Surface properties of ${\mathrm{SmB}}_6$ from x-ray photoelectron spectroscopy

We have investigated the properties of cleaved ${\mathrm{SmB}}_6$ single crystals by x-ray photoelectron spectroscopy. At low temperatures and freshly cleaved samples a surface core level shift is observed which vanishes when the temperature is increased. A Sm valence between 2.5 and 2.6 is derived from the relative intensities of the ${\mathrm{Sm}}^{2+}$ and ${\mathrm{Sm}}^{3+}$ multiplets. The B/Sm intensity ratio obtained from the core levels is always larger than the stoichiometric value. Possible reasons for this deviation are discussed. The B 1s signal shows an unexpected complexity: An anomalous low energy component appears with increasing temperature and is assigned to the formation of a suboxide at the surface. While several interesting intrinsic and extrinsic properties of the ${\mathrm{SmB}}_6$ surface are elucidated in this manuscript, no clear indication of a trivial mechanism for the prominent surface conductivity is found.

Figure 1

Valence band of freshly cleaved ${\mathrm{SmB}}_6$ at $T=17$ K taken with a photon energy of $h\nu =330$ eV compared to multiplet states derived from tabulated values (Ref. [29]).

Figure 2

Comparison of sample 1 and sample 2 valence bands and their difference. Inset (a): Comparison of sample 2 valence band and difference spectrum shifted by $\Delta E=0.6$ eV. Inset (b): Temperature dependence of the sample 1 valence band, showing the reduction of the surface state emission at room temperature.

Figure 3

Comparison of (a) sample 1 and (b) sample 2 valence bands taken with different photon energies at low temperature to vary the depth sensitivity. Inset: Inelastic mean free path (IMFP) as a function of electron kinetic energy (Ref. [31]).

Figure 4

Stoichiometry extracted from the B 1s and Sm 4d core levels for different photon energies and samples. The blue shaded area on the right side shows the values for sample 2 after exposure to ambient atmosphere. The red dashed line represents the nominal stoichiometric value.

Figure 5

(a) B/Sm intensity ratio extracted from XPS (see text) as a function of the polar angle. (b) Comparison of the Sm 4d line right after cleavage and after exposure to ambient atmosphere. The line shape is dominated by ${\mathrm{Sm}}^{3+}$ and ${\mathrm{Sm}}^{2+}$ related multiplet structures.

Figure 6

Boron 1s line as a function of photon energy, temperature, and emission angle.

Figure 7

Boron 1s line for different hexaborides at low and room temperature.

Figure 8

Markers: Intensity ratio of the boron surface components vs the generic hexaboride component as a function of the IMFP calculated from the various photon energies and emission angles used in Fig. 6. Solid line: Fit of the data with Eq. (1), see text.