Detection of Spin-Polarized Electrons: Spin-polarized Photoemission and Spin Polarized Inverse Photoemission Photoemission Mott Detectors: Spin Polarized Inverse Photoemission Other Techniques

Pass Energy = C(V0-VI) Conventional Detector/Spin Integrated Photoemission Only electrons with E = Epass+/- δE get thru the analyzer δE increases with Epass Outer Hemisphere (VO) Inner Hemisphere VI Note: Intensity Increases with Pass energy, resolution decreases! e- E = Epass Detector= Channeltron or Channelplate Retards Electrons to Epass Retarding/focussing lens KE-Vretard = Epass (Vretard varied, Epass constant) hv e- E = KE

M Analyzer Exciting 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 e- M

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 n̂1 Heavy atom nuclei

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

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 n̂1) 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:

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

Conventional Detector/Spin Integrated Photoemission 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. Outer Hemisphere (VO) Inner Hemisphere VI e- E = Epass L R Retards Electrons to Epass Retarding/focussing lens Detectors= Channeltron or Channelplate hv e- E = KE

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!

Mott Detectors based on designs similar to this are sold commercially See Gray, Dunning and ref. therein

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

(E. Vescovo, et al)

Oxygen exposure: Polarization of surface near Fermi level disappears.(E. Vescovo, et al)

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.

Dowben Group Facility for spin-polarized inverse photoemission 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.

Photoemission from GaAs is enhanced with a thin coating of Cs to yield a negative electron affinity (CBM below vacuum level)

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

From www.zurich.ibm.com

Spin polarized SEM useful for measuring magnetic domains. Browning, et al. J. Elect. Spect. & Related Phen. 51(1990) 315

  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.