2.6 Elliott–Yafet Mechanism
51
this type of scattering wavevector changes by a lot, and thereby will be very effective
in relaxing spin.
2.6.3 Where Does the Elliott–Yafet Mechanism of Spin
Scattering Occur?
At the onset, Elliott–Yafet relaxation process is considered to be primary mechanism
for spin relaxation in low mobility semiconductors, e.g., organic materials. This is
because in organics the momentum-relaxing scattering events are frequent.
To summarize, in this relaxation mechanism the spin relaxation does not occur
due to the mere presence of spin–orbit interaction in the system. In this case, spin
relaxation occurs only if the scattering of carriers takes place during transport. This
implies higher is the spin scattering rate subject to the higher momentum scattering
rate. Noteworthy, this spin relaxation mechanism must be associated with some
degree of momentum relaxation because it is the change in wavevector that results
in the change in spin.
2.7 D’yakonov-Perel’ Mechanism
2.7.1 What Is the Prerequisite Condition
for D’yakonov-Perel’ Mechanism?
The origin of D’yakonov-Perel’ spin relaxation mechanism is also spin-orbit
coupling. It is the lack of inversion symmetry in some solids that causes this spin
relaxation process to become the dominant relaxation mechanism in those systems.
Let us segregate the discussion in two divisions:
(i) In some cases, for example in compound semiconductors, inversion symmetry
becomes absent owing to its crystallographic structure. Examples are inorganic
semiconductors like GaAs (Group III–V semiconductors), ZnSe (Group II–VI
semiconductors). In these semiconductors, the very presence of two distinct
atoms in the Bravais lattice breaks their inversion symmetry. This kind of crystallographic inversion asymmetry, also known as bulk inversion asymmetry,
leads to Dresselhaus spin–orbit interaction (Dresselhaus 1955). Furthermore,
bulk inversion symmetry has also been found to be absent in disordered organic
semiconductors.
(ii) In a different scenario, external electric field could be applied or there may
be a built-in electric field in a solid. Such external or built-in electric fields
can also break the inversion symmetry. As a result, conduction band energy
profile becomes inversion asymmetric along that electric field direction. This
51
this type of scattering wavevector changes by a lot, and thereby will be very effective
in relaxing spin.
2.6.3 Where Does the Elliott–Yafet Mechanism of Spin
Scattering Occur?
At the onset, Elliott–Yafet relaxation process is considered to be primary mechanism
for spin relaxation in low mobility semiconductors, e.g., organic materials. This is
because in organics the momentum-relaxing scattering events are frequent.
To summarize, in this relaxation mechanism the spin relaxation does not occur
due to the mere presence of spin–orbit interaction in the system. In this case, spin
relaxation occurs only if the scattering of carriers takes place during transport. This
implies higher is the spin scattering rate subject to the higher momentum scattering
rate. Noteworthy, this spin relaxation mechanism must be associated with some
degree of momentum relaxation because it is the change in wavevector that results
in the change in spin.
2.7 D’yakonov-Perel’ Mechanism
2.7.1 What Is the Prerequisite Condition
for D’yakonov-Perel’ Mechanism?
The origin of D’yakonov-Perel’ spin relaxation mechanism is also spin-orbit
coupling. It is the lack of inversion symmetry in some solids that causes this spin
relaxation process to become the dominant relaxation mechanism in those systems.
Let us segregate the discussion in two divisions:
(i) In some cases, for example in compound semiconductors, inversion symmetry
becomes absent owing to its crystallographic structure. Examples are inorganic
semiconductors like GaAs (Group III–V semiconductors), ZnSe (Group II–VI
semiconductors). In these semiconductors, the very presence of two distinct
atoms in the Bravais lattice breaks their inversion symmetry. This kind of crystallographic inversion asymmetry, also known as bulk inversion asymmetry,
leads to Dresselhaus spin–orbit interaction (Dresselhaus 1955). Furthermore,
bulk inversion symmetry has also been found to be absent in disordered organic
semiconductors.
(ii) In a different scenario, external electric field could be applied or there may
be a built-in electric field in a solid. Such external or built-in electric fields
can also break the inversion symmetry. As a result, conduction band energy
profile becomes inversion asymmetric along that electric field direction. This