Breakdown of spatial inversion symmetry in core-level photoemission of Pt(001)

We have measured the spin polarization of the $4d$ and $4f$ core-level photoelectrons from Pt(001) by using unpolarized laboratory x-ray sources under a highly bulk sensitive condition. The $4d$ and $4f$ photoelectrons are highly spin polarized perpendicular to the reaction plane as defined by the incident photons and the outgoing electrons. The measured spin polarization and a close look at the core-level photoemission process demonstrate that the bulk core-level photoemission with unpolarized light contributes to the measured spin polarization. This result is in contrast to the valence band photoemission from nonmagnetic solids, wherein the bulk cannot contribute to the measured spin polarization due to the existence of spatial inversion symmetry. Thus, the argument based on spatial inversion symmetry does not apply to the core-level photoemission. The measured spin polarization is in good agreement with an atomic model.

Figure 1

(Color online) Sketch for the spin resolved photoemission setup recently installed at Lawrence Livermore National Laboratory. Unpolarized x rays hit the sample at an angle of $45°$ with respect to the surface normal. The x-ray tube is located on the $Y\text{−}Z$ plane, which is the reaction plane. The energies and the spins of the normally emitted photoelectrons are analyzed by a hemispherical electron energy analyzer and the Mott detector, which has a thorium target operated at 25 keV and a Sherman function of $0.16\pm 0.04$, respectively. Two transversal spin components $P_X$ and $P_Y$ can be simultaneously measured in the Mott detector.

Figure 2

(Color online) Spin resolved $4d$ core-level spectra from Pt(001) with unpolarized x rays of $h\nu =1253.6\text{eV}$. The left (right) panel presents the $X\left(Y\right)$ component. The partial intensities $I_{\pm }$ are derived from the spin integrated total intensity $I$ and the polarization $P$ by $I_{\pm }=\left(I/2\right)\left(1\pm P\right)$. The error bars give the statistical uncertainties.

Figure 3

(Color online) Subtraction of background (BG) from the $4d$ core-level photoemission spectrum from Pt(001) with unpolarized x rays of $h\nu =1253.6\text{eV}$ for the $X$ component. Two different backgrounds are assumed as a straight line and a Shirley type, as shown in the upper panel. After background subtractions, the corrected polarizations are plotted in the lower panel as up triangles for the straight line subtraction and as down triangles for the Shirley type subtraction. The circles are the polarizations without background subtraction.

Figure 4

(Color online) Comparison between the experimental spin polarizations measured from the $4d$ and $4f$ core levels of Pt(001) with unpolarized x rays of 1253.6 and 1486.6 eV and the theoretical values calculated by using the atomic model. Equation (1) with the tabulated radial matrix elements, the phase shifts, and the ${\beta }^j$ from Ref. 29 is used for the theoretical spin polarization. The experimental spin polarizations are plotted after background subtractions. The upper panel is for $4d$ and the lower panel is for $4f$.