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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Outer Hemisphere (VO)

Inner Hemisphere VI

e- E = KE

Retarding/focussing lens

Retards Electrons to Epass

KE-Vretard = Epass (Vretard varied, Epass constant)

e- E = Epass

Detector= Channeltron or Channelplate

Pass Energy = C(V0-VI)

Only electrons with E = Epass+/- δE get thru the analyzer

δE increases with Epass

Note: Intensity Increases with Pass energy, resolution decreases!

Conventional Detector/Spin Integrated Photoemission

hv

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M

e-

AnalyzerExciting photon does NOT directly couple with electron spin

Therefore, Spin is conserved during photoemission process (e.g., J. Osterwalder: Spin-Polarized Photoemission, Lect. Notes Phys. 697, 95–120 (2006)

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) 1635See 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 becomesσ↑(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:

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

For coplanar scattering, NL α N↑(1+S(θ2)) + N↓(1-S(θ2)), etc. and:

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Outer Hemisphere (VO)

Inner Hemisphere VI

e- E = KE

Retarding/focussing lens

Retards Electrons to Epass

e- E = Epass

Detectors= Channeltron or Channelplate

Conventional Detector/Spin Integrated Photoemission

hv

R

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.

L

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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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θ

Au

e-P=0

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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Rotator

Laser

GaAs B

G-MDetector

G-M Detectors

Spin Gun

MagnetGaAsCrystal

Cs Source

O2

Lin.Transport

Sample MottDetector

h

Dowben Group Facility for spin-polarized inverse photoemission

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Dowben group uses photoelectrons from GaAs

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.com19

Browning, et al.J. Elect. Spect. & Related Phen. 51(1990) 315

Spin polarized SEM useful for measuring magnetic domains.

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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.

  

  from unit.aist.go.jp

Correlated electron research center

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