Schematic diagram of ESPRESSO machine. The highly efficient VLEED spin detector is connected to high-energy resolution hemispherical analyzer (VG-SCIENTA R4000) via 90° electron deflector (). The ferromagnetic target (T) can be magnetized by electric coils along z () and x (; not shown in the figure) directions. The reflected electrons by the target are detected by the channeltron () beside the exit of the deflector. By using the electron deflector both in-plane (x direction) and out-of-plane (z direction) spin components can be observed. Spin-integrated ARPES measurement is also available by observing the portion of photoelectron with the two-dimensional electron detector, multi-channel plate (MCP: (M)), and phosphor screen (P), through a CCD camera. Since the size of the aperture (A) for the spin-resolved measurement channel and the entrance slit (S) is variable, one can select the optimum energy resolution and angular resolution for the SARPES measurement.
(a) Fermi surface mapping of the Bi(111) film obtained by the two-dimensional detector of spin-integrated ARPES mode with the He discharge lamp (hν = 21.22 eV). (b) Electron band dispersions of Bi(111) film along – (left) and – (right) directions which correspond to the vertical and horizontal dashed lines in (a).
(Upper panel) Row SARPES spectra of Bi(111) film taken with plus (I +: (red) solid line) and minus (I −: (blue) dashed line) magnetized target at the specific k point that is indicated by dotted circle in Fig. 2(a) and dotted lines in Fig. 2(b). (Bottom panel) The obtained spin-polarization, P, derived from the asymmetry between I + and I − using 0.21 as the value of effective Sherman function (S eff ) (see text). Error is indicated by bars. The quantization axis of the SARPES measurement is along x direction of the sample (– direction in Fig. 2(a)). (Middle panel) Spin up (I ↑: (red) solid line) and spin down (I ↓: (blue) dashed line) spectra are obtained from the total spectrum (I tot = I + + I −, (black) thick line) and the polarization, P, by the usual procedure, I ↑, ↓ = (1 ± P)I tot /2. The angular resolution and energy resolution of the measurement are set as Δθ = ±0.70° and ΔE ∼ 30 meV (E p = 5 eV, slit = 1.5, aperture = 2×3 and acceptance angle = ±7.0°) in this measurement.
Intensity of reflected electrons by the positively ((red) up triangle: I +) and negatively ((blue) down triangle: I −) magnetized target as the function of electron kinetic energy (upper panel). The circle (I 0) is the total intensity of the impinged electrons onto the target position estimated by the measurement with the back channeltron ( in Fig. 1). From the observed electron intensities (I +, I −, and I 0) the electron reflectivity of the target (second panel), and the asymmetry between positively and negatively magnetized target A = (I + − I −)/(I + + I −) which is proportional to the effective Sherman function S eff (third panel) can be obtained. The total efficiency, the figure of merit (FOM) which is the products of the reflectivity and effective Sherman function is calculated as in the bottom panel (see text in detail).
(a) SARPES spectra along line of the surface states of Bi(111) film taken with He discharge lamp (hν = 21.22 eV). The SARPES spectra are from −4° to 4° with the step of 0.5°. The observed area is indicated by the box in Fig. 2(b). Spin up and spin down states as well as the total intensity are indicated by (red) solid line, (blue) dashed line, and (black) thick solid line respectively. (b) Corresponding spin-polarization of (a). The spin-quantization axis is x direction (– direction in Fig. 2(a)). The maximum (minimum) spin polarization is almost ±100% at around ±1.5° and ±4°.
(a) Spin-integrated band mapping obtained from the total intensity of Fig. 5(a) which is in good agreement with the band dispersion obtained by the two-dimensional detector for the spin-integrated ARPES mode (box in Fig. 2(b)). (b) Spin-polarization mapping obtained from Fig. 5(b). The color scale indicates the spin-polarization. (c) and (d) The band mapping of (c) spin up and (d) spin down states that are obtained from the spin up and spin down SARPES spectra in Fig. 5(a).
Fermi distribution curve measured by the ESPRESSO machine with Au polycrystal excited by the He discharge lamp (hν = 21.22 eV) at the sample temperature of about 10 K.
Angular resolution of SARPES measurement as the function of electron acceptance angle in different analyzer lens mode with different aperture size for spin detection channel. One can select optimum acceptance angle for spin-integrated ARPES and aperture size for SARPES according to the experimental requirement.
The calculated energy resolution of SARPES measurement as the function of the aperture and the entrance slit sizes at the analyzer pass energy of E p = 2 eV. Note that the expected energy resolution is proportional to the analyzer pass energy and those for the different pass energies are not indicated. The experimentally determined energy resolution of some setups is also indicated in the parentheses. The energy resolution is governed by the entrance slit in case of the larger aperture size and vice versa.
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