High-Resolution Three-Dimensional Spin- and Angle-Resolved Photoelectron Spectrometer Using Vacuum Ultraviolet Laser Light
Komori and Shin Groups
Spin-polarized electrons in solids have been intensively studied not only for fundamental scientific interests but also for technological applications such as to spintronic devices utilizing the spin degree of freedom. Recently, highly spin-polarized surface states have been discovered under the topological concepts associated with the electronic band structure of materials with strong spin-orbit interaction. Spin- and angle-resolved photoelectron spectroscopy (SARPES) is a powerful technique for investigating such spin-dependent electronic bands. We have constructed a SARPES apparatus [1] with high energy (1.7 meV) and angle resolutions (0.7˚) by combining a very-low-energy-electron-diffraction-type (VLEED) spin detector using the oxygen-adsorbed Fe surface [2] and a high-photon-flux vacuum-ultraviolet (VUV) laser, the 6th harmonic of a basic wave of Nd:YVO4 quasi-continuous wave laser with a non-linear optical crystal KBe2BOF2. (hν = 6.994 eV) [3]. The spectrometer consists of a hemispherical photoelectron analyzer equipped with a photoelectron deflector function and twin VLEED detectors, as shown in Fig. 1. The latter allows us to analyze the spin vector of a photoelectron three-dimensionally.
We have shown that the present laser-SARPES machine realizes a quick SARPES on the spin-split surface band structure of a Bi(111) film even with 7 meV energy and 0.7˚ angular resolutions. Figure 2 demonstrates three-dimensional detection of the spin polarization on the Bi(111) surface. We performed three-dimensional SARPES at a point between Γ and K where we can expect non-zero spin polarizations in the x, y, and z directions because of the absence of mirror symmetry on the ΓK line of the Bi(111) surface. In Figs. 2(a)-(f), two spin-polarized surface states, labeled S1 and S2, are clearly seen, and the amplitude of the spin vector is summarized in Fig. 2(g). From the spin polarization analysis, we can draw the energy dependence of the spin vector as in Fig. 2(h).
References
- [1] K. Yaji et al., Rev. Sci. Instrum. 87, 053111 (2016).
- [2] T. Okuda et al., Rev. Sci. Instrum. 82, 103302 (2011).
- [3] T. Kiss et al., Rev. Sci. Instrum. 73, 1875 (2002).