Figure 1

(a) Schematic diagram of the PES spectrometer system using a laser as a photon source (Laser-PES) and a demonstration of the sub-meV energy resolution. The second harmonic of a quasi-CW frequency-tripled $\mathrm{N}\mathrm{d}:\mathrm{Y}\mathrm{V}{\mathrm{O}}_3$ laser by using an optically-contacted prism-coupled $\mathrm{K}\mathrm{B}{\mathrm{e}}_2\mathrm{B}{\mathrm{O}}_3{\mathrm{F}}_2$ (KBBF) crystal is focused on a sample with a $\mathrm{C}\mathrm{a}{\mathrm{F}}_2$ lens [25]. The kinetic energies of electrons emitted from the sample are measured with a high-precision hemispherical electron analyzer (GAMMADATA-SCIENTA R-4000). The PES spectrometer and the laser systems are vacuum-separated by a $\mathrm{C}\mathrm{a}{\mathrm{F}}_2$ view port, through which the produced 6.994 eV light can be transmitted. (b) Ultrahigh-resolution PES spectrum of an evaporated gold film measured at 2.9 K (red circles), together with the FD function at 2.9 K convolved by a Gaussian with full width at half maximum of $360\mu \mathrm{e}\mathrm{V}$ (a blue line). Total energy resolution of $360\mu \mathrm{e}\mathrm{V}$ was confirmed from the very good match between the experimental and calculated spectra.Reuse & Permissions

Figure 2

Ultrahigh-resolution PES data of $\mathrm{C}\mathrm{e}\mathrm{R}{\mathrm{u}}_2$. (a) $T$-dependent ultrahigh-resolution spectra near $E_F$ with an inset showing the symmetrized spectra from the same data. (b) Normalized SC DOS at 3.9 K compared with calculated spectra. Error bars correspond to square root of intensity. Blue, red, and green lines are results of fittings with anisotropic gaps of a form $\Delta (\theta )={\Delta }_{\mathrm{m}\mathrm{i}\mathrm{n}}+({\Delta }_{\mathrm{m}\mathrm{a}\mathrm{x}}-{\Delta }_{\mathrm{m}\mathrm{i}\mathrm{n}})\times \cos (2\theta )$ of (${\Delta }_{\mathrm{m}\mathrm{a}\mathrm{x}},{\Delta }_{\mathrm{m}\mathrm{i}\mathrm{n}},\Gamma$) $=$ (0.98, 0.98, 0.13), (1.12, 0.50, 0.07), and (1.18, 0.00, 0.001) in units of meV, where $\Gamma$ is a thermal broadening parameter and $\theta$ the polar angle ($0\le \theta \le \pi /4$). The inset schematically illustrates the anisotropic gap, where a red broken circle represents FS and the distance between blue lines corresponds to a gap value. (c) $T$-dependent SC gap values. The red diamond is the averaged gap value (${\Delta }_{\mathrm{a}\mathrm{v}\mathrm{e}}=⟨\Delta (\theta )⟩$) determined from the anisotropic $s$-wave gap fitting for the data shown in (b), while circles are ${\Delta }_{\mathrm{a}\mathrm{v}\mathrm{e}}$ determined from the same fitting for another high-quality sample ($T_c=6.2\mathrm{K}$) with an energy resolution of 1.15 meV assuming the same ${\Delta }_{\mathrm{m}\mathrm{i}\mathrm{n}}/{\Delta }_{\mathrm{m}\mathrm{a}\mathrm{x}}$ ratio.Reuse & Permissions