54
2 Basic Elements of Spintronics
2.7.3 Is Frequent Momentum Scattering Actually Be
Beneficial for Spin Longevity!!
It is the Dresselhaus and Rashba spin–orbit interactions that generated velocitydependent magnetic field
B
v
, which triggers D’yakonov-Perel’ relaxation. Therefore, reduction in
B
v
might suppress D’yakonov-Perel’ relaxation. Now, if the
carrier experiences momentum relaxing scattering phenomenon quite frequently,
i.e., carriers are having low mobility and small momentum relaxation time, then
v is
small implying
B
v
is small. This means that slower moving electrons experience a
smaller
B
v
. Thus, they should be less susceptible to D’yakonov-Perel’ relaxation
and consequently their spin lifetimes should be large. This phenomenon originally
noted by D’yakonov and Perel’ has come to be known as ‘motional narrowing’.
It seems to imply that high carrier mobility or high saturation velocity of charge
carriers might lead to the dephasing of their spin polarizations. In most of the cases
this phenomenon has been found out to be indeed true. In a different scenario, very
strong momentum-relaxing mechanism has been found to cause a larger spread in
the velocity of the carriers, and thereby tend to put the spin polarizations of different
electrons immediately out of phase with respect to one another. Thus, it cannot
be asserted in absolute scale a priori whether momentum-relaxing collisions are
beneficial or detrimental to spin lifetimes. The consequence may vary from system
to system. Noteworthy, in case of Elliott–Yafet mechanism, spin relaxation rate is
directly proportional to the momentum scattering rate.
Furthermore, as we recognize that D’yakonov-Perel’ spin relaxation mechanism
is less effective in case of low mobility materials than that of high mobility materials,
thus in D’yakonov-Perel’ process spin relaxation rate could be considered as inversely
proportional to the momentum scattering rate. Significantly, this constitutes the basis
to distinguish Elliott–Yafet spin relaxation process from D’yakonov-Perel’ process.
We emphasize that distinction between these two spin relaxation processes is based
on the fact that the dependences of their spin relaxation rates on their momentum
scattering rate, i.e., mobility is opposite.
2.7.4 Where Does D’yakonov-Perel’ Mechanism Occur?
The primary spin relaxation process in high mobility semiconductors is usually the
D’yakonov-Perel’ mechanism.
2 Basic Elements of Spintronics
2.7.3 Is Frequent Momentum Scattering Actually Be
Beneficial for Spin Longevity!!
It is the Dresselhaus and Rashba spin–orbit interactions that generated velocitydependent magnetic field
B
v
, which triggers D’yakonov-Perel’ relaxation. Therefore, reduction in
B
v
might suppress D’yakonov-Perel’ relaxation. Now, if the
carrier experiences momentum relaxing scattering phenomenon quite frequently,
i.e., carriers are having low mobility and small momentum relaxation time, then
v is
small implying
B
v
is small. This means that slower moving electrons experience a
smaller
B
v
. Thus, they should be less susceptible to D’yakonov-Perel’ relaxation
and consequently their spin lifetimes should be large. This phenomenon originally
noted by D’yakonov and Perel’ has come to be known as ‘motional narrowing’.
It seems to imply that high carrier mobility or high saturation velocity of charge
carriers might lead to the dephasing of their spin polarizations. In most of the cases
this phenomenon has been found out to be indeed true. In a different scenario, very
strong momentum-relaxing mechanism has been found to cause a larger spread in
the velocity of the carriers, and thereby tend to put the spin polarizations of different
electrons immediately out of phase with respect to one another. Thus, it cannot
be asserted in absolute scale a priori whether momentum-relaxing collisions are
beneficial or detrimental to spin lifetimes. The consequence may vary from system
to system. Noteworthy, in case of Elliott–Yafet mechanism, spin relaxation rate is
directly proportional to the momentum scattering rate.
Furthermore, as we recognize that D’yakonov-Perel’ spin relaxation mechanism
is less effective in case of low mobility materials than that of high mobility materials,
thus in D’yakonov-Perel’ process spin relaxation rate could be considered as inversely
proportional to the momentum scattering rate. Significantly, this constitutes the basis
to distinguish Elliott–Yafet spin relaxation process from D’yakonov-Perel’ process.
We emphasize that distinction between these two spin relaxation processes is based
on the fact that the dependences of their spin relaxation rates on their momentum
scattering rate, i.e., mobility is opposite.
2.7.4 Where Does D’yakonov-Perel’ Mechanism Occur?
The primary spin relaxation process in high mobility semiconductors is usually the
D’yakonov-Perel’ mechanism.