat {R}}={\widehat {R}}(\theta ,{\hat {\mathbf {n} }})$ be a
rotation matrix. According to the
Rodrigues' rotation formula, the rotation operator then amounts to
The orthonormal basis set for total angular momentum is $|j,m\rangle$, where j is the total angular momentum quantum number and m is the magnetic angular momentum quantum number, which takes values −j, −j + 1, ..., j − 1, j. A general state within the j subspace
where P_{ℓ}^{m} is an
associated Legendre polynomial, ℓ is the orbital angular momentum quantum number, and m is the orbital magnetic
quantum number which takes the values −ℓ, −ℓ + 1, ... ℓ − 1, ℓ The formalism of spherical harmonics have wide applications in applied mathematics, and are closely related to the formalism of spherical tensors, as shown below.
Spherical harmonics are functions of the polar and azimuthal angles, ϕ and θ respectively, which can be conveniently collected into a unit vector n(θ, ϕ) pointing in the direction of those angles, in the Cartesian basis it is:
So a spherical harmonic can also be written $Y_{\ell }^{m}=\langle \mathbf {n} |\ell m\rangle$. Spherical harmonic states $|m,\ell \rangle$ rotate according to the inverse rotation matrix $U(R^{-1})$, while $|\ell ,m\rangle$ rotates by the initial rotation matrix ${\widehat {U}}(R)$.
We define the Rotation of an operator by requiring that the expectation value of the original operator ${\widehat {\mathbf {A} }}$ with respect to the initial state be equal to the expectation value of the rotated operator with respect to the rotated state,
Any observable vector quantity of a quantum mechanical system should be invariant of the choice of frame of reference. The transformation of expectation value vector which applies for any wavefunction, ensures the above equality. In Dirac notation:
where the RHS is due to the rotation transformation acting on the vector formed by expectation values. Since |Ψ⟩ is any quantum state, the same result follows:
Note that here, the term "vector" is used two different ways: kets such as |ψ⟩ are elements of abstract Hilbert spaces, while the vector operator is defined as a quantity whose components transform in a certain way under rotations.
From the above relation for infinitesimal rotations and the
Baker Hausdorff lemma, by equating coefficients of order $\delta \theta$, one can derive the commutation relation with the rotation generator:^{
[2]}
where ε_{ijk} is the
Levi-Civita symbol, which all vector operators must satisfy, by construction. The above commutator rule can also be used as an alternative definition for vector operators which can be shown by using the
Baker Hausdorff lemma. As the symbol ε_{ijk} is a
pseudotensor, pseudovector operators are invariant
up to a sign: +1 for
proper rotations and −1 for
improper rotations.
Since operators can be shown to form a vector operator by their commutation relation with angular momentum components (which are generators of rotation), its examples include:
Where the RHS is the ${\vec {V}}\cdot {\vec {W}}$ operator originally defined. Since the dot product defined is invariant under rotation transformation, it is said to be a scalar operator.
Spherical vector operators
A vector operator in the
spherical basis is V = (V_{+1}, V_{0}, V_{−1}) where the components are:^{
[2]}
it can be argued that the commutator with operator replaces the action of operator on state for transformations of operators as compared with that of states:
The rotation transformation in the spherical basis (originally written in the Cartesian basis) is then, due to similarity of commutation and operator shown above:
where the basis are transformed by $R^{-1}$ or the vector components transform by $R$.
In the subsequent discussion surrounding tensor operators, the index notation regarding covariant/contravariant behavior is ignored entirely. Instead, contravariant components is implied by context. Hence for an n times contravariant tensor:^{
[2]}
Note: In general, a tensor operator cannot be written as the tensor product of other tensor operators as given in the above example.
Tensor operator from vector operators
If ${\vec {V}}$ and ${\vec {W}}$ are two three dimensional vector operators, then a rank 2 Cartesian dyadic tensors can be formed from nine operators of form ${\hat {T}}_{ij}={\hat {V_{i}}}{\hat {W_{j}}}$,
The RHS of the equation is change of basis equation for twice contravariant tensors where the basis are transformed by $R^{-1}$ or the vector components transform by $R$ which matches transformation of vector operator components. Hence the operator tensor described forms a rank 2 tensor, in tensor representation,
Similarly, an n-times contravariant tensor operator can be formed similarly by n vector operators.
We observe that the subspace spanned by linear combinations of the rank two tensor components form an invariant subspace, ie. the subspace does not change under rotation since the transformed components itself is a linear combination of the tensor components. However, this subspace is not irreducible ie. it can be further divided into invariant subspaces under rotation. Otherwise, the subspace is called reducible. In other words, there exists specific sets of different linear combinations of the components such that they transforms into a linear combination of the same set under rotation.^{
[3]} In the above example, we will show that the 9 independent tensor components can be divided into a set of 1, 3 and 5 combination of operators that each form irreducible invariant subspaces.
