Antiferromagnets have large potential for ultrafast coherent switching of
magnetic order with minimum heat dissipation. In novel materials such as
Mn2Au and CuMnAs, electric rather than magnetic fields may control
antiferromagnetic order by N\'eel spin-orbit torques (NSOTs), which have,
however, not been observed on ultrafast time scales yet. Here, we excite
Mn2Au thin films with phase-locked single-cycle terahertz electromagnetic
pulses and monitor the spin response with femtosecond magneto-optic probes. We
observe signals whose symmetry, dynamics, terahertz-field scaling and
dependence on sample structure are fully consistent with a uniform in-plane
antiferromagnetic magnon driven by field-like terahertz NSOTs with a torkance
of (150±50) cm2/A s. At incident terahertz electric fields above 500
kV/cm, we find pronounced nonlinear dynamics with massive N\'eel-vector
deflections by as much as 30{\deg}. Our data are in excellent agreement with a
micromagnetic model which indicates that fully coherent N\'eel-vector switching
by 90{\deg} within 1 ps is within close reach.Comment: 16 pages, 4 figure