Nodal gap symmetry in Ba(Fe$_{0.93}$Co$_{0.07}$)$_{2}$As$_{2}$ from NMR measurement in high magnetic field

There is conflicting evidence in pnictide superconductors for fully gapped s$_\pm$ wave symmetry or gap nodes at the Fermi surface. Here we report measurements of the $^{75}$As NMR Knight shift that suggest existence of gap nodes on an optimally doped single crystal of Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$. The measurements were done from 2 K to 300 K with external magnetic fields from 6.4 T to 16.8 T. The spin part of the Knight shift at low temperature($