Momentum Dependence of the Superconducting Gap in ${\mathrm{NdFeAsO}}_{0.9}{\mathrm{F}}_{0.1}$ Single Crystals Measured by Angle Resolved Photoemission Spectroscopy

We use angle resolved photoemission spectroscopy to study the momentum dependence of the superconducting gap in ${\mathrm{NdFeAsO}}_{0.9}{\mathrm{F}}_{0.1}$ single crystals. We find that the $\Gamma$ hole pocket is fully gapped below the superconducting transition temperature. The value of the superconducting gap is $15\pm 1.5\mathrm{meV}$ and its anisotropy around the hole pocket is smaller than 20% of this value—consistent with an isotropic or anisotropic $s$-wave symmetry of the order parameter. This is a significant departure from the situation in the cuprates, pointing to the possibility that the superconductivity in the iron arsenic based system arises from a different mechanism.

Figure 1

The superconducting gap along the high symmetry directions ($T=20\mathrm{K}$). (a),(b) ARPES intensity map along the $\Gamma \mathrm{\text{−}}X$ and $\Gamma \mathrm{\text{−}}M$ directions, respectively [directions shown in panel (e)]. Black dotted lines are a guide to the eye, outlining the dispersion of the band and the coherent peak. (c),(d) EDC’s for panels (a) and (b), respectively. The momentum range is indicated by the white arrows in panels (a) and (b). The colored curves mark the EDC at $k_{\mathrm{F}}$. (e) ARPES intensity map as a function of $k_x$ and $k_y$ momentum, integrated within 20 meV about the Fermi energy. Dotted circles are guides to the eye. (f) Dispersion of the coherent peak showing the back bending characteristic of the SC state obtained by EDC fits to data in panel (b). (g) Comparison of the EDC’s in superconducting state at $k_{\mathrm{F}}$ points along $\Gamma \mathrm{\text{−}}X$ and $\Gamma \mathrm{\text{−}}M$ directions. (h) Symmetrized EDCs from the data in panel (g).

Figure 2

The magnitude of the superconducting gap along the $\Gamma$ hole pocket. (a) Raw- and (b) symmetrized-EDCs at the Fermi crossing momenta marked by green (or gray) dots in panel (c), where the definition of the $\varphi$ angle is shown. (d) The value of the superconducting gap extracted from the data in panel (b) using the coherent peak position method. Peak positions are indicated by arrows.

Figure 3

Magnitude of the superconducting gap around the $\Gamma$ hole pocket in polar coordinates. Blue (or dark gray) plots indicate the measured data, and red (or gray) dots are the reflected ones. It is clear from this graph that the gap is always open, with a magnitude (${\Delta }_{\mathrm{peak}}$) that varies between $\sim 13$ and $\sim 18\mathrm{meV}$, indicating conventional $s$-wave or slightly anisotropic $s$-wave behavior at the $\Gamma$ pocket. The green (or gray) line indicates a model with a slight gap anisotropy of 20% that would still be consistent with this data.