Zero-field splitting

Template:RefimproveTemplate:Technical Zero-field splitting (ZFS) describes various interactions of the energy levels of a molecule or ion resulting from the presence of more than one unpaired electron. In quantum mechanics, an energy level is called degenerate if it corresponds to two or more different measurable states of a quantum system. In the presence of a magnetic field, the Zeeman effect is well known to split degenerate states. In quantum mechanics terminology, the degeneracy is said to be "lifted" by the presence of the magnetic field. In the presence of more than one unpaired electron, the electrons mutually interact to give rise to two or more energy states. Zero-field splitting refers to this lifting of degeneracy even in the absence of a magnetic field. ZFS is responsible for many effects related to the magnetic properties of materials, as manifested in their electron spin resonance spectra and magnetism.

The classic case for ZFS is the spin triplet, i.e., the S = 1 spin system. In the presence of a magnetic field, the levels with different values of magnetic spin quantum number (M_S = 0, ±1) are separated, and the Zeeman splitting dictates their separation. In the absence of magnetic field, the 3 levels of the triplet are isoenergetic to the first order. However, when the effects of inter-electron repulsions are considered, the energy of the three sublevels of the triplet can be seen to have separated. This effect is thus an example of ZFS. The degree of separation depends on the symmetry of the system.

Quantum-mechanical description

The corresponding Hamiltonian can be written as

$\hat{\mathscr{H}}=D(S_z^2-\frac{1}{3}S(S+1))+E(S_x^2-S_y^2),$

where S is the total spin quantum number, and $S_{x,y,z}$ are the spin matrices.

The value of the ZFS parameter are usually defined via D and E parameters. D describes the axial component of the magnetic dipole–dipole interaction, and E the transversal component. Values of D have been obtained for a wide number of organic biradicals by EPR measurements. This value may be measured by other magnetometry techniques such as SQUID; however, EPR measurements provide more accurate data in most cases. This value can also be obtained with other techniques such as optically detected magnetic resonance (ODMR; a double-resonance technique which combines EPR with measurements such as fluorescence, phosphorescence and absorption), with sensitivity down to a single molecule or defect in solids like diamond (e.g. N-V center) or silicon carbide.

Algebraic derivation

The start is the corresponding Hamiltonian ${\hat{\mathscr{H}}}_D=𝐒𝐃𝐒$. $𝐃$ describes the dipolar spin–spin interaction between two unpaired spins ($S_1$ and $S_2$). Where $S=S_1+S_2$ is the total spin, and Template:NumBlk is a symmetric and traceless ($D_{xx}+D_{yy}+D_{zz}=0$, when is arises from dipole–dipole interaction) matrix, which means that it is diagonalizable.

With $D_{jj}$ denoted as $D_j$ for simplicity, the Hamiltonian becomes Template:NumBlk

The key is to express $D_x{S}_x^2+D_y{S}_y^2$ as its mean value and a deviation $\mathrm{\Delta }$, Template:NumBlk to find the value for the deviation $\mathrm{\Delta }$, which is then by rearranging equation (Template:EquationNote) Template:NumBlk

Inserting (Template:EquationNote) and (Template:EquationNote) into (Template:EquationNote) yields Template:NumBlk

Note that $S_z^2-S_z^2$ was added in the second line in (Template:EquationNote). By doing so, $S_x^2+S_y^2+S_z^2=S(S+1)$ can be further used.

By using the fact that $𝐃$ is traceless ($\frac{1}{2}{D}_x+\frac{1}{2}{D}_y=-\frac{1}{2}{D}_z$), equation (Template:EquationNote) simplifies to Template:NumBlk

By defining D and E parameters, equation (Template:EquationNote) becomes Template:NumBlk with $D=\frac{3}{2}{D}_z$ and $E=\frac{1}{2}(D_x-D_y)$Template:Snd the measurable zero-field splitting values.

Further reading

• Principles of electron spin resonance: By N. M. Atherton. p. 585. Ellis Horwood PTR Prentice Hall. 1993 Template:ISBN.
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External links

• Description of the origins of zero-field splitting