Angle-resolved photoemission observation of the superconducting-gap minimum and its relation to the nesting vector in the phonon-mediated superconductor ${\text{YNi}}_2{\text{B}}_2\text{C}$

We have performed ultrahigh-resolution angle-resolved photoemission spectroscopy to directly study the large superconducting (SC) gap anisotropy of ${\text{YNi}}_2{\text{B}}_2\text{C}$. We succeed in measuring momentum $\left(k\right)$ dependence of SC gap for individual Fermi surface (FS) sheets, which demonstrates complexity of SC gap in a phonon-mediated superconductor. Within measured $k$ regions on FS sheets, we find a pointlike minimum of SC gap, whose $k$ positions can be connected by the known nesting vector. This shows close correlation between the nesting vector and node formation.

Figure 1

(Color online) (a) First derivative Auger spectrum of ${\text{YNi}}_2{\text{B}}_2\text{C}$ (001). Inset: the LEED pattern of ${\text{YNi}}_2{\text{B}}_2\text{C}$ $\left(001\right)\text{−}1\times 1$ at 64 eV electron-beam energy. (b) Typical spherical path through the BZ at 21.218 eV $\left(\text{He}I\alpha \right)$ for states at $E_{\text{F}}$. Upper right corner: BZ and the $\Gamma$-X-P-Z plane.

Figure 2

(Color online) (a) The ARPES intensity map at $E_{\text{F}}$ $\left(\text{energy}\text{window}=E_{\text{F}}\pm 12.5\text{meV}\right)$ of ${\text{YNi}}_2{\text{B}}_2\text{C}$ (001), along with the calculated FSs. [(b)–(d)] Calculated FSs from 17th, 18th, and 19th bands, respectively.

Figure 3

(Color online) (a) Regions of FSs for which we measured SC gaps. (b) The selected low-$T$ high-resolution ARPES spectra. Blue (dark) and red (light) circles represent the experimental results measured at 6 K (SC) and 18 K (normal), respectively. Green and yellow lines (on the SC and normal state spectra) represent the fitting results by using the modified BCS function and the FD function, respectively. (c) A plot of SC gap values on those FSs, where the color (tone) of symbols corresponds to the color (tone) of measured $k$ regions shown in Fig. 3a.

Figure 4

(Color online) Symmetrized FS. The arrow I and II represent the vector connected to the pointlike SC gap minima on $\Gamma \left(\text{Z}\right)$-centered 17th FS and local gap minima on X(P)-centered 17th FS, respectively.