Total reflection hard x-ray photoelectron spectroscopy: Applications to strongly correlated electron systems

We demonstrate that total reflection hard x-ray photoelectron spectroscopy (TR-HAXPES) is a versatile method for elucidating a difference between surface and bulk electronic states of strongly correlated electron systems, complementing conventional bulk sensitive hard x-ray photoelectron spectroscopy (HAXPES). To demonstrate the experimental feasibility of the method, we investigated ${\mathrm{La}}_{0.6}{\mathrm{Sr}}_{0.4}{\mathrm{MnO}}_3$ and the electron-doped high-$T_{\mathrm{C}}$ superconductor ${\mathrm{La}}_{1.9}{\mathrm{Ce}}_{0.1}{\mathrm{CuO}}_4$. From the incident angle dependence of the spectral line shapes, we found that the surface-sensitive TR-HAXPES measurement equivalent to soft x-ray photoelectron spectroscopy is possible in the total reflection condition. The results strongly suggest that this method allows us to measure both surface and bulk electronic states without making any changes to experimental setup such as the energy resolution, x-ray energy, and the beamline.

Figure 1

Probing depths of HAXPES at a sample surface as functions of the glancing angle. Left: Bulk sensitive HAXPES (labeled “NR-HAXPES”): the penetration depth of x ray is much larger than the escape depth of the photoelectron. Right: Surface sensitive HAXPES (labeled “TR-HAXPES”): the penetration depth of the x ray is smaller than the escape depth of the x ray. The incident angles of the TR- and NR-HAXPES for LSMO (LCCO) are $1.3^{\circ }$ and $1.8^{\circ }$ ($1.5^{\circ }$ and $2.0^{\circ }$) from the sample surface, respectively. However, these values largely depend on how the sample is mounted on the sample holder.

Figure 2

Comparison of wide-scan HAXPES spectra between total reflection and normal conditions. (a) ${\mathrm{La}}_{0.6}{\mathrm{Sr}}_{0.4}{\mathrm{MnO}}_3$. (b) ${\mathrm{La}}_{1.9}{\mathrm{Ce}}_{0.1}{\mathrm{CuO}}_4$.

Figure 3

Measured HAXPES spectra of a LSMO thin film at 160 K. (a) Comparison of Mn $2p$ core-level HAXPES spectra between surface-sensitive TR-HAXPES and bulk-sensitive NR-HAXPES. (b) Enlargements of the energy range corresponding to the Mn $2p_{2/3}$ line. (c) Comparison of O $1s$ core-level HAXPES spectra between TR- and NR- HAXPES.

Figure 4

Measured HAXPES spectra of the LCCO thin film at room temperature. (a) Comparison of Cu $2p$ core-level HAXPES spectra between surface-sensitive TR-HAXPES and bulk-sensitive NR-HAXPES. (b) Comparison of O $1s$ core-level HAXPES spectra between TR- and NR- HAXPES.