Direct observation of a Fermi surface and superconducting gap in ${\text{LuNi}}_2{\text{B}}_2\text{C}$

We measured the Fermi surface (FS), band dispersion, and superconducting gap in ${\text{LuNi}}_2{\text{B}}_2\text{C}$ using angle resolved photoemission spectroscopy. Experimental data were compared to the tight-binding version of the linear muffin-tin orbital (LMTO) method and linearized augmented plane-wave (LAPW) calculations. We found reasonable agreement between the two calculations and experimental data. The measured FS exhibits large parallel regions with a nesting vector that agrees with a previous positron annihilation study and calculations of the generalized susceptibility. The measured dispersion curves also agree reasonably well with the TB-LMTO calculations, although with some differences in the strength of the hybridization. In addition, the spectrum in the superconducting state revealed a 2 meV superconducting gap. The data also clearly show the presence of a coherent peak above the chemical potential $\mu$, which originates from thermally excited electrons above the energy of $2\Delta$. This feature was not previously observed in the Lu-based material.

Figure 1

(Color online) Comparison between the Fermi surface maps measured by ARPES and the linear muffin-tin orbital (TB-LMTO) calculation. (a) Sketch of the first Brillouin zone for ${\text{LuNi}}_2{\text{B}}_2\text{C}$. [(b)–(d)] ARPES mapping at the chemical potential for incident photon energies of 128.13 eV, 119.44 eV, and 102.98 eV, respectively. [(e)–(g)] The Fermi surface maps obtained by TB-LMTO calculations for constant $k_z$ values equal to (e) 0.2, (f) 0.15, and (g) 0.8 expressed in the units of $\Gamma \text{−}Z$ distance.

Figure 2

Dispersion of the conduction bands obtained with a photon energy of 102.98 eV [same as Fig. 1d], compared with the TB-LMTO calculation. (a) Fermi surface map with momentum cuts indicated by the solid lines, in which cuts (c) and (i) pass through the $\Gamma$ and $X$ points, respectively. (b) Fermi surface contours obtained by the TB-LMTO calculation for the value of $k_z$ corresponding to the data in panel (a). Panels (c)–(i): measured band dispersion along the cuts indicated in (a). Panels (j)–(p): calculated band dispersion along the cuts marked in panel (b).

Figure 3

(Color online) Superconducting gap of ${\text{LuNi}}_2{\text{B}}_2\text{C}$ measured in the $a\text{−}b$ plane (a) at $T=11\pm 1\text{K}$, compared with the normal state at $T=40\text{K}$. (b) The Dynes function (solid red line) with the parameters $\Delta =1.5\text{meV}$ and $\Gamma =0.05\text{meV}$ fitted to the symmetrized spectrum (solid black circles). (c) Enlarged portion of superconducting spectra from (a) close to the chemical potential. The data was integrated over the momentum range shown in the inset.