Improved systematic uncertainty evaluation of the ${}^{171}$Yb optical lattice clock NMIJ-Yb1 with uncertainty of $2.6\times {10}^{-17}$

We report a systematic uncertainty evaluation of the ${}^{171}$Yb optical lattice clock NMIJ-Yb1 (NMIJ: National Metrology Institute of Japan) with a fractional frequency uncertainty of $2.6\times {10}^{-17}$, improved by a factor of 3.8 compared with our previous uncertainty. The uncertainty of the lattice light shift is reduced to $1.6\times {10}^{-17}$ by preparing axially sideband-cooled atoms in a shallow optical lattice. Different models to calculate the lattice light shift are compared, taking account of possible model-dependent biases due to the treatment of the radial motion of atoms trapped in the optical lattice. The uncertainty of the blackbody radiation shift reaches $7.0\times {10}^{-18}$ by measuring the radiative temperature at the position of atoms with an in-vacuum temperature sensor. We also demonstrate the nearly continuous operation of NMIJ-Yb1 with an uptime of 91.3 $\mathrm{\%}$ for 10 days, showing the potential for future improvement of the calibration uncertainty of International Atomic Time.