  Low-frequency quasi-periodic oscillations (LFQPOs) are commonly found in
black hole X-ray binaries, and their origin is still under debate. The
properties of LFQPOs at high energies (above 30 keV) are closely related to the
nature of the accretion flow in the innermost regions, and thus play a crucial
role in critically testing various theoretical models. The Hard X-ray
Modulation Telescope (Insight-HXMT) is capable of detecting emissions above 30
keV, and is therefore an ideal instrument to do so. Here we report the
discovery of LFQPOs above 200 keV in the new black hole MAXI J1820+070 in the
X-ray hard state, which allows us to understand the behaviours of LFQPOs at
hundreds of kiloelectronvolts. The phase lag of the LFQPO is constant around
zero below 30 keV, and becomes a soft lag (that is, the high-energy photons
arrive first) above 30 keV. The soft lag gradually increases with energy and
reaches ~0.9s in the 150-200 keV band. The detection at energies above 200 keV,
the large soft lag and the energy-related behaviors of the LFQPO pose a great
challenge for most currently existing models, but suggest that the LFQPO
probably originates from the precession of a small-scale jet.
