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.
In the presence of a magnetic field, the levels with different values of magnetic spin quantum number (MS = 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.
The degree of separation depends on the symmetry of the system.
The corresponding Hamiltonian can be written as where S is the total spin quantum number, and
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.
describes the dipolar spin–spin interaction between two unpaired spins (
is the total spin, and is a symmetric and traceless (
{\displaystyle D_{xx}+D_{yy}+D_{zz}=0}
, when is arises from dipole–dipole interaction) matrix, which means that it is diagonalizable.
for simplicity, the Hamiltonian becomes The key is to express
as its mean value and a deviation
, which is then by rearranging equation (3) Inserting (4) and (3) into (2) yields Note that
– the measurable zero-field splitting values.