Transcript Spin Polarized Electron - Jordan University of Science and

Jordan University of Science and Technology
Edit by :- Ala’a Ali Qudah .
ID :- 20053092012.
Advicor :- Dr. Hasan Al-Khateeb.
Research Of Spin Polarized Electron (Phys 791).
Spin Polarized Electron
Spin polarized electron (SPE) beams have wide applications
1) Condensed matter
2) Atomic and molecular
3) High energy physics
Historically, the development of a suitable SPE source and
have been tried in attempts to produce beams of spin
polarized electrons
1) Scattering from unpolarized target
2) Photoemission from polarized atoms
3) Fano effect
4) The most suitable source photoemission from GaAs.
polarized electron :If the electron spins have a preferential orientation so that there
exists a direction for which the two possible spin states are not
equally populated .
Polarization is defined :-
P
N  N 
N  N 
For example :10 electrons ,8 electrons spin up and 2 electrons spin down
P = (8-2)/(8+2)
= 0.6
In the mid-1970s Pierce, Meier, and Zurcher developed an intense source
of polarized electrons based on photoemission from p-type GaAs,
because have many advantages.
Advantages of GaAs source:1) Narrow energy width.
2) Quantum efficiency.
3) Good degree of polarization .
4) High brightness.
Photoemission from GaAs can be described particular three step model:1) Photoexcitation.
2) Transport.
3) Escape.
Optical Spin Orientation
Transport and Escape
The electron affinity χ of p-type GaAs is large (about 4eV). It represents the
difference in energy between the vacuum level and the conducting band
prevents electrons from leaving the crystal surface. adding layers of cesium to
the crystal will reduce the electron affinity to values close to zero.
Furthermore, adding layers of cesium and oxygen will reduce the electron
affinity to negative values (negative electron affinity (NEA)).
NEA GaAs is able to extract photoelectrons with high quantum efficiency.
Increase the efficiency of current
Then Adding several layers of cesium and oxygen (is called activating the
crystal) is able to increase the photocurrent of electrons to many orders of
magnitude
Important of polarized electrons
1)
2)
3)
it is important to know the spin state of incident electron as
it is important to know it is energy and momentum .
studying spin-dependent electron-atom scattering processes
such as spin-orbit interaction between the incident electron
and the atom as well as the exchanges between them.
the polarized-electron atom collisions can provide valuable
information about collision mechanisms and electron
correlation.
electron ()  Atom  ( Atom )  electron  photons( )
The polarization of the light emitted from excited state provides
us with a lot of information about the processes during the
collision between the electron and atom.
To determine the polarization properties of the light completely, we must
measure the Stokes parameters and To determine these parameters, we
have to measure the intensity of the light after passing through rotatable
retarder and linear polarizer (optical polarimeter ).
The intensity of the transmitted light of the incident light beam which has
Stokes parameters , and
I P
I ( ,  ,  )  1  1 cos2(   ) cos2   sin 2(   ) sin  cos 
2 I
P
+ 2 cos 2(   ) sin 2   sin 2(   ) cos 2  cos 
I
+
sin 2(   ) sin  P3 
I 
To measure the linear polarization of light and
I P (0 )  I P (90 )
P1 
I P (0 )  I P (90 )
I P (45  )  I P (135  )
P2 
I P (45  )  I P (135  )
And the circular polarization
P3 
I (0  ,45  ,90  )  I (0  ,135  ,90  )
I (0  ,45  ,90  )  I (0  ,135  ,90  )
Then it possible to express the intergrated Stokes parameters in terms of state
multipoles
1
P1  3 / 2 
J
1
J
2 
 T (J ) / 
J f  20
1
P2  3 / 2 
J
1
J
2 
 Re T21 ( J ) / 
Jf 
1
P3   3 / 2 
J
1
J
2 
 Im T21 ( J ) / 
Jf 
Where
J J
2(1)
1 1
I
T00 ( J ) 

3 2J  1
6 J
f
J
TKQ ( J )   (1) J  M (2 K  1)
M
M
J K J
and
J
M
1
J
2 
 T20 ( J )
Jf 
K 
 JM  JM
Q 
K Q K
Summary
Electron beam is said to be polarized if there exists a direction for
which the two possible spin states are not equally populated.
it is important to know the initial and final states of the system
and to know the spin states of the incident electron and can
provide valuables information about collision mechanisms.
The most suitable source photoemission from GaAs , because
have many advantages .and can be described this source in
three steps photoemission, transport and escape.