Irreducible tensor operators
The subspace spanned by $\{{\hat {T}}_{ij}\}$ can be divided two subspaces; three independent antisymmetric components $\{{\hat {A}}_{ij}\}$ and six independent symmetric component $\{{\hat {S}}_{ij}\}$, defined as ${\hat {A}}_{ij}={\frac {1}{2}}({\hat {T}}_{ij}-{\hat {T}}_{ji})$ and ${\hat {S}}_{ij}={\frac {1}{2}}({\hat {T}}_{ij}+{\hat {T}}_{ji})$. Using the $\{{\hat {T}}_{ij}\}$ transformation under rotation formula, it can be shown that both $\{{\hat {A}}_{ij}\}$ and $\{{\hat {S}}_{ij}\}$ are transformed into a linear combination of members of its own sets. Although $\{{\hat {A}}_{ij}\}$ is irreducible, the same cannot be said about $\{{\hat {S}}_{ij}\}$.
The six independent symmetric component set can be divided into five independent traceless symmetric component and the invariant trace can be its own subspace.
Hence, the invariant subspaces of $\{{\hat {T}}_{ij}\}$ are formed respectively by:
One invariant trace of the tensor, ${\hat {t}}=\sum _{k=1}^{3}{\hat {T}}_{kk}$
Five linearly independent traceless symmetric components from ${\hat {S}}_{ij}={\frac {1}{2}}({\hat {T}}_{ij}+{\hat {T}}_{ji})-{\frac {1}{3}}{\hat {t}}\delta _{ij}$
If ${\hat {T}}_{ij}={\hat {V_{i}}}{\hat {W_{j}}}$, the invariant subspaces of $\{{\hat {T}}_{ij}\}$ formed are represented by:^{
[4]}
One invariant scalar operator ${\vec {V}}\cdot {\vec {W}}$
Three linearly independent components from ${\frac {1}{2}}({\hat {V}}_{i}{\hat {W}}_{j}-{\hat {V}}_{j}{\hat {W}}_{i})$
Five linearly independent components from ${\frac {1}{2}}({\hat {V}}_{i}{\hat {W}}_{j}+{\hat {V}}_{j}{\hat {W}}_{i})-{\frac {1}{3}}({\vec {V}}\cdot {\vec {W}})\delta _{ij}$
From the above examples, the nine component $\{{\hat {T}}_{ij}\}$ are split into subspaces formed by one, three and five components. These numbers add up to the number of components of the original tensor in a manner similar to the dimension of vector subspaces adding to the dimension of the space that is a direct sum of these subspaces. Similarly, every element of $\{{\hat {T}}_{ij}\}$ can be expressed in terms of a linear combination of components from its invariant subspaces:
In general cartesian tensors of rank greater than 1 are reducible. In quantum mechanics, this particular example bears resemblance to the addition of two spin one particles where both are 3 dimensional, hence the total space being 9 dimensional, can be formed by spin 0, spin 1 and spin 2 systems each having 1 dimensional, 3 dimensional and 5 dimensional space respectively.^{
[4]} These three terms are irreducible, which means they cannot be decomposed further and still be tensors satisfying the defining transformation laws under which they must be invariant. Each of the irreducible representations T^{(0)}, T^{(1)}, T^{(2)} ... transform like angular momentum eigenstates according to the number of independent components.
It is possible that a given tensor may have one or more of these components vanish. For example, the quadrupole moment tensor is already symmetric and traceless, and hence has only 5 independent components to begin with.^{
[3]}
Since $J_{x}$ and $J_{y}$ are linear combinations of $J_{\pm }$, they share the same similarity due to linearity.
If, only the commutation relations hold, using the following relation, $|j,m\rangle \rightarrow U(R)|j,m\rangle =exp\left(-{i{\frac {\theta }{\hbar }}{\hat {n}}\cdot {\vec {J}}}\right)|j,m\rangle =\sum _{m'}D_{m'm}^{(j)}(R)|j,m'\rangle$
we find due to similarity of actions of $J$ on wavefunction $|j,m\rangle$ and the commutation relations on ${\widehat {T}}_{m}^{(j)}$, that:
where the exponential form is given by
Baker–Hausdorff lemma. Hence, the above commutation relations and the transformation property are equivalent definitions of spherical tensor operators. It can also be shown that $\{ad_{{\hat {J}}_{i}}\}$ transform like a vector due to their commutation relation.
In the following section, construction of spherical tensors will be discussed. For example, since example of spherical vector operators is shown, it can be used to construct higher order spherical tensor operators. In general, spherical tensor operators can be constructed from two perspectives.^{
[5]} One way is to specify how spherical tensors transform under a physical rotation - a
group theoretical definition. A rotated angular momentum eigenstate can be decomposed into a linear combination of the initial eigenstates: the coefficients in the linear combination consist of Wigner rotation matrix entries. Or by continuing the previous example of the second order dyadic tensor T = a ⊗ b, casting each of a and b into the spherical basis and substituting into T gives the spherical tensor operators of the second order.^{[
citation needed]}
Construction using Clebsch–Gordan coefficients
Combination of two spherical tensors $A_{q_{1}}^{(k_{1})}$ and $B_{q_{2}}^{(k_{2})}$in the following manner involving the
Clebsch–Gordan coefficients can be proved to give another spherical tensor of the form:^{
[4]}
This equation can be used to construct higher order spherical tensor operators, for example, second order spherical tensor operators using two first order spherical tensor operators, say A and B, discussed previously:
Then $\Upsilon _{l}^{m}({\vec {V}})$, where ${\vec {V}}$ is a vector operator, also transforms in the same manner ie, is a spherical tensor operator. The process involves expressing $\Upsilon _{l}^{m}({\vec {r}})=r^{l}Y_{l}^{m}(\theta ,\phi )=\Upsilon _{l}^{m}(x,y,z)$ in terms of x, y and z and replacing x, y and z with operators V_{x} V_{y} and V_{z} which from vector operator. The resultant operator is hence a spherical tensor operator ${\hat {T}}_{m}^{(l)}$.^ This may include constant due to normalization from spherical harmonics which is meaningless in context of operators.
There is some arbitrariness in the choice of the phase factor: any factor containing (−1)^{±q} will satisfy the commutation relations.^{
[6]} The above choice of phase has the advantages of being real and that the tensor product of two commuting
Hermitian operators is still Hermitian.^{
[7]} Some authors define it with a different sign on q, without the k, or use only the
floor of k.^{
[8]}
which raise or lower the orbital magnetic quantum number m_{ℓ} by one unit. This has almost exactly the same form as the spherical basis, aside from constant multiplicative factors.
Spherical tensors can also be formed from algebraic combinations of the spin operators S_{x}, S_{y}, S_{z}, as matrices, for a spin system with total quantum number j = ℓ + s (and ℓ = 0). Spin operators have the ladder operators:
$S_{\pm }=S_{x}\pm iS_{y}$
which raise or lower the spin magnetic quantum number m_{s} by one unit.
Applications
Spherical bases have broad applications in pure and applied mathematics and physical sciences where spherical geometries occur.
Dipole radiative transitions in a single-electron atom (alkali)
The transition amplitude is proportional to matrix elements of the dipole operator between the initial and final states. We use an electrostatic, spinless model for the atom and we consider the transition from the initial energy level E_{nℓ} to final level E_{n′ℓ′}. These levels are degenerate, since the energy does not depend on the magnetic quantum number m or m′. The wave functions have the form,
The dipole operator is proportional to the position operator of the electron, so we must evaluate matrix elements of the form,
$\langle n'\ell 'm'|\mathbf {r} |n\ell m\rangle$
where, the initial state is on the right and the final one on the left. The position operator r has three components, and the initial and final levels consist of 2ℓ + 1 and 2ℓ′ + 1 degenerate states, respectively. Therefore if we wish to evaluate the intensity of a spectral line as it would be observed, we really have to evaluate 3(2ℓ′+ 1)(2ℓ+ 1) matrix elements, for example, 3×3×5 = 45 in a 3d → 2p transition. This is actually an exaggeration, as we shall see, because many of the matrix elements vanish, but there are still many non-vanishing matrix elements to be calculated.
A great simplification can be achieved by expressing the components of r, not with respect to the Cartesian basis, but with respect to the spherical basis. First we define,
where, we have multiplied each Y_{1m} by the radius r. On the right hand side we see the spherical components r_{q} of the position vector r. The results can be summarized by,
for q = 1, 0, −1, where q appears explicitly as a magnetic quantum number. This equation reveals a relationship between vector operators and the angular momentum value ℓ = 1, something we will have more to say about presently. Now the matrix elements become a product of a radial integral times an angular integral,
We see that all the dependence on the three magnetic quantum numbers (m′,q,m) is contained in the angular part of the integral. Moreover, the angular integral can be evaluated by the three-Y_{ℓm} formula, whereupon it becomes proportional to the Clebsch-Gordan coefficient,
$\langle \ell 'm'|\ell 1mq\rangle$
The radial integral is independent of the three magnetic quantum numbers (m′, q, m), and the trick we have just used does not help us to evaluate it. But it is only one integral, and after it has been done, all the other integrals can be evaluated just by computing or looking up Clebsch–Gordan coefficients.
The selection rule m′ = q + m in the Clebsch–Gordan coefficient means that many of the integrals vanish, so we have exaggerated the total number of integrals that need to be done. But had we worked with the Cartesian components r_{i} of r, this selection rule might not have been obvious. In any case, even with the selection rule, there may still be many nonzero integrals to be done (nine, in the case 3d → 2p).
The example we have just given of simplifying the calculation of matrix elements for a dipole transition is really an application of the Wigner–Eckart theorem, which we take up later in these notes.
Magnetic resonance
The spherical tensor formalism provides a common platform for treating coherence and relaxation in
nuclear magnetic resonance. In
NMR and
EPR, spherical tensor operators are employed to express the quantum dynamics of
particle spin, by means of an equation of motion for the
density matrix entries, or to formulate dynamics in terms of an equation of motion in
Liouville space. The Liouville space equation of motion governs the observable averages of spin variables. When relaxation is formulated using a spherical tensor basis in Liouville space, insight is gained because the relaxation matrix exhibits the cross-relaxation of spin observables directly.^{
[5]}
^
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