# Electron Beam Characterization of REBCO-Coated Conductors at Cryogenic Conditions
## 2 Material and Methods
### Sample Analysis
#### 2.2.3 Composition Analysis via XPS  
The surface chemical analysis by XPS was performed in a room temperature UHV system equipped with a monochromated Al K\({}_{\alpha}\) X-ray source and a hemispherical electron energy analyzer collecting photoelectrons at normal emission angles. The energy scale was calibrated by setting the Cu 2p\({}_{3/2}\) and the Au 4f\({}_{7/2}\) states of sputter-cleaned metals to 932.6 and 84.0 eV, respectively. A mild charging effect was observed (deformed C 1s line) for the RCE-deposited sample. No charge compensation was applied. The atomic concentrations of the present elements were obtained assuming a uniform material and weighting the respective peak area of the XPS spectrum after background subtraction by the corresponding sensitivity factors. Table 2 lists the main elements found on the surface of the two REBCO samples. Elements such as Cl, P, F, Zn, and Ca were also detected, amounting to less than 10 atomic % in total. These elements likely come from post-production sample handling; for instance, Cl is introduced by epoxy-gluing. Meanwhile, the carbon, also observed in EDS spectra, is a common and dominant surface contaminant that has a naturally low SEY value if present in the sp2 state.  
The analysis depth in the nanometer range renders the XPS highly sensitive to the oxidized surface layer, making it challenging to infer the bulk REBCO composition. Still, the XPS surface chemical composition analysis, such as the EDS, could identify the main chemical constituents of the Gd-Ba-Cu-O coating. The chemical states of the different elements are known to greatly influence the SEY of a surface [33; 34]. This is particularly the case for barium, which shows a maximum SEY below unity for the pure metallic state, while BaO displays a maximum SEY beyond 2 [33]. From the XPS spectra, the position of the Ba 3d\({}_{5/2}\) core level at a binding energy of 780.1 eV for the PLD-deposited sample  
\begin{table}
\begin{tabular}{l c c c c c c} \hline \hline
**Sample** & **C** & **O** & **Cu** & **Ba** & **Gd** & **N** & **Ag** & **Si** \\ \hline PLD-deposited: & 33.2 & 42.7 & 2.8 & 8.0 & 0.9 & 2.1 & 1.2 & 0.0 \\ RCE–deposited: & 49.3 & 36.3 & 1.7 & 1.9 & 0.4 & 1.4 & 0.0 & 5.3 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Atomic concentrations in % of elements detected on the surface of as–received PLD–deposited and RCE–deposited samples by the XPS method. Analysis depth in the \(\sim\)nm range.  
\begin{table}
\begin{tabular}{l c c c c c} \hline \hline
**Sample** & **C** & **O** & **Cu** & **Ba** & **Gd** \\ \hline PLD-deposited: precipitate & 4.5 & 13.0 & 21.0 & 33.5 & 28.0 \\ PLD-deposited: Gd–Ba–Cu–O film & 5.7 & 13.2 & 21.0 & 33.3 & 26.8 \\ RCE–deposited: precipitate & 6.6 & 17.4 & 62.0 & 9.4 & 4.6 \\ RCE–deposited: Gd–Ba-Cu-O film & 4.4 & 13.1 & 19.1 & 29.2 & 34.2 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Weight concentrations in percentage (%) of elements detected by the EDS method on the surface of PLD-deposited and RCE-deposited samples, at spots indicated in Figure 2. Analysis depth in \(\sim\)5 \(\upmu\)m range.  
is compatible with the presence of BaO oxide [35]. The additional shift of the Ba 3d\({}_{5/2}\) line to 780.4 eV for the RCE-deposited sample can be explained by a mild charging effect. Hence, in the presence of BaO with intrinsically high SEY, it is likely the elevated carbon surface concentration on the RCE-deposited sample that is responsible for the low SEY reported below.