Transcript Lecture 11 spin polarized photoemission

Detection of Spin-Polarized Electrons:
Spin-polarized Photoemission and Spin Polarized Inverse Photoemission
I.
Photoemission
II.
Mott Detectors:
III. Spin Polarized Inverse Photoemission
IV. Other Techniques
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Pass Energy = C(V0-VI)
Conventional Detector/Spin Integrated
Photoemission
Only electrons with E = Epass+/- δE get thru
the analyzer
Outer Hemisphere (VO)
δE increases with Epass
Inner Hemisphere
VI
e- E = Epass
Retards
Electrons to
Epass
Retarding/focussing
lens
Note: Intensity Increases
with Pass energy,
resolution decreases!
Detector= Channeltron
or Channelplate
KE-Vretard = Epass (Vretard varied, Epass constant)
hv
e- E = KE
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Analyzer
Exciting photon does NOT
directly couple with electron
spin
e-
Therefore, Spin is conserved
during photoemission process
(e.g., J. Osterwalder: SpinPolarized Photoemission, Lect.
Notes Phys. 697, 95–120 (2006)
M
Need a detector that detects
electron spin direction: The
MOTT detector
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Detection of Spin-Polarized Electrons:
See T. J. Gay and F. B. Dunning, Rev. Sci. Inst. 63 (1992) 1635
See also N. F. Mott., Proc. Roy. Soc. A 135 (1932) 429
Consider Fig. 1 (Gay,
Dunning). An
unpolarized electron
beam with equal
numbers of spins P and
AP to vector n̂ 1
Heavy atom
nuclei
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To a reasonable approximation, high
energy electrons will interact with the
bare nucleus charge Z.
The motion of the electrons in the
presence of the field E due to the
nuclear charge sets up a magnetic
field (B) on the electrons
v = electron velocity
The nuclear field is (Gay, Dunning) given as (Ze/r3)r. (r = elec.-nucleus
distance.)
Since rxv = L, the orbital angular momentum of the electron, we have
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The interaction of B (L) with the electron spin S introduces
an asymmetry in the direction of scattering to left or right
in the plane of the figure:
For spin up (parallel to n̂ 1) the
scattering becomes
σ↑(θ1) = I (θ1) (1+S(θ1))
and
σ↓(θ1) = I (θ1) (1-S(θ1))
The net polarization (P) of the beam striking the second
target is:
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Scattering through the second target
(also a high z nucleus) will yield a
similar asymmetry, given by:
For coplanar scattering,
NL α N↑(1+S(θ2)) + N↓(1-S(θ2)),
etc. and:
Thus, if one knows (or
independently measures) S
for a given scattering angle,
measuring the number of
electrons scattered to the left
and right will give you the
polarization of the incoming
beam
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To detect spins of
incoming polarized
electrons, we need a
single heavy target
(gold does not
oxidize) , and 2 (or 4
if fancy)
channeltrons to
measure beam
polariztion.
Retards
Electrons to
Epass
Conventional Detector/Spin Integrated
Photoemission
Outer Hemisphere (VO)
Inner Hemisphere
VI
e- E = Epass
L
Retarding/focussing
lens
R
Detectors= Channeltron
or Channelplate
hv
e- E = KE
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Schematic of Mott Detector with retarding grids
in front of the Channeltrons. (Incoming
electrons are accelerated after energy selection
in hemispherical analyzer to provide good
scattering assymmetries, see Gay and Dunning)
The efficiency (Є) of the
detector is given by:
Unfortunately, the efficiency
of typical Mott Detectors is~
10-3 – 10-4
Patience is a virtue during spin
polarized photoemission!
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Mott Detectors based on designs similar to this are sold commercially
See Gray, Dunning and
ref. therein
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Because of low count rates in Spin polarized measurements, it is
possible to combine a conventional detector for spin integrated
measurements with a Mott detetector and switch back and forth
between them!
H. Berntsen, et al., REVIEW OF
SCIENTIFIC INSTRUMENTS 81, 035104
2010
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(E. Vescovo, et al)
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Oxygen exposure:
Polarization of surface
near Fermi level
disappears.(E. Vescovo,
et al)
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P=0
e-
θ
Au
Polarized beam
One can create a beam of polarized electrons for
inverse photoemission experiments by colliding an
unpolarized beam of a heavy (W, Au, etc) target.
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Dowben Group Facility for spin-polarized inverse
photoemission
Dowben group uses photoelectrons from GaAs
GaAs
B
Rotator
h
G-M
Detector
Sample
Spin Gun
Mott
Detector
Cs Source
Laser
Lin.
Transport
O2
GaAs
Crystal
G-M
Detectors
Magnet
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Photoemission using circularly polarized light at hv = Eg gives heavily
polarized light (direct band gap) because of dipole selection rules (it’s a bit
complicated, see D. T. Pierce, F. Meier, PRB 13 (1976)
However, polarized photoelectrons (P > 50%) can be made in this way.
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Photoemission from GaAs is enhanced with a thin coating of Cs
to yield a negative electron affinity (CBM below vacuum level)
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Other techniques:
Magnetic circular dichroism (change in polariztion of
reflected light)
Spin polarized (SEM)—looking at magnetic domains
Spin polarized LEED—hard to measure net polarization,
but can detect surface magnetic lattices
Spin polarized neutron detection magnetic unit cells
of bulk lattices
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From www.zurich.ibm.com
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Spin polarized SEM useful for measuring magnetic domains.
Browning, et al.
J. Elect. Spect. & Related Phen. 51(1990)
315
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from unit.aist.go.jp
Correlated
electron research
center
Figure 1-3. Magnetic domain image of the Fe(001), where
magnetization direction is shown by the gray level. The
relationship between the magnetization direction and the
gray level is given by the graded band above the domain
image; central gray shows a right direction, lighter gray
shows a counterclockwise direction, and darker gray shows
a clockwise direction.
